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fume 34, Number 4^Winter 1991/92

*^N

. .

Mid-Qcean
Ridges

Index to Volume 34 (1991)

Number 1, Spring

Ocean Engineering & Technology

Introduction: Ocean Engineering, Albert J. Williams 3rd
High Resolution Optical and Acoustic Remote Sensing for

Underwater Exploration, W. Kenneth Stewart
SOFAR Floats Give a New View of Ocean Eddies,

Philip L. Richardson

Robotic Undersea Technology/, Dana R. Yoerger
A Telescope at the Bottom of the Sea, George Wilkens,

John Learned, and Dan O'Connor
Ocean Data Telemetry: New Methods for Real-Time

Ocean Observation, Daniel E. Frye, W. Brechner

Owens, James R. Valdes
The Role of the Microcontroller in Ocean Research

Instruments, Albert M. Bradley
The History of Salinometers and CTD Sensor Systems,

Neil Brown

Underwater Technology in the USSR, Deam Given
Toward a Global Ocean Observing System, D. James Baker
Modernizing NOAA's Ocean Service, Virginia K. Tippie

and John H. Cawley
Artificial Reefs: Emerging Science and Technology,

Iver W. Duedall and Michael A. Champ
Douglas Chester Webb: A Profile, Henry M. Stommel

Number 2, Summer
An Open Door:
Soviet-American Cooperation

From the Editor: An Open Door: Soviet-American

Cooperation in Marine Science, Vicky Cullen
Diving the Soviet Mir Submersibles,

Cindy Lee Van Dover
The Oceans and Environmental Security,

James M. Broadus and Raphael V. Vartanov
The History of Soviet Oceanology, Leonid M.

Brekhovskikh and Victor G. Neiman
Living Marine Resources, Viatcheslav K. Zilanov
The USSR and the International Law of the Sea,

Yuri G. Barsegov

Soviet Polar Research, Arthur Chilingarov
Exploring Pacific Seafloor Ashore: Magadan

Province, USSR, Wilfred B. Bryan
Developing a New Soviet Ocean Policy,

Raphael V. Vartanov
Dynamics of Ocean Ecosystems: A National Program in

Soviet Biooceanology, Mikhail E. Vinogradov
Satellite Oceanography, Vladimir V. Aksenov

and Alex B. Karasev
Good Morning, Comrades, Hugh D. Livingston and

Stella J. Livingston
Physical Oceanography: A Review of Recent

Soviet Research, Yuri A. Ivanov
A History of USSR-US Cooperation in Ocean Research,

N.A. Ostenso, A. P. Metalnikov, and B.I. Imerekov

Number 3, Fall

Reproductive Adaptations
in Marine Organisms

An Introduction to Reproductive Adaptations

in Marine Organisms, Lisa Clark
Caribbean Reef Corals, Alina M. Szmant

and Nancy J. Gassman

Mating Strategies of Coastal Marine Fishes, Phillip S. Lobel
Sex (and Asex) in the Jellies, Katherine A.C. Madin

and Laurence P. Madin
Larval Forms with Zoological Verses, Walter Garstang,

illustrated by Rudolph Scheltema
The Story of the Coelacanth, Keith S. Thomson
Elasmobranch Fish: Oviparous, Viviparous, and

Ovoviviparous, Carl A. Luer and Perry W. Gilbert
Challenging the Challenger, Craig M. Young
Hydrothermal Vent Plumes: Larval Highways?, Lauren S.

Mullineaux, Peter H. Wiebe, and Edward T. Baker
Photoessay: A World of Art Beneath the Waves,

Kathy Sharp Frisbee

Number 4, Winter
Mid-Ocean Ridges

Introduction — Mid-Ocean Ridges: The Quest for Order,

Ken C. Macdonald

The Segmented Mid-Atlantic Ridge, Jian Lin
Modeling Ridge Segmentation... A Possible Mechanism ,

Hans Schouten and Jack Whitehead
RIDGE: Cooperative Studies of Mid-Ocean Ridges (plus

Box on InterRIDGE), Donna Blackman
Map of Mid-Ocean Ridges and Research Locations
Ridges and Rises: A Global View, Peter Lonsdale and

Chris Small
Onions and Leaks: Magma at Mid-Ocean Ridges, A Very

Personal View, Joe Cann
From Pillow Lava to Sheet Flow, Evolution of Deep-Sea

Volcanology, Wilfred B. Bryan
Tectonics of Slow-Spreading Ridges, Jeffrey A. Karson
Mid-Ocean Ridge Seismicity, Eric A. Bergman
Hydrothermal Vent Systems, Margaret K. Tivey
The Biology of Deep-Sea Vents and Seeps

Alvin's Magical Mystery Tour, Richard A. Lutz
Megaplumes, Edward T. Baker
Tomographic Imaging of Spreading Centers,

Douglas R. Toomey
Bruce C. Heezen, A Profile, Paul J. Fox

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MID-OCEAN RIDGES

9 Introduction
Mid-Ocean Ridges: The Quest for Order
Ken C. Macdonald

The last decade has brought significant advances in
understanding of the seafloor and its spreading processes.

nThe Segmented Mid-Atlantic Ridge
Jian Lin
A recent Mid- Atlantic Ridge expedition and
other studies contribute to expanding knowledge of
ridge segmentation.

page 11

Ridge Segmentation: A Possible Mechanism

Hans Schouten and Jack Wlritehead

A laboratory experiment with glycerine and

water provides a model for ridge segmentation resulting

from the rise of hot mantle material.

RIDGE and InterRidge

Donna Blackman and Trileigh Stroh
RIDGE is a cooperative effort to study the mid-
ocean ridges and InterRidge is its international counterpart.

Ridges and Rises: A Global View

Peter Lonsdale and Chris Small
An overview of current knowledge of the
patterns, mechanisms, and the relief of mid-ocean ridges.

page 68

Onions and Leaks: Magma at Mid-Ocean Ridges

"% Joe Cann

\^S\J A 35-year review of a dynamic period of Earth
science and ridge models succeeding ridge models.

M ^% From Pillow Lava to Sheet Flow

J Evolution of Deep-Sea Volcanology

"^— Wilfred B. Bryan

An historic, current, and future look at knowledge of the
rocks that make up mid-ocean ridges.

page 84

Copyright © 1991 by the Woods Hole Oceanographic Institution. Oceanus (ISSN 0029-8182) is published in March, June,
September, and December by the Woods Hole Oceanographic Institution, 9 Maury Lane, Woods Hole, Massachusetts

02543. Second-class postage paid at Falmouth, Massachusetts and additional mailing points.
POSTMASTER: Send address change to Oceanus Subscriber Service Center, P.O. Box 6419, Syracuse, NY 13217-6419.

Oceanus

Headings and Readings

Tectonics of Slow-Spreading Ridges

Jeffrey A. Karson

Slow-spreading and fast-spreading ridges build
structure that are quite different from one another.

Mid-Ocean Rise Seismicity

Eric A. Bergman

Seismic waves signal earthquake locations and
expand knowledge of ridge structures.

page 75

"\ Hydrothermal Vent Systems

Margaret K. Tivey

In the 15 years since the first "black smoker" was
sighted, much has been learned of hydrothermal vents.

The Biology of Deep-Sea Vents and Seeps

Richard A. Lutz

\te>' Extensive submersible work in the past two
years has brought new knowledge of deep-sea vent and
seep communities.

Megaplumes
fc / 1 ... Edward T. Baker

The megaplume dectectives are on the case
studying a recently discovered vent phenomenon.

page 100

Editor's Note I

Glossary ',

Map: Ridges & Rises 24
Books & Videos 108
Creature Feature 112

The Oceanus Annual Index

may be found on the inside

front cover.

"\ ^^ Tomographic Imaging of Spreading Centers

J Douglas R. Toomey

^X sLum A new tool yields three-dimensional images of
Earth's dynamic processes working deep within mid-ocean
ridge spreading centers.

Profile

Bruce C. Heezen

Paul J. Fox
A man of extraordinary vision and enormous research

capacity changed thinking about the seafloor.

ON THE COVER: An artist's concept of the mighty Mid-Atlantic Ridge and a glimpse of the
Pacific Ridge are highlighted in the colors scientists use to indicate elevation.
(Watercolor by E. Paul Oberlander, WHOI Graphics)

Winter 1991/92

Vicky Cullen
Editor

Lisa Clark

Assistant Editor

Kathy Sharp Frisbee

Editorial Assistant

Robert W. Bragdon

Advertising & Business Coordinator

Lisa Poole

Publishing Intern

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Oceanus

International Perspectives on Our Ocean Environment

Volume 34, Number 4, Winter 1991 /92 ISSN 0029-8182

193O

Published Quarterly by the

Woods Hole Oceanographic Institution

Guy W. Nichols, Chairman of the Board of Trustees
James M. Clark, President of the Corporation
Charles A. Dana, III, President of the Associates

Craig E. Dorman, Director of the Institution
Sallie K. Riggs, Director of Communications

Editorial Advisory Board

Robert D. Ballard,

Director of the Center for Marine Exploration, WHOI
James M. Broadus,

Director of the Marine Policy Center, WHOI
Henry Charnock,

Professor of Physical Oceanography, University of Southampton, England
Gotthilf Hempel,

Director of the Alfred Wegener Institute for Polar Research, Germany
John Imbrie,

Henry L. Doherty Professor of Oceanography, Brown University
John A. Knauss,

US Undersecretary for the Oceans and Atmosphere, NOAA
Arthur E. Maxwell,

Director of the Institute for Geophysics, University of Texas
Timothy R. Parsons,

Professor, Institute of Oceanography, University of

British Columbia, Canada
Allan R. Robinson,

Gordon McKay Professor of Geophysical Fluid Dynamics,

Harvard University
David A. Ross,

Senior Scientist and Sea Grant Coordinator, WHOI

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Oceanus

From the Editor

On Mid-Ocean Ridges

he theory of plate tectonics, the idea that the surface of the earth is
made up of eight large and several small rigid plates that are in
constant motion (at least in geologic time), was born in the late
1960s, a synthesis of the concepts of continental drift and seafloor
spreading. Observations on the apparent fit of the bulge of eastern South
America into the indentation of Africa date back at least 300 years. The
first detailed theory of continental drift was proposed in 1912 by Alfred
Wegener, a German meteorologist. He suggested that a single supercon-
tinent he called Pangaea existed through most of geological time and
that it began to break up about 180 million years ago. In 1937, Alexander
DuToit, a South African geologist, suggested that rather than one pri-
mordial continent perhaps there
were two, Gondwanaland in the
south and Laurasia in the north.
Neither proposed a mechanism for
continental motion.

New work in the 1950s
brought mounting evidence for
continental drift. During the early
1960s the theory of seafloor
spreading was advanced by US
geophysicist Harry Hess, who
suggested that new crust is con-
tinually being generated by
volcanic activity at the crests of
mid-ocean ridges.

In a 1970 paper, US scientists
Robert Dietz and John Holden
reconstructed Pangaea and de-
scribed a plausible sequence of continental dispersion, depicted overleaf,
over the past 200 million years. "Continental drift,"' they wrote, "is a
necessary consequence of plate tectonics in that the continents would be
passively rafted on the backs of the conveyor-belt-like crustal plates. The
drift of the continents may be conveniently thought of as a summation of
sea-floor spreading."

Plate motion and the tectonic forces that create and destroy the
plates are still being defined and understood, and we share with you in
this issue of Oceanus some of the continuing excitement among Earth
scientists about these forces and the phenomena they create. It is along
the mid-ocean ridges, as Harry Hess surmised, that the crustal plates are
created as molten material, called magma, from deep beneath the surface
rises to fill the gaps or rifts created between plates that are moving or
"spreading" apart. Our authors tell us that different spreading rates
result in different surface expressions: The slowly spreading Mid-
Atlantic Ridge, where about 30 millimeters of crust are created annually, -
rises steeply from surrounding seafloor and has a characteristic depres-

Boundaries of the large

and some of the small

crustal plates are

diagrammed here.

Winter 1991/92

PERMIAN -225 million years ago

TRIASSIC -200 million years ago

JURASSIC -135 million yearsago

Acknowledgment:

The Oceanus staff

thanks G. Michael

Purdy, Woods Hole

Oceanographic
Institution Depart-
ment of Geology and
Geophysics Chairman,
for his advice on
this issue.

sion at its crest that is called a rift valley, while the fast-spreading East
Pacific Rise that expands some 60 to 170 millimeters per year presents a
more gently rolling topography and no rift valley. Seawater circulates
down through the porous new volcanic crust, heating as it moves and
accumulating elements absorbed from the rock. Eventually the heated,
altered water rises again to erupt through vents in the seafloor and
support colonies of unusual animals.

The new crust being created at ridges is balanced by destruction
of crust on the opposite sides of the plates where deep-sea trenches mark
subduction zones, areas where the heavier of two colliding plates is
shoved back down toward the center of the earth. Characteristics of
theses zones include curving island chains or island arcs, such as the
Aleutian Islands and the islands of Japan, and volcanoes born of the
melting edges of subducting plates. Alternatively, when there is no
subduction, mountains result, such as the Himalayas that mark the
colliding boundaries of the Indian and Eurasian plates.

Readers new to these concepts may find it helpful to begin with a
review of the glossary that begins opposite and the profile of Bruce
Heezen (page 100) in which Heezen's student, Jeff Fox, describes the
meticulous assembly of mid-ocean ridge topography by Heezen and
Marie Tharp. Their successive "physiographic maps" revealed the sharp
relief of a seafloor previously thought to be largely flat and uninterest-
ing. A world map on pages 24 and 25 shows ridges and research areas to
help readers follow this issue's wide-ranging discussions of ridge
research.

Author Joe Cann (Onions and Leaks. . .on page 36) notes that early in
his career it was still possible to become well acquainted with every
scientist working in marine geology while now that would be quite
impossible. "Thirty-five years ago," he writes, "most geologists were
secure in the knowledge that continents did not move." As we mark just
over two decades since the theory of plate tectonics gained wide accep-
tance, we invite you to join us for a review of the exciting, and still new,
realm of a planet in motion.

Oceanus

CRETACEOUS -65 million years ago

CENOZOIC Present

Glossary

accreting plate boundary— the border between two separating crustal

plates where new oceanic lithosphere or crust is being created
asthenosphere-the layer of the earth that extends roughly from 100 to

300 kilometers below the surface where temperature and pressure

cause the rock to flow plastically.
axial — relating to a line that bisects a mid-ocean ridge. For example, an

axial rift valley runs down the center of the Mid-Atlantic Ridge.
basalt — medium gray to black igneous rock that constitutes the upper-
most 2 to 3 kilometers of oceanic crust and is the chief component of

isolated oceanic islands, rich in iron, magnesium, and calcium
bathymetry — measurement and charting of ocean depths using echo

soundings plotted on a chart to show seafloor contours
crust — the outermost layer of the earth, 6 to 8 kilometers thick beneath

the ocean and 30 to 35 kilometers thick beneath the continents
diapir — a vertical columnar plug of less dense rock or magma that has

risen through more dense rock

dike — a tabular body of igneous rock that intrudes pre-existing struc-
tures (see photo page 38)
echo sounding — generation of sound in water and recording the time

lapse of the return or echo of the sound from a reflecting surface as a

measure of depth
fault — rock fracturing that displaces the sides of the fracture relative to

one another. Fault movement may be continuous creep or a series of

abrupt jumps (earthquakes).

fault scarp — steep cliff formed by movement along one side of a fault
fissure — an extensive crack in a rock formation
fracture zone — area surrounding a large fault that crosses and displaces

a mid-ocean ridge, often the site of intense seismic activity
gabbro — a group of granular, dark-colored igneous rocks composed

largely of plagioclase and clinopyroxene
graben — a crustal block that is depressed relative to neighboring blocks,

which are called horsts
horst — a crustal block that is raised relative to neighboring blocks, which

are called grabens
hot spot — heat source from deep within the earth's mantle, surface

manifestation of a rising plume of hot mantle material
hydrothermal — relating to heated water and its actions or products
igneous rock — rock of several types formed of molten material (magma)

that upwells from the deeper part of Earth's crust and comprises

most of the oceanic crust

World maps similar to

these were published in

1970 by Robert Dietz

and John Holden
depicting the breakup of
the primordial super-
continent Pangaea and
subsequent dispersion
of continents over the
past 200 million years.
The action began when
the southwest Indian
Ocean rifted, splitting
West Gondwana (South
America and Africa)
from East Gondwana,
and India lifted off
Antarctica. Then
Laurasia (Nortli
America and Eurasia)
separated from South
America and the bulge
of Africa. Later South
America and Africa
split. Spain rotated to
form the Bay of Biscay,
and Madagascar split
from Africa. India
continued its north-
ward trek, and Austra-
lia separated from
Antarctica. As Antarc-
tica rotated westward,

Australia made a

remarkable northward

flight and New Zealand

dropped off its east

coast. The North and

South Atlantic oceans

continued to open,

Greenland parted

company with Europe,

Africa moved slightly

northward and rotated,

and India collided with

Asia, raising the
Himalayan Mountains.

Winter 1991/92

hydro thermal-
relating to
heated water
and its actions
or products

hydrothermal
vent

isostasy — state of equilibrium with the earth's crust buoyantly sup-
ported by the plastic material in the mantle

lithosphere — Earth's outer shell including the crust and uppermost rigid
layer of mantle. In plate tectonics, the lithospheric plates move over
the plastic asthenosphere below.

magma — molten, mobile rock that is the product of melting deep within
the earth's crust or upper mantle, the source of igneous rocks. Lava
is magma that reaches the earth's surface. Magma that solidifies
below the surface is called intrusive or plutonic and that emerging
and solidifying above the surface is called extrusive or volcanic rock.

mantle — zone of the earth extending from below the crust to the core.
The upper mantle extends to 400 kilometers depth followed by a
transition zone from 400 to 1,000 kilometers and the lower mantle
from 1,000 to 2,900 kilometers.

Moho — abbreviation of Mohorovicic discontinuity, the boundary
between the crust and the mantle

offset — horizontal displacement of a topographic trend, commonly
along a fault; also a spur or branch from a mountain range

ophiolite — masses of igneous rocks of oceanic crustal origin that have
been pushed up onto continents by plate collisions

peridotite — coarse-grained igneous rock thought to be the primary
component of the upper mantle, often associated with ophiolites

rift valley — the deep central cleft with a mountainous floor in the crest of
a mid-ocean ridge. The valley results from plate separation; at fast-
spreading ridges upwelling magma fills the rift and smooths the
topography while at slow-spreading ridges the upwelling magma
does not fill the rift but adheres to the trailing edge of the separating
plates.

rift — a narrow opening in a rock caused by cracking or splitting

Ring of Fire — chain of volcanoes occurring in a rough circle around the
perimeter of the Pacific Ocean

scarp — sequence of cliffs resulting from faulting

seep — place of contact between deep-sea sediments and limestone walls
where hypersaline waters seep onto the seafloor and feed sulfide-
dependent biological communities

seismic waves — the form (like sound waves) of the energy released by
fracturing or abrupt slipping of rock along fault planes during an
earthquake. Seismic waves provide valuable information about the
regions they travel through; most importantly they map reflecting
discontinuities, and measurement of the velocity at which the waves
travel through different layers of rocks allows inferences to be made
concerning the extent of various rock types within the earth.

sheeting — ruptures in massive rocks characterized by tabular surfaces
that are slightly curved and parallel to the topographic surface

shield volcano — gently sloping volcano built by flows of very fluid
basaltic lava erupted from a large number of closely spaced vents
and fissures

strike — the direction taken by a structural surface, directional trend

strike-slip — movement parallel with the strike of a fault

Glossary continues on page 112

Oceanus

Introduction

Mid-Ocean Ridges:
The Quest for Order

Ken C. Macdonald

very year, the chain of active volcanoes that comprises the mid-
ocean ridge erupts, on average, ten times as much lava as the

dramatic and disastrous Mt. St. Helens eruption in 1980. This is

enough lava to pave the entire US Interstate freeway system with a layer
of rock 10 feet thick. The largest and most volcanically active chain of
mountains in the solar system, the mid-ocean ridge wraps around the
globe for over 70,000 kilometers, much like the seam of a baseball. Along
the ridge, brittle plates that comprise Earth's surface separate at rates of
10 to 170 millimeters per year. As the plates move apart, rock melts,
separates from the solid residuum, and wells up from tens of kilometers
deep. Some of the molten rock ascends all the way to the seafloor, pro-
ducing extensive volcanic eruptions and building volcanoes, while the rest
adheres to the edges of the parting plates. The late Bruce Heezen (see profile
on page 100) aptly called this world-
encircling system of ridges "the
wound that never heals."

Over the last decade, an ex-
traordinary confluence of diverse
observations at mid-ocean ridges
has led to a series of advances in
our understanding of the seafloor
and its spreading processes.
Swath-mapping tools have been
developed that can image large ar-
eas of the deep seafloor accurately.
Structural maps based on these
charts, combined with geochemical
studies of rock samples, seismic
and gravitational studies of veloc-
ity and density variations beneath
the ridge, seafloor magnetization
studies, and near-bottom imaging
of hydrothermal-vent distribution,
have revealed a fundamental parti-
tioning of the ridge into segments
bounded by discontinuities. These
segments behave like giant cracks
in the seafloor that can lengthen or
shorten, and have cycles of in-
creased volcanic, hydrothermal,
and tectonic activity.

Most observations support the
concept of a hierarchy in the

Merging Sea Beam and SeaMARC II swath bathymetry
produced this shaded-relief image of the Office of Naval
Research East Pacific Rise Natural Laboratory. In the fore-
ground is the Siqueiros transform fault (a first-order disconti-
nuity) and the 8°20'N seamount chain; the fast-spreading
East Pacific Rise and the 9°N overlapping spreading centers
are in the middle; and the Clipperton Transform Fault is in
the background. Notice the numerous seamount chains. The
actual image is approximately 300 by 300 kilometers, viewed
toward the northeast. (This image is based on data from
expeditions funded by the US Office of Naval Research.)

Winter 1991/92

Is the
architecture of

the global

mid-ocean ridge

system really so

orderly, or is

this concept

of a
"segmentation

hierarchy"

merely a human

construct?

segmentation of mid-ocean ridges. First-order segments are generally
hundreds of kilometers long, persist for millions to tens of millions of
years, and are bounded by relatively permanent, rigid, plate-transform
faults (first-order discontinuities). A first-order segment is usually
divided into several second-order segments. These segments are shorter,
survive for less than several million years, and are bounded by nonrigid,
second-order discontinuities that can migrate along the length of the
ridge. Thus second-order segments lengthen, shorten, and even disap-
pear. There are third- and fourth-order segments (and discontinuities
bounding them) that are increasingly short, short lived, and peripatetic.
For example, fourth-order segments, approximately 10 kilometers long,
may survive as distinct structures for only 100 to 10,000 years. The
longevity of individual segments and associated cycles of volcanic/
hydrothermal/ tectonic activity must influence the distribution and
survival of exotic faunal communities that flourish at mid-ocean ridge
hot springs (see The Biology of Deep Sea Vents and Seeps, page 75). For
example, a violent eruption on the East Pacific Rise near 9°50'N in March
and April 1991 wiped out a large community of tube worms, mussels,
and other benthic fauna (and might have done the same to divers in the
submersible DSV Alvin who arrived only hours to days later!).

Thus, amidst frequent volcanic eruptions and seafloor temblors,
there seems to be an orderly spatial and temporal pattern to magmatic,
volcanic, hydrothermal, and tectonic processes associated with the birth
of new ocean floor. Is the architecture of the global mid-ocean ridge
system really so orderly, or is this concept of a "segmentation hierarchy"
merely a human construct? To be sure, this model may be vastly modi-
fied or even abandoned, as new information and new minds contribute
to the ongoing debate. In his superb book, The Principles of Physical
Geology, Arthur Holmes recalled the words of Alfred North Whitehead,
which are still appropriate to our exploration of the Mid-Ocean Ridge
today: "There can be no living science unless there is a widespread
instinctive conviction in the existence of an Order of Things and, in
particular, of an Order of Nature."

Ken C. Macdonald is Professor of Marine Geophysics at the University of
California, Santa Barbara, and a member of the Woods Hole Oceanographic
Institution Corporation. Over the last 20 years he has focused on the tectonics on
mid-ocean ridges and has been fortunate enough to participate in some of the
first explorations of the ridge, using swath-mapping systems, remotely controlled
vehicles, and submersibles.

Oceanus

The Segmented
Mid- Atlantic Ridge

Jian Lin

early three decades ago, in 1964, an Oceanus article by
Richard M. Pratt described an exciting R/V Chain expedi-
tion to the Mid-Atlantic Ridge. On echo-sounding profiles
across the ridge crest some 2,500 meters beneath the ocean
surface, Pratt and his colleagues saw familiar mountains
and valleys on the ocean floor. But a peculiar feature caught his eye: The
rift valley in one area had shifted laterally for tens of kilometers. Pratt, a
Woods Hole Oceanographic Institution (WHOI) scientist, speculated in
his article (Volume 11, Number 2, December 1964) that the shift of the ridge
may have been caused by a "transverse" feature of unknown origin.

In the early 1960s, H. William Menard and Bruce Heezen discovered
similar features on other parts of the Mid-Atlantic Ridge and on the East
Pacific Rise. In 1965, J. Tuzo Wilson identified these transverse features
as "transform faults:" boundaries formed perpendicular to the length of
the Ridge. The anomalous transverse feature noted by Pratt is now
known as the Atlantis Transform Fault. It is just one of many that offset
the 60,000-kilometer-long global mid-ocean ridge system. The discovery
and recognition of transform faults played an essential role in the develop-
ment of the plate-tectonic theory in the late 1960s and early 1970s.

In plate-tectonic theory, Earth's outer 100 to 250 kilometers, called
the lithosphere, breaks up into a set of rigid plates that move with
respect to each other. The lithospheric plates, such as those of North
America and Africa, drift over underlying, less rigid asthenosphere,

According to the theory

of plate tectonics,
Earth's lithosphere is
broken into plates that
move with respect to
each other. The plates
originate at mid-ocean
ridges (A), subduct into
the underlying astheno-
sphere at trenches (B),
and slide by each other
at transform faults (C).

ASTHENOSPHERE

MESOSPHERE

Winter 1991/92

A computer-generated
relief image of the rift
valley of the Mid-
Atlantic Ridge near
29°50'N (view towards
the south). The rift
valley is 20 to 30
kilometers wide and a
few kilometers deep.
The inner rift valley is
covered by elongated
volcanic hills and
circular volcanoes.
Large steplike normal
faults run along the
ridge axis, here shown
at three-times vertical
exaggeration.

28°N

Rift Valley

42°W

43°W

1000

Speading Center Axis

PR. Shaw/WHOI

much like
icebergs
floating in the
ocean. Plates
are created at
mid-ocean
ridges, are
consumed at
subduction
trenches, and
slide by each
other along

transform faults. When plates interact at
their boundaries, earthquakes strike,
volcanoes erupt, and mountains grow. The
plate-tectonic theory provided a fundamen-
tal link between global tectonics, from ridges
to trenches and from continents to ocean
floors. It unified geology in the same way that
the principle of evolution unified biology.
The development of the rigid-plate concept and
early, sparse observations of ocean ridges led to a simple
idea of how the ridges worked. They were thought to be
linear spreading segments, periodically offset by trans-
form faults. Each spreading segment was hundreds of
kilometers long and had the same two-dimensional

In this computer-generated relief image of the Mid-Atlantic Ridge
between 28°N and 30°45'N, thin red lines along the rift valley approxi-
mate spreading axes based on magnetic data. Note the prominent rift
valley, deep Atlantis Transform Fault trough (just below 43° W), and
small non-transform offsets (regions where red lines do not meet).

Oceanus

Atlantis Transform Fault

cross-sectional view along its strike.

Today, two decades after the birth of plate-tectonic theory, this view
is rapidly changing. This article begins with a report on a recent expedi-
tion to the Mid-Atlantic Ridge, where Pratt visited 30 years ago. Using
this and other recent studies as background, I will review current ideas
on seafloor spreading at mid-ocean ridges and explain how the earlier
two-dimensional ridge model must be expanded to allow for variations
along strikes and with time.

The Mid-Atlantic Ridge

The huge Mid-Atlantic Ridge (MAR) mountain range runs down the
middle of the Atlantic Ocean from Iceland in the north to near Antarctica
in the south. Since it was first studied in 1873 by the British survey ship
HMS Challenger, the ridge has been the focus of intense scientific curios-
ity. Perhaps the most detailed survey was Project FAMOUS (French-
American Mid-Ocean Ridge Undersea Study) of the early 1970s, in which
oceanographers investigated a 100-kilometer-long stretch of the rift
valley near 37°N.

Early exploration of the Mid-Atlantic Ridge only identified trans-
form faults of very large offsets. From observations of Earth's magnetic
field, however, oceanographers in the early 1980s proposed that the
MAR was composed of a string of about 50-kilometer-long spreading
segments separated by small "zero-offset" transform faults.

To look closely at these "zero-offset" features, in 1988 and 1989,
scientists from WHOI and the University of Washington examined a 900-
kilometer-long stretch of the ridge. Our survey, which started near the
Atlantis Transform Fault, included detailed ocean floor mapping and
precise measurements of Earth's gravitational and magnetic fields. Our
sonar sounding system, called Sea Beam, was much more capable than
that used by Pratt in the early 1960s: It can map a 2-kilometer- wide

This relief image of the

Atlantis Transform
Fault is viewed toward

the east. The deepest
parts of the trajisform

valley are more than
six kilometers below sea
level. The vertical relief

from the top to the
bottom of the valley is

more than four
kilometers, and is

shown at three-times

vertical exaggeration.

The red vertical bars on

the top-right corner are

an imaging artifact.

Winter 1991/92

Seismic
Velocity

(m/s)

4.5-5.5
6.5-7.0

7.8-8.4 1

Below a ridge, the

mantle of the astheno-

sphere (orange) rises to

fill the gap between two

separating lithospheric

plates (blue). As they

rise, some rocks melt to

form magmas. The

buoyant magmas or

melts then surge into a

magma chamber (red).

Material in the magma

chamber further
segregates into various

layers of the oceanic
crust (dark green). The
crust is less dense than
the mantle. In the
mantle, density
decreases with increas-
ing temperature and
with depth.

swath of the seafloor in a single pulse. Never before had oceanographers
seen such a long stretch of a slow-spreading ridge with such high-
resolution sonar.

With Sea Beam sending back numerous ocean-floor images, we soon
recognized many familiar features. The central rift valley, which is 20 to
30 kilometers wide and a few kilometers deep, runs nicely along the
ridge crest. Volcanic hills and circular volcanoes — each tens to a few
hundreds meters tall — blanket the rift valley's inner floor. The crust

cracks along steep steps of normal
faults, some of which run along
the ridge for tens of kilometers.

We were intrigued by the im-
mense size of the trough inside the
Atlantis Transform Fault. This fault
offsets the ridge axis by almost 70
kilometers, and the deepest parts of
its floor are more than six kilome-
ters below sea level. Its vertical relief
from top to bottom is more than
four kilometers, or twice the depth
of the Grand Canyon.

Further down the ridge axis,
however, we were totally sur-
prised: The "zero-offset" features
that we were searching for were
not transform faults after all. The

ridge breaks into many 20- to 80-kilometer-long spreading segments, but
these segments often overlap one another at "nontransform" offsets.
Unlike a transform fault, a nontransform offset does not contain a trough
perpendicular to the ridge axis. By themselves, these nontransform
offsets constitute a new type of unstable, transitory plate boundary.

Why do the MAR volcanic chains break into short spreading seg-
ments? Is this phenomenon related to deep structures of the earth
beneath the spreading axis? To answer these questions, we must first
examine how oceanic crust is generated.

The Origin of Oceanic Crust

The mantle beneath lithospheric plates, known as the asthenosphere,
creeps plastically because its temperature stays near its melting point.
Below a ridge, however, the asthenosphere rises to fill the gap between
two separating plates. While ascending, some mantle rocks fuse to form
basaltic magmas, or melts, and the buoyant magmas float to the top of
the mantle to form oceanic crust. Meanwhile, the unmelted mantle
residual accretes to the oceanic lithosphere bottom. From studying the
chemical composition of rocks dredged from the ocean floor, oceanogra-
phers have determined that melts are produced at depths of 20 to 80
kilometers and at temperatures of 1,150° to 1,400°C. Theoretical models
further suggest that the rising asthenosphere reaches its maximum
velocity in a partial melting zone, inside which the mantle has its mini-
mum density and viscosity.

Buoyant melts from all depths surge into a magma chamber at the

Oceanus

Transform Zones
\

CD
.C
CL
(f>
O

Asthenosphere

base of the crust. Inside the chamber, melts further separate into layers
according to their densities. The least-dense lava erupts to form volca-
noes on the ocean floor. The most-dense, called gabbro, accumulates at
the chamber floor to form the lower crust. Between these two layers lies
a layer of vertical dikes, narrow slabs of cooled melt that have risen to
fill fissures in the crust. The end product of the melt segregation process,
therefore, is a stable layering of light crust (its density is expressed as 2.5
to 2.8 grams per cubic centimeter) overlying heavy mantle (3.1 to 3.3
grams per cubic centimeter).

On the other hand, the
partially molten astheno-
sphere under the lithospheric
plates is not stable. This is be-
cause its density and viscos-
ity are less than that of the
overlying lithosphere, a situ-
ation analogous to a layer of
high-density fluid overlying
low density fluid. Laboratory
experiments show that if the
density and viscosity con-
trasts between two fluids are
great enough, the less-dense
fluid will rise and protrude
into the upper layer, in the
form of regularly spaced dia-
pirs (see Box, page 19). In the

mid-1980s, oceanographers proposed that diapirs may occur below mid-
ocean ridges. They reasoned that the partial melting zone of the astheno-
sphere can develop gravitational instability, inducing regularly spaced
diapirs of melts. The melt diapirs then percolate toward the surface to
form discrete magma chambers that feed individual spreading segments.

Such diapir-induced segmentation models predict that ocean-floor
topography should be shallowest at segment centers and deepest at seg-
ment boundaries. The models also predict that crustal thickness, which
is a measure of melt production, should be greatest at segment centers
and decrease toward segment edges. The first prediction was confirmed
readily by detailed ridge-crest topography, including that obtained in our
survey. To confirm the second prediction, however, techniques for prob-
ing into the earth are required. Two commonly used techniques are the
studies of Earth's gravitational field and of seismic waves.

Gravity, Seismic, and Faulting Evidence

Geophysicists often use sensitive gravity meters to probe unseen mate-
rial below ground. To examine the crust and mantle below spreading
segments, we must first employ modeling to remove the gravitational
effects of seawater and a model crust. The leftover signal, called the
mantle Bouguer anomaly, should then reveal information about the
mantle. Using this technique, researchers have detected an unusual
"bull's-eye" shaped gravity low over a spreading segment in the South
Atlantic. During our 1988 to 1989 survey, we found a string of such

Crust

Melt

Depleted

Mantle

In this model of

magma diapirs beneath

a ridge, the partially

molten asthenosphere

(red) is not stable
under the cold lithos-
phere (green). The
gravitational instabil-
ity of this partial
melting zone will
induce regularly
spaced diapirs of
magmas. The magma
diapirs then percolate
toward the surface to
form discrete spreading
segments.

Winter 1991 J92

44°W

30°N

• III

(MGAL)

Gravity data reveals

gravity lows beneath

the spreading segments

of the North Mid-
Atlantic Ridge. This
pattern may be caused
by thicker crust or less
dense mantle beneath
the midpoints of the
spreading segments.

bull's-eyes in the North Atlantic.

In both the North and South Atlantic, most of the circular regions of
gravity lows are centered near the shallow middle points of the spread-
ing segments. In contrast, large positive values are located over the
Atlantis Transform Fault and the nontransform offsets. There are two
possible explanations for this gravity pattern. The first is that the crust,
which is less dense than the mantle, decreases in thickness from segment
centers to segment edges. The second is that the mantle beneath the

42°W segment mid-points is of unusu-

ally low density, most likely due
to a combined effect of high
temperature, the presence of
limited melts, and density changes
in the melted mantle. Both possi-
bilities are consistent with the
concept of diapir-induced seg-
mentation. Similar gravity pat-
terns have now been observed in
other sections of the Mid-Atlantic
Ridge, although substantial local
variations exist.

The study of seismic waves
provides another powerful tool for
probing the deep structure of
ridges. In the 1980s, seismologists
studied a half-dozen large-offset
transform faults and a few
nontransform offsets at the Mid-
Atlantic Ridge. Their results
generally confirmed that the
oceanic crust is abnormally thin
beneath segment boundaries,
especially under large-offset
transform faults. The gravity and
seismic data together, therefore,
confirmed that the punctuation of
the ridge topography by offset
features is indeed indicative of
deep-seated, along-axis periodicity in the melt supply.

In addition to the gravity and seismic data, imprints of segmentation
were found in the pattern of tectonic faulting and earthquakes on the
ocean floor. Based on deep-sea observations from submersibles, re-
searchers in the 1970s and 1980s determined that lithospheric plates do
not spread steadily; instead, they move in a "stop-and-go" fashion, with
long periods of tectonic stretching interrupted by short periods of
volcanic construction. During prolonged periods of stretching, tectonic
faults developed at the Mid-Atlantic Ridge. Recently we observed that
tectonic faults are quite linear within spreading segments, indicating that
the crust of each segment breaks in parallel fault arrays. In contrast,
oblique faults are common near segment offsets, suggesting that the
tectonic-volcanic cycles of neighboring segments are not synchronous
with each other. The major faults of each spreading segment are

f ^DISCONT
DISCONT.

28°N

en
o

Oceauus

seismically active, as indicated by large numbers of moderate-sized
modern earthquakes (See Mid-Ocean Ridge Seismicity, page 60).

The Spreading-Rate Factor

The overall magma supply, as well as the style of segmentation, vary
dramatically from slow- to fast-spreading ridges. From early, sparse
observations, oceanographers noted that the gross axial topography of a
mid-ocean ridge depends on the plate-spreading rate. The crest of the
slow-spreading Mid-Atlantic Ridge, with full spreading rates of about 30
millimeters per year, is rugged, with faulted crust and a median valley.
In contrast, the axis of the fast-spreading East Pacific Rise, with full rates
of 60 to 170 millimeters per year, is much smoother, and is defined by an
elevated crust a few hundred meters high. Theoretical models suggest
that the rift valley of a slow-spreading ridge may result from thinning of
the lithospheric plate similar to the "necking"of a plastic beam under
tension. For fast-spreading ridges, topography is caused mainly by the
upward push of the buoyant magma chamber.

There are other major differences between slow- and fast-spreading
ridges. Geophysicists have imaged the top of magma chambers at the
fast-spreading East Pacific Rise and the intermediately fast-spreading
Juan de Fuca Ridge and the Valu Fa Ridge of the Lau Basin, but no
comparable structure was found at the slow-spreading Mid-Atlantic
Ridge. The lithospheric plate at the MAR has a 3- to 10-kilometer-thick
brittle lid in which moderate to large earthquakes can nucleate; at the
fast-spreading ridges, the brittle lid is thinner than 2 kilometers, and
moderate and large earthquakes are essentially absent. Other differences
include gravity and bathymetry, which vary substantially along the
slow-spreading ridges, but only slightly along intermediate- and fast-
spreading ridges. There is mounting evidence, then, that overall magma
supply is greater at fast-spreading ridges than slow ones.

Despite major differences in magma supply, both fast- and slow-
spreading ridges break into spreading segments. Various types of ridge-
crest offsets have been found at the East Pacific Rise, including transform
faults, overlapping spreading centers, and deviations from axial
lineality. Chemical-composition studies of seafloor rocks indicate that
even small ridge offsets mark boundaries between two distinctive
magma-supply units.

Global variability in ridge magma supply is, in some ways, analo-
gous to global variability in climate. The mean air temperature (or
overall climate) of Earth's polar regions is substantially lower than that
of the equatorial oceans. In both the cold polar and warm equatorial
regions, however, the temperature varies from one local area to another.
Similarly, the mean magma supply at the slow-spreading Mid-Atlantic
Ridge is lower than that of the fast-spreading East Pacific Rise. But in
both ridges the magma supply varies locally from one segment to
another, and from one part of the segment to another part.

The geophysical evidence discussed above and the present segmen-
tation theories illustrate only the gross structure of ridge segmentation
on length scales of a few to a few hundred kilometers. Smaller, shorter-
lived segmentation features, which are beyond the detectabilitly of current
instruments, are certainly possible. To further understand and eventually

Tlie overall

magma supply,

as well as the

style of
segmentation,

vary

dramatically

from slow- to

fast-spreading

ridges.

Winter 1991/92

Why do slow-
spreading ridges

differ

dramatically

from fast ones?

How do

spreading

segments evolve

in time over tens

of millions of

years?

predict the global pattern of magma supply and ridge segmentation, we
must develop better instruments, expand the data base, and formulate
new theories.

The Future of Ridge-Magma Dynamics

In the past decade, theories of ridge-magma dynamics have advanced
rapidly in conjunction with new oceanographic instruments and discoveries
of exciting ridge features. As a result, we have a better understanding of
how two-thirds of Earth's solid surface — the oceanic crust — was created.

The 1990s promise even greater understanding of ridge dynamics, a
system once described by the late Bruce Heezen (see the Profile on page
100) as "the wound that never heals." Oceanographers have designed
and are implementing an international decade-long program called
RIDGE (Ridge Inter-Disciplinary Global Experiments; see the article on
page 21) to study the interactions among complex ridge processes from
magma dynamics to earthquakes, water column chemistry, and biology.
In particular, the RIDGE program has designed specific oceanographic
experiments to continuously explore the origin of mid-ocean ridge
segments, posing such questions as, Why do slow-spreading ridges
differ dramatically from fast ones, but both break into segments? How
do spreading segments evolve in time over tens of millions of years?
Why are transform faults longer and stable while nontransform offsets
are shorter and transitory? Does segmentation in the volcanic ridge
correlate with changes in ridge-crest hydrothermal vents, or even with
the biological population at the ridge crest? These and many other
questions await exploration in a new era of oceanographic studies.

Acknowledgments: The research reported in this article was supported by the
National Science Foundation and the Office of Naval Research. G. Michael Purdy
and Hans Schouten of WHOI, and J.-C. Sempere of the University of Washington
were the leaders of the 1988-1989 Mid- Atlantic Ridge expedition. Thanks to
Brian Tucholke and Tom Reed for permission to use the previously unpublished
images at the top of page 12 and on page 13 and to P.R. Shaw for permission to
use the previously unpublished image at bottom of page 12. The figure on page
11 is after Isacks, Oliver, and Sykes, 1968; that on page 14 is modified after
RIDGE Steering Committee, 1989; on page 15 after Whitehead, Dick, and
Schouten, 1985, and Dick, 1989; and on page 16 after Lin, Purdy, Schouten,
Sempere, and Zervas, 1990.

Jian Lin is an Assistant Scientist at the Department of Geology and Geophysics,
Woods Hole Oceanographic Institution. He was born when H. William Menard
and Bruce Heezen discovered the first transform faults in the world's ocean
basins. Today he enjoys the opportunity to further develop and challenge aspects
of the great theory of plate tectonics. His research activity ranges from oceano-
graphic measurements at sea to building quantitative ridge models on super
computers. When not exploring undersea volcanoes, he studies earthquake
faults in southern California and their threats to metropolitan areas. He is an
Associate Editor of the Journal of Geophysical Research.

Oceamis

MODELING RIDGE SEGMENTATION.

ALL MID-OCEAN RIDGES seem to be
segmented. In many places the ridges
consist of a series of relatively straight
segments divided by fracture zones. In
other places they are divided by overlap-
ping spreading centers. The spreading
plates display a pattern of fairly orderly
cellular structure, with spacing between
the cells of approximately 30 to 80 kilo-
meters. The spacing varies with the speed
of spreading of the ridge. Those study-
ing the ridge had often suggested that
ridge segmentation was due to thermal
contraction of the cooling plates as they
spread apart.

In 1984 and 1985 we, along with
Henry Dick, hypothesized something
quite different — that the segmentation
results from forces produced by hot
mantle material rising under spreading
centers and liberating melt. We knew
that a layer of material with either en-
hanced melt or a higher temperature
tends to develop a lower density, and
there was reason to believe that it would
also have a lower viscosity than the sur-
rounding regions. We also knew that
such a region is prone to develop fluid-
dynamic instabilities. One example is
called Rayleigh-Taylor instability. This
happens when a layer of lower-density
fluid underlies a layer of higher-density
fluid. The interface between the two flu-
ids develops undulations so that the
lower density fluid can float upward
through the denser fluid.

To demonstrate this we conducted
some simple experiments in which a
water-glycerine mixture was quickly in-
jected into glycerine with a hypodermic
syringe along a horizontal line. Although

this line gradually rises because the wa-
ter-glycerine mixture is less dense than
the glycerine, an instability also devel-
ops (see the photos overleaf) and leads to
the formation of semi-spherical pockets.
It is reasonable to expect that a linear
region of partially molten mantle in the
earth will behave in a similar manner
and will lead to fairly regularly spaced
protrusions from which the melt will
ascend to form magma chambers. We
suggested this example as a possible
model of what might be happening un-
der oceanic ridges. To be specific, the
idea was that segmentation was pro-
duced by buoyancy-induced instability
(which ultimately leads to volcanism)
rather than by thermal contraction of the
cold plates.

Clearly, the model was too crude to
apply to ridges in detail. However, at
that time numerical models of spreading
ridges were always taken to be two di-
mensional for simplicity and ignored seg-
mentation. Unfortunately, if the segmen-
tation is a process that enhances up-
welling the two dimensional models
would be incomplete. Recently, observa-
tions indicate that segmentation is in-
deed not just a surface feature of cooling
plates but extends "deep" (tens of kilo-
meters) under ridges. In addition, recent
three-dimensional numerical models
have been developed with flows that do
break up into segments. Thus the crude
idea that segmentation has deep origins
seems to be borne out even though the
detailed mechanics of the break up may
be different in detail than our Rayleigh-
Taylor models. "^N

(continued on next page)

HANS SCHOUTEN AND JACK WHITEHEAD

WOODS HOLE OCEANOGRAPHIC INSTITUTION

Winter 1991/92

.A Possible Mechanism

These two photographs were taken 30 seconds apart. In the upper photo-
graph, the injecting needle was dragged from right to left. The gravitational
instability of a horizontal line of water I glycerine mixture in a bath of pure

glycerine is shown below.

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International Perspectives on Our Ocean Environment

Oceanus

RIDGE:

Cooperative Studies of Mid-Ocean Ridges

Donna Blackman and Trileigh Stroh

he Ridge Inter-Disciplinary Global Experiments (RIDGE)
Initiative is a cooperative effort to study the mid-ocean
ridges as a dynamic global system of focused energy flow
from Earth's interior outward. The National Science Founda-
tion supports the RIDGE Initiative, part of the US Global

Change Research Program, through both its Global Change and Ocean

Sciences divisions.

The program's key goals include:

• characterizing the global ridge structure,

• understanding crustal accretion and upper-mantle dynamics,

• charting the variability over time of volcanic and hydrothermal
systems,

• mapping biological colonization and evolution at ridge crests,

• determining the properties of multiphase materials at ridge crests, and

• developing technology for ridge-crest experimentation.

By characterizing the ridge structure, researchers intend to provide a
global perspective for the mechanics of plate separation, variable lava
types, circulation of hot seawater, and biological characteristic of indi-
vidual ridge sections. Swath bathymetry, sidescan sonar imagery, and
widely spaced geologic and hydrologic samples will be used to develop
large-scale maps of the ridge system. This will provide a basis for
estimating the total flux of materials through ridge crests (hydrothermal
input to the oceans, for example), as well as for making site-location
decisions for more detailed study.

Crustal accretion results from convective upwelling of the mantle
beneath a spreading center (see Onions and Leaks. ../page 36). Basaltic
melt segregates from the rising, decompressing mantle, and is delivered
to a magma chamber at the ridge axis where it solidifies to form oceanic
crust. Understanding this process requires a variety of information: the
mantle's flow geometry, temperature, and composition beneath mid-
ocean ridges; the nature of the subaxial magma chamber and the mode
of volcanic extrusion on the seafloor; and the role of hydrothermal
circulation in cooling the crust. Constraints on the upper-mantle struc-
ture are obtained from computer modeling and seismic- and electromag-
netic-imaging studies that use large arrays of seafloor instruments. A
magma chamber's size and shape are revealed by geophysical measure-
ments including seismic refraction /reflection and gravity data; geo-
chemical studies of rock samples from the ridge axes help determine the

RIDGE

Winter 1991/92

Megaplume ^p

^ "-

- ;'J*t*-?- ?«» SHS^J

Mantle

Peridotite

Basaltic melt arriving
from the mantle either

reestablishes or
replenishes a crustal
magma chamber that
solidifies to produce
gabbro, diabase dikes,
or basaltic lava flows.
The geometry, longev-
ity, and circulation of a

subaxial magma

chamber are topics of

active inquiry.

history of the basaltic melt as it
separates from the mantle and
cools within the crust. Mapping the
hydrothermal vent fields at ridge
axes and the associated faulting of
the seafloor illustrates the seawa-
ter-circulation pattern in the upper
crust.

Although seafloor spreading is
continuous on a geologic time
scale, individual earthquakes,
eruptions, and venting episodes
affect only a short length of the
ridge for a short time. Neither the
spatial nor the temporal scales of
specific ridge-axis events are
currently known in detail, but both
play critical roles in shaping
seafloor morphology, local seawa-
ter properties, and biological
diversity. Two important means
for studying temporal variability in
ridge processes are event detection

and response and long-term deployment of instruments in a seafloor
observatory. When a ridge-crest volcanic event, such as an earthquake
swarm, is detected, airborne and shipboard instruments can be deployed
to chart the activity pattern and map any new eruptions. Long-term
monitoring can reveal linkages among complex, interrelated physical,
geological, and biological processes at ridges. Diverse coordinated
measurements made at permanent ridge-crest observatories will be
essential in studying these relationships and developing improved
theoretical models for ridge processes.

Mapping biological communities along mid-ocean ridge crests is
fundamental for understanding the thermal and chemical requirements
of these unique ecosystems. Sampling and laboratory studies will reveal
the physiological and genetic requirements for living without sunlight at
water depths exceeding 2,000 meters. Integrating biological and chemical
studies will elucidate the dependence of different organisms on the tem-
peratures and the chemical characteristics of hydrothermal vents. Determin-
ing a biological community's response time to changes in volcanic and
hydrothermal activities will be an important aspect of this research.

Multiphase materials are present in virtually every part of the mid-
ocean ridge system, from the upwelling mantle that contains basaltic
melt, through the magma chamber where molten rock is crystallized, to
the hydrothermal systems in which both liquid and gas aqueous solu-
tions are likely to exist. Laboratory experiments on silicate aggregates
under various temperature and pressure conditions are needed to define
the behavior of the ascending mantle (viscosity, melt content, and
composition). The chemical properties and crystallization sequences of
mid-ocean ridge basalts must be determined at pressures appropriate for
a crustal magma chamber. Modeling the effects of combined vapor and
fluid phases in hydrothermal circulation should aid in understanding

Oceanus

interactions between the cooling crust and seawater.

Developing extended-deployment seafloor instrumentation that can
accurately measure the changing conditions at the ridge crest is an
integral part of many of the above research topics. Examples of new
technological advances include chemical sensors that detect minute
changes in trace elements and compounds (such as hydrogen sulfide,
methane, iron, manganese, and oxygen), geodetic instruments to mea-
sure uplift and tilt of volcano flanks, broadband ocean-bottom seismom-
eters, and deep-water temperature and chemical profiling systems.
Systems that can deploy and manipulate these sensitive instruments will
also be required and may take the form of remotely operated seafloor
vehicles or manned submersibles.

Donna Blackman is a post-doctoral research associate at the University of
Washington studying mid-ocean ridge processes. At the time of writing, she is
assisting with several projects at the RIDGE office.

Trileigh Stroh has been the RIDGE Coordinator and Editor of RIDGE Events
newsletter since 1988. In 1992 she will instead serve as Executive Administrator
for the InterRidge office.

InterRidge

INTERRIDGE IS an international effort
to coordinate and expand ridge-crest re-
search. Representatives from scientific
communities in Australia, Canada,
France, Germany, Iceland, Japan, Nor-
way, Portugal, UK, USA, and USSR have
been meeting since 1989 to establish
means for effective communication, pro-
gram coordination, and data exchange
among various national programs for
mid-ocean ridge research. Ratification of
an InterRidge program plan and estab-
lishment of an InterRidge office are ex-
pected in 1992. The Program Plan will
propose three primary program elements:
global studies, observatory development,
and regional dynamics studies.

A recent response to events south-

west of Iceland showed that international
cooperation can produce insights into
ridge-crest processes and their transient
oceanographic signals. In November
1990, Icelandic scientists reported a se-
ries of earthquakes on the Reykjanes
Ridge. Their report was followed by de-
ployment of sonobuoys and expendable
bathythermographs from a US Navy P3
aircraft. British, Icelandic, and American
scientists used an Icelandic research ship
to conduct several days of on-site map-
ping and sampling. Using the combined
assets of several countries enhances the
ability to quickly assemble a team of
investigators at an eruption site, provid-
ing valuable opportunities to document
ongoing ridge-crest activity.

Winter 1991/92

Earth's RiftS, Ridges & Rises: Areas of interest referred to within this issue

1-Albatross Plateau
2-Atlantis Transform Fault
3-Aleutian Trench
4-Carlsberg Ridge
5-Chile Rise
6-Clipperton Fracture
7-Cox Transform Fault

8-East Pacific Rise
9-Easter Island
10-Galapagos Rift
11-Gorda Ridge
12-Guaymas Basin
13-Guaymas Transform Ridge
14-Gulf of Aden

15-Gulf of California
16-Hawaiian Islands
17-Juan de Fuca Ridge
18-Kula Rise
19-Labrador Sea
20-Lesser Antilles Arc
21-Louisiana Slope

Oceanns

This computer-generated map is courtesy of Peter W Sloss, NOAA National Geophysical Data Center. Boulder. CO.

22-AMAR Area

FAMOUS Area
23-TAG Area
24-MARK Area
25-Mariana Trench
26-Mathematician Rise
27-Mendoza Rise
28-Mid-Atlantic Ridge

29-Middle America Trench
30-Monterey Canyon
31-Pacific-Antarctic Ridge
32-Red Sea

33-Siqueiros Transform Fault
34-South Shetland Arc
35-Troodos Ophiolite
36-Valu Fa Ridge

37-West Florida Escarpment

38-Iceland

39- Azores

40-Tristan da Cunha

41-Vema Transform

42-1 5°20' Transform

43-Kane Transform

Winter 1991/92

Slow spreading

produces
relatively steep-
sided "ridges,"
while fast
spreading
produces more
gently sloping

"rises."

Ridges and Rises:
A Global View

Peter Lonsdale and Chris Small

eafloor spreading — the process that creates new material to fill
in gaps between Earth's separating crustal plates — results in
broad elevations with spreading centers along their crests. This
is simply because crust formed by volcanic activity deepens as
it moves away from the axes, cools, and contracts. The young-
est, hottest crust stands highest, and the rate of deepening, which
determines the regional slope gradients away from the spreading center,
is proportional to the horizontal rate of crustal aging. Slow spreading
produces relatively steep-sided "ridges," while fast spreading produces
more gently sloping "rises." The regional side-slopes of spreading ridges
and rises are concave curves, flattening out to imperceptible gradients
where the crust is about 100 million years old. Even on the steepest,
youngest part of slow-spreading ridge flanks, the regional gradients are
actually so low that "ridge" may seem a misnomer; the popular concept
of the Mid-Atlantic Ridge as a mighty chain of undersea mountains
seems somewhat overblown given that the regional slope halfway down
its flanks is no steeper than the eastward slope of the North American
Great Plains. Of course, the small-scale topography of the Mid-Atlantic
Ridge is rougher, but it took the truly global perspective provided by the
very low resolution of exploratory bathymetric data (piano-wire sound-
ings, hundreds of kilometers apart) for 19th-century oceanographers to
recognize this "ridge" as a major feature of Earth's surface.

Long before oceanic crust attains an age of 100 million years, the
cooling-induced slope of its upper surface is altered by other seafloor
processes; the outer margins of mid-ocean ridges are generally defined
by the topographic boundary between the landward structural slope of
cooling lithosphere and the seaward depositional slope of continent-
derived sediment. In the Atlantic Ocean, this boundary usually occurs
where the crust was created about 70 million years ago, so the Mid-
Atlantic Ridge is a 1,500- to 2,000-kilometer- wide structure that includes
all the crust created since then. It covers half of the seafloor. Narrower
basins have proportionately narrower mid-ocean ridges. Young ex-
amples are in the Gulf of Aden and the mouth of the Gulf of California,
where the ridges are less than 100 kilometers wide. Ridges may even be
disproportionately narrow where basins are exposed to the rapid influx
of sediment from adjacent continents, and in extreme cases (such as
within the Gulf of California and the Red Sea) smothering sediment
completely inhibits ridge-building volcanic eruptions, and seafloor
spreading proceeds without construction of a mid-ocean ridge. Rapid

Ocentins

burial of a ridge and obliteration of its characteristic relief can also occur
if spreading stops because of a change in continental drift patterns. A
buried mid-ocean ridge underlies a sediment plain in the center of the
Labrador Sea, where spreading between Greenland and Labrador
stopped 45 million years ago.

Mid-Ocean Ridges and Ocean Basins

These examples can be arranged in sequence to illustrate a familiar
model of the development and demise of mid-ocean ridges in growing
"Atlantic-type" intercontinental ocean basins. Such basins have been
abundant on Earth for the last 300 million years, as the supercontinent
Pangaea broke up and its fragments drifted apart. There are other types
of ocean basins, with other types of spreading ridges, especially in the
whole hemisphere occupied by the Pacific Ocean.

Speculative reconstructions of supercontinents that preceded
Pangaea suggest that the Pacific Ocean may have originated by spread-
ing at a mid-ocean ridge between North America and Antarctica, but
that was in late Precambrian times, about 700 million years ago, and all
the crust formed during this expansionary phase has long since been re-
cycled into Earth's mantle at subduction zones. None of the Pacific
Ocean's present floor is older than 250 million years, and all the time it
was being created the ocean basin was getting smaller as adjacent conti-
nents converged on it. During this prolonged contractional phase, two
distinctive types of spreading ridges have been active: Pacific rises (typi-
fied by the East Pacific Rise), and back-arc ridges (such as the Mariana
Trough Ridge). Rises are generally considered variants of mid-ocean

Rift Valley

New Ocean Basin

Mid-Ocean Ridge

Fossil Ridge

Ocean
Water

Sediment

Continental
Crust

Oceanic
Crust

Mantle
Rocks

Jack Cook/WHOI Graphics

Winter 1991/92

The development and

demise of a mid-ocean

ridge. A: Incipient

separation of two

continental blocks

causes faulting and

thinning of the

continental crust, and

development of a rift

valley (e.g., East

African Rift).

B: Continued crustal

separation produces a

gap that is partly filled

by sediment washed off

the continents and
partly by melting of the

mantle to produce
oceanic crust (e.g., Gulf
of California). C:As
the gap between the
separating continents
increases, oceanic crust
formation by seafloor
spreading at the crest of
a rifted mid-ocean ridge
becomes fully developed
(e.g., North Atlantic).
D: If continental
separation stops,
seafloor spreading
ceases and the mid-
ocean ridge subsides as
it cools and gradually
becomes covered
with sediment.

These equal-area
projections reveal the
global distribution of
spreading centers on

mid-ocean ridges,
Pacific rises, and back-
arc ridges. The relative
importance of the three
types varies from
region to region.

ridges, mere components of the same
global ridge system that connects
divergent plate boundaries,
though they neither originate
as intercontinental rifts nor
occupy mid-ocean posi-
tions. Back-arc ridges are
not even connected to a
global ridge system, and
instead of straddling
boundaries between major
diverging plates they adjoin
subduction
zones that
mark

sites
of plate
convergence

and destruction of oceanic
crust or lithosphere.
Whereas authors with an
"Atlantic fixation" have
adopted Pacific rises as an
eccentric, errant variety of
the familiar mid-ocean
ridge, to the amusement of
researchers studying
nearshore parts of the East Pa-
cific Rise or the Juan de Fuca
Ridge, back-arc ridges are often
slighted as isolated second-order com-
plications to the global scheme. The striking
contrasts in the geographic distributions of the three
types of spreading ridge encourages such parochial
assessments of their relative importance, and the
view from Tokyo or San Diego can be quite differ-
ent than that from Boston or London.

•— Mid- Ocean Ridge

Back-Arc Ridge

• • • • • Inactive Back- Arc Ridge
Pacific Rise

Pacific Rise Spreading and Destruction

Pacific rises are concentrated in the eastern half of
the ocean, where multiple, branching rise crests have
developed at the boundaries of purely oceanic plates.
They tend to spread faster than mid-ocean ridges,
because oceanic plates move faster than partly continental
ones. Despite this faster rate of crustal accretion, there is a
net loss of Pacific crust each year, because the rate of
recycling into the mantle, by the process of subduction at
marginal trenches, is even greater. As oceanic plates
descend into subduction zones, one or both flanks of the
rises that grow on their trailing edges eventually enter the
trenches and are destroyed. If one flank is completely
consumed, the actively spreading rise crest may collide

Oceanus

with the continental margin, as parts of the Chile Rise are now doing off
southern Chile.

More commonly, a rise crest drifting toward the margin of the ocean
basin ceases spreading before it ever enters the trench, and it is an
inactive or "fossil" rise crest that is consumed. The northernmost part of
the Pacific Basin is now occupied by the south flank of a rise (Kula Rise)
that was 2,000 kilometers long and 2,000 kilometers wide when it
stopped spreading 42 million years ago; since that time the Aleutian
Trench has consumed almost all of its north flank, and all but 75 kilome-
ters of the fossil rise crest. In the same period a trench along the western
margin of North and Central America has consumed most of the east
flank of the northern East Pacific Rise. Along parts of this margin, off
northern and southern California, there was a collision between the
active rise crest and the continent, but off central California and Baja
California, spreading ceased when the rise crest was within 50 to 100
kilometers of the trench. Fortunately, subduction (removal of oceanic
crust) ceased at the same time, so the record of 20- to 10-million-y ear-old
fossil-rise crests is preserved on the present ocean floor. Further north,
off Oregon and Washington, part of the East Pacific Rise that had been
approaching North America began to move away from it about 20
million years ago, thereby escaping subduction and surviving as the rise
system now called the Juan de Fuca Ridge.

The Juan de Fuca Ridge acquired its new identity, and a unique rate
and pattern of spreading, when it became isolated from the main rise
system by continental collision. Some altogether new rises got started
with the fission of an oceanic plate that was being pulled in two different
directions toward trenches along different parts of its margin. About 25
million years ago this was the fate of the largest eastern Pacific plate,
which split into the "Cocos Plate" moving toward the Middle America
Trench and the "Nazca Plate," which built a rise that extends east-west
just north of the equator. A more common origin is when new spreading
centers open up along the flanks of existing rises, generally in response
to changes in plate-motion direction. After a few million years with both

Crest of
Mature Rise

Incipient Spreading
Center on Rise Flank

Trench

Rift Valley

New Rise

Trench

Jack Cook/WHOI Graphics

Winter 1991/92

The development of a
new Pacific rise by
replacement of an old
one. A: A Pacific rise,
producing crust that is
reentering the mantle
at a marginal trench,
develops a new
extensional plate
boundary (with rifted
oceanic crust) on its
flank. B: The site of
seafloor spreading
shifts to the new plate
boundary, where young
hot crust forms a new
rise (e.g., East Pacific
Rise), right. This new
rise may in turn
become inactive,
especially if it ap-
proaches a trench too
closely. The original
rise crest becomes
inactive and its axis
forms a rift valley (e.g.,
Mathematician Rise),
left. The color key is
identical to the one for
the figure on page 27.

The origin of a back-arc

ridge. A: Crust
overlying a downgoing

slab of old oceanic

crust is thickened by

arc volcanism. B: The

seaward movement of

the trench and adjacent

part of volcanic arc
causes rifting of the arc
and growth of a back-
arc ridge in a
new basin.

Trench

rise crests active, the older one is generally replaced by the new one and
becomes extinct.

This process commonly recurs at different times along various parts
of the same rise, making these parts of different age, and thereby confus-
ing the nomenclature. Some authors consider almost all the Pacific floor
east of the Hawaiian Islands to be the partly subducted remnant of the
East Pacific Rise, while others restrict the term to crust that has formed
since the most recent new start and reorientation, which occurred 20
million years ago at latitude 20°S (replacing the now-fossil Mendoza
Rise), 10 million years ago at 10°S (replacing the Galapagos Rise), and 5
million years ago at 18°N (replacing the Mathematician Rise). As further
illustration of this rise's hybrid origin, the northernmost and youngest
part of the present East Pacific Rise crest, in the mouth of the Gulf of
California, is an exceptional 200-kilometer-long mid-ocean ridge, where
intercontinental spreading between Baja California and the Mexican
mainland has occurred for the past 3.5 million years. This local expan-
sion of the Pacific Basin occurred when the tip of the spreading center,

which had intersected the
continental margin, propa-
gated a short distance into the
interior to link up with fault
systems developing in the
Gulf of California.

Seaward

Mantle melts

to supply
volcanic arc

Volcanism

ceases on

this fragment

of the former arc

Back- arc
Ridge

Arc Platelet

Volcanic

Arc -Trench

Mantle supplies

both volcanic arc

and back-arc ridge

Water

Sediment

Back-Arc Ridges:
Episodic Rifting

The Pacific Ocean's complex
western margin is the locale
for most "back-arc ridges," a
phrase that describes their
tectonic setting at the back or
landward side of the rows or
arcs of subduction-zone
volcanoes that form the "Ring
of Fire," where oceanic plates
underthrust the Pacific Rim at
marginal trenches. Around a
contracting ocean basin, the
subduction zones must
migrate seaward. As they do,
a narrow sliver of the rim,

^ including the landward side

of the trench and the volcanic arc, migrates with them. Detachment of
this "arc platelet" and its subsequent drift away from the parent land-
ward plate causes back-arc spreading. The process is characteristically
episodic. Rifting begins in the volcanically weakened arc crust, which is
split lengthwise, and the landward half becomes inactive. Spreading
between the volcanically active and inactive halves of the arc opens up a
back-arc basin, with a spreading ridge whose crest migrates seaward at
only about half the speed of the arc platelet, thereby becoming increas-

Old Oceanic
Crust

New Oceanic
Crust

Mantle

Jack Cook/WHOI Graphics

Oceanus

ingly distant from the trench. The back-arc ridge generally becomes
inactive after spreading and building the basin floor for several million
years, but if subduction and arc volcanism continue, the process may
repeat, with rifting renewed in the arc. In this manner, a series of succes-
sively younger back-arc basins, floored by extinct or actively spreading
ridges, has been added to the western margin of the Pacific. Exceptions
to the rule that back-arc ridges are features of contracting trench-ringed
ocean basins are two isolated examples associated with Atlantic island
arcs: an inactive (and sediment-smothered) one behind the Lesser
Antilles Arc in the eastern Caribbean, and an active back-arc ridge
behind the remote South Shetland Arc.

Characteristics of Spreading Ridges

How do mid-ocean ridges, Pacific rises, and back-arc ridges differ in
gross topography? Their diverse histories and tectonic settings result in a
variety of sizes, sediment covers, and symmetries. Short-lived, back-arc
ridges tend to be narrower, their ocean-margin location makes them
more vulnerable to sediment smothering, and an asymmetric sediment
supply (mainly from adjacent volcanic arcs) threatens that even if ridge
development is not suppressed, the seaward flank may be buried by
sediment fans. In many back-arc basins there is even evidence for
encroachment of arc volcanism onto the seaward flank of the ridge.
Asymmetric topography, in contrast to the striking bilateral symmetry of
mid-ocean ridges, is also a feature of some nearshore Pacific rises that
have unequal sediment loading on the two flanks; for example, most of
the landward east flank of the Juan de Fuca Ridge lies beneath a thick
lens of sediment brought by turbidity currents from the nearby continen-
tal margin, whereas the west flank is sheltered from the effects of such
currents by the relief of the rise crest itself. A more fundamental cause of
asymmetry on many Pacific rises is varying amounts of flank removal at
marginal subduction zones. Once a whole flank has been consumed and
the crest collides with the continental margin, the "rise" is merely a
steadily deepening ramp leading from the margin to the continental
interior, as exemplified by the westward slope of the seafloor between
California and Hawaii.

The processes of sediment burial and plate consumption that cause
some gross differences between the three genetic types of spreading
ridges are secondary to the volcanic, tectonic, and hydrothermal pro-
cesses that create oceanic crust and shape the medium- and small-scale
topography at ridge and rise crests. These processes seem to work in
remarkably similar ways at all three types. Variations in ridge-crest
structure and relief are more clearly related to the rate of crustal accre-
tion than to the origin and history of the spreading center.

The current spreading rate (the width of the crustal strip added per
unit of time) at the divergent boundaries of major plates, varies from 12
to 14 kilometers per million years (or, 12 to 14 millimeters per year)
along mid-ocean ridges in the Arctic and between the slow-moving
African and Antarctic plates. It is more than 10 times this speed along
most of the East Pacific Rise. Active back-arc spreading centers cover a
similar spectrum of spreading velocities, and geologic study of old

The Pacific

Ocean's

complex western

margin is the

locale for most

"back-arc
ridges/' at the

back or
landward side
of subduction-
zone volcanoes

Winter 1991/92

oceanic crust indicates that the same range of rates has prevailed
throughout the past 200 million years, though the worldwide average
rate has fluctuated significantly during this period. The spreading rates
of the mid-ocean ridges and Pacific rises that are now active fall into four
classes: slow, medium, fast, and ultra-fast. The longest ridges are in the
slow-spreading class, which includes many Atlantic and Indian mid-
ocean ridges; but the most productive in terms of total area of crust
added each year is the ultra-fast class, which only includes the central
part of the East Pacific Rise.

Unrifted Rises and Rifted Ridges

Although there are four speed classes, and speed of opening affects ridge
crest structure, we recognize just two fundamental structural types:
unrifted rises and rifted ridges. The former, characteristic of both the fast
and the ultra-fast classes, have narrow 100- to 300-meter-high, 2- to 10-
kilometer-wide "axial ridges" along their spreading axes. The elevation
of the axial ridge is readily explained by its location over a body of hot,
partly molten rock, including a thin, narrow magma chamber (see
Tomographic Imaging of Spreading Centers, page 92). Despite the
"unrifted" apellation, the axial ridge contains a volcanic rift zone much
like the rift zones on Hawaiian volcanoes: an elongated zone of weak-
ness into which vertical bladelike sheets of molten rock are injected from
underlying magma sources. When the sheets freeze within the ridge,
tabular intrusions known as "dikes" are formed, and gaping fissures
open up in the overlying seabed. If the magma pressure is great enough,
the sheets reach the seafloor and lava erupts from fissures along the axial
ridge crest. The remarkably narrow zone of intense dike injection and
fissure eruptions, typically less than 1 kilometer wide, is usually marked
by a shallow "axial summit graben" only 10 to 100 meters deep and
formed by collapse of the axial ridge crest between major eruption
events. The fissured floor of this summit graben is a favored site for
mineral-precipitating discharges of hydrothermal fluids that are heated

Both rifted ridges and

unrifted rises occur in

this gravity anomaly

plot.

• Axis of Rifted Ridge

• Axis of Unrifted Rise

Gravity Anomaly (mgal)

, Normal Fracture Zone
t Oblique Fracture Zone

-30

-20

-10

180

-ISO

Oceanus

270 E

180 E

mGal

<-30

by contact with the hot frac-
tured seabed.

Rifted ridges are those mid-
ocean and back-arc ridges on
which the plate boundary does
not crop out along the crest of the
highest volcanic ridge, but along
the floor of a 100- to 3,000-meter-
deep axial rift valley that is
typically 10 to 40 kilometers
wide, and bordered by uplifted
fault blocks called rift mountains.
Much has been written on the
nature and origin of axial rift
valleys since they were discov-
ered by reconnaissance echo
sounding more than 50 years ago
(see Onions and Leaks. . ., page 36
and Tectonics of Slow Spreading
Ridges, page 51 for some of the
newer ideas and observations).
The discussion here summarizes
their global distribution, which
has recently been clarified by
satellite observations.

Radar altimeters carried by
some mapping satellites,

notably Seasat, Geosat, and ERS-1, measure variations in the shape of the
sea surface, which is affected by seafloor topography because water piles
up over ridges that exert a gravitational attraction. After processing to
remove the influence of waves, tides, and long-wavelength variations,
altimeter profiles can be displayed as maps of sea-surface gravity
anomalies, with positive anomalies over axial ridges and negative
anomalies over the floors of axial rift valleys. Data from systematic
global coverage by satellite altimeters combined with more patchy
mapping by ships equipped with echo sounders shows that axial rift
valleys are most impressive on slow-spreading ridges; at spreading rates
of 15 to 60 millimeters per year, there is a crude negative correlation
between spreading rate and rift-valley depth. However, a rift valley's
presence and size are probably controlled by the rate at which molten
rock is supplied from the mantle to the accreting plate boundary, and not
necessarily the correlated spreading rate. Where the magma supply is
voluminous and steady (a prerequisite for fast spreading and a feature of
slow-spreading ridges that happen to be near unusual "hot spot" magma
sources), a permanent reservoir of partly molten rock underlies the
spreading axis and creates an axial ridge. At spreading centers with
smaller, more episodic magma supplies, like most slow-spreading ridges
that are far from unusual "hot spot" magma sources, plate separation
and crustal extension leads to rift valley formation. In the medium-
spreading speed class, many axes have well developed, albeit shallow,
rift valleys, but axial ridges similar to those of fast-spreading rises also
occur, sometimes on adjacent parts of the same spreading center. At

90 E

>+30

T/7/s map depicts the
gravity field of the
Southern Ocean, as
derived from Geosat
altimetry. The south-
ernmost portions of the
Pacific-Antarctic and
SoutJieast Indian ridges
appear as a complex
chain of positive
gravity anomalies
between 150° and
210° E. This map is
available in both digital
and poster form from
the NOAA National

Geophysical Data
Center (Report MGG-

6, Marks and
McAdoo, 1992).

Winter 1991/92

Brazil,
South America

o
I

1000

Continent &
Continental
Slope

Sediment
Plain

Mid -Atlantic
Ridge

Speeding
Segment

Transform
Fault Offset

Fracture
Zone

Spreading
Direction

Where the overall

strike of the ridge is

highly oblique to the

spreading direction, as

in the equatorial part of

the Mid-Atlantic

Ridge, the offsets

between spreading

segments are longer

and more closely

spaced.

these intermediate spreading rates (60 to 80 millimeters per year) the
presence or absence of a rift valley is probably sensitive to local and spatial
and temporal changes in the magma supply rate.

Ridge-Crest Segmentation

Fracture zones, and the rise-crest offsets that create them, are also
essential features. Ridge-crest segmentation is one of the most funda-
mental features of spreading centers and one of the most active areas of
current research. The crustal-accretion belt along a ridge crest is not
continuous, but is broken by several types and sizes of ridge offsets into
laterally displaced segments tens or hundreds of kilometers long.
Individual spreading segments are oriented at right angles to the spread-
ing direction, and are frequently arranged in a staircase with offsets
systematically stepping left or right. In such cases the relative lengths of
spreading segments and offsets is determined by how oblique the ridge
is to the spreading direction; for instance, the equatorial part of the Mid-
Atlantic Ridge strikes almost east-west, and has long left-stepping offsets
linking short spreading segments, while further south, where the overall
strike is more nearly north-south, lateral offsets are shorter and more
widely spaced. There is also empirical evidence of an inverse correlation
between segment length (offset spacing) and spreading rate. Only a
small fraction of the global ridge system has been surveyed with the
high-resolution tools needed to locate small offsets, however, so their
mapped abundance partly reflects the relatively small survey effort. The
spreading rate certainly influences the structure of the ridge offsets. On
slow-spreading ridges, all but the shortest offsets contain transform
faults, which are narrow zones of "strike-slip" (horizontally sliding)
faulting at right angles to the spreading segments. Similar transform
faults subdivide fast-spreading rise crests, but only where offsets are
longer than 50 to 100 kilometers. Shorter steps in the plate boundary are
much more abundant, and have broad zones of deformation that are said
to be "nontransform" because they lack strike-slip faults. Both transform
and nontransform offsets, known as "fracture zones," leave recognizable

Oceanus

trails on the rise flanks, belts of distinctive topography that interrupt the
abyssal hill pattern because they have spread from disruptions of the
spreading center. Fracture zones produced at long transform faults have
high relief that was easy to discern even with early echo sounders.
Conversely, locating fracture zones produced at nontransform offsets
generally requires high-resolution mapping of abyssal-hill or crustal-age
patterns, because their subtle relief is unpredictable: Nontransform
offsets migrate along the rise crest at variable speeds and directions,
rather than maintaining a stable location, as transform offsets do. The V--
shaped feature in the middle is a good example of a pair of oblique
fracture zones produced by a migrating nontransform offset, in this case
one that has migrated steadily southwest at a rate about twice that of the
spreading rate.

Much remains to be learned about the origin of ridge-crest segmen-
tation, and the reasons for segmentation-pattern changes. Why, for
instance, do some spreading segments grow in length at the expense of
their neighbors, causing the offsets between them to migrate along the
plate boundary? Different approaches being used to tackle this problem
include making detailed studies of the rise-crest processes at a few
convenient locations and preparing a global inventory of all such offsets,
to see how their directions and rates of migration correlate with such
factors as spreading rate, segment length, rise-crest depth, etc. For the
latter task, a much more complete description of the global ridge system
is needed, implying many more months of survey effort with the sophis-
ticated multibeam mapping sonars now available on research ships.
Satellite observations may have sufficient resolution to partly supple-
ment the shipboard work, and provide an immediate global perspective.
Unfortunately, the best satellite data now available has been classified as
a military secret (except in the "nonstrategic" Antarctic region), and is
therefore unavailable to most researchers, just as the results of
multibeam sonars were a decade ago. This impediment to understanding
the pattern and relief of spreading ridges will disappear as military
satellites are replaced by civilian ones, like the ERS-1 that is now in orbit
and busy collecting altimeter profiles across all the world's ridges.

Peter Lonsdale is a Professor of Oceanography and Research Geologist with the
Scripps Institution of Oceanography (SIO). He has spent two or three months in
each of the past 20 years examining the ocean floor with echo sounders,
cameras, and submersibles, about half of this effort being on Pacific rises and
back-arc ridges.

Chris Small is a graduate student at SIO, with a special interest in using satellite
altimeters for structural studies of mid-ocean ridges.

Ridge-crest

segmentation is

one of the most

fundamental

features of

spreading

centers and one

of the most

active areas of

current research.

Winter 1991 /92

Thirty -five
years ago, most
geologists were

secure in the

knowledge that

continents did

not move.

Onions and Leaks:

Magma at
Mid-Ocean Ridges

A Very Personal View

Joe Cann

n 1992 we see mid-ocean ridges clearly, forming a complex,
50,000-kilometer-long web of seafloor mountain chains that
encircle Earth. Along the mountain crests there is a narrow belt of
activity, marked by shallow earthquakes, seafloor volcanic
eruptions, and hot springs, where new ocean crust is constructed
at the rate of a few centimeters every year (about as fast as fingernails
grow). Recent intense study of this zone has sharpened our picture,
refocused it here and there, brought sudden insights, and revealed errors
of perception, until we have reached new levels of clarity.

This year seems especially propitious for reviewing mid-ocean
ridges. We are pleased that our new models are good, that our under-
standing is secure. There are difficulties to be sorted out, but most are
within our grasp. Now we should settle down to explain what we know.
And in that spirit we write, and you read, this issue of Oceanus.

Our certainty is not new. Thirty-five years ago, most geologists were
secure in the knowledge that continents did not move, that the oceans
were permanent, unchanging features of Earth's surface, containing
sediments as old as ocean water itself and interleaved here and there
with lava flows. Mid-ocean ridges might be fold-mountain belts like
submarine Rockies, or rift mountains like submerged East African
highlands, but were certainly explainable in sensible continental terms.
Within a few years this comfortable picture was to be turned upside
down by the very people who then possessed such certainty of belief.

Thirty five years ago I was a geology undergraduate. Our first-year
text was by Arthur Holmes who, in about 1930, focused attention on the
mid-ocean ridges with his concept that Earth's deep interior might be
slowly convecting as it was heated by the radioactive decay of potas-
sium, uranium, and thorium. He thought that deep-Earth convection
currents might move the continents apart, and that upwelling currents
might rise in the centers of those oceans that had matching coastlines on
either side, such as the Atlantic and Indian oceans. At first, Holmes
thought that the Mid- Atlantic Ridge was a strip of continent left behind
as Africa and America split apart, but in our 1944 textbook he replaced

Oceanus

the continent with oceanic crust. That diagram looks very much like the
sketches we draw today.

In student seminars we talked about Wegener and du Toit, pioneers
of continental drift theories. We argued whether the oceans might be
young, as they said, or ancient, and whether animals had crossed the
oceans on land bridges or floating tree trunks, or maybe had wandered
from place to place when the continents were joined to form
Gondwanaland, Laurasia, or, earlier,
Pangaea. Our professors cautioned us
against believing Holmes too literally,
indicating that ideas about drift were based
on woolly speculation. Harold Jeffreys, the
most eminent geophysicist of the time, had
proved that drift was impossible. How could
we disagree? The pioneers of rock magne-
tism certainly did so. They showed that
ancient rocks are magnetized very differ-
ently from recent ones, suggesting that they
originated at latitudes other than those they
now occupied. Could Earth's spin axis have
changed? Or had the continents moved?
Jeffreys skeptically pointed out that iron
could be remagnetized by striking it with a
hammer, a tool traditional with geologists.

Then marine geology and geophysics
began to take a hand. Inspired by service at
sea and trained in antisubmarine warfare,

young marine scientists brought new talents and instruments to the
oceans. It was a curious time. Marine scientists were few, and nearly
everyone knew nearly everyone else. Even when I came into marine
geology in 1962 as a young post doc, I was able to rapidly meet almost
all of the players in the game. That would be quite impossible today.
Many of the leaders in the field worried more about their next expedition
than about publishing the results of their last, but all were willing to talk.
Ideas developed by word of mouth, shortcutting publication, so that often it
was — and still is— difficult to lay credit where it properly belongs.

Harry Hess, one of the most charismatic scientists of his time, was a
reluctant but remarkable speaker: Quiet, with a cigarette dangling from
his fingers, he was seemingly casual, yet profoundly convincing. He first
took a semi-fixist view, in which convection currents stirred the mantle,
but continents did not move. He ascribed the existence of mid-ocean
ridges to transformation of dense mantle peridotite to light serpentinite,
by way of water seeping up from the rising convection currents. Soon he
moved into the mobilist camp, and allowed that convection moved
continents. In the first breakthrough since Holmes, Hess suggested that
new ocean crust is continually created at mid-ocean ridges and spreads
away as the ocean grows.

This became the theory of ocean-floor spreading, and from it
emerged the first model of mid-ocean-ridge processes. Hess thought that
the rising limbs of deep-Earth convection currents not only split the
ocean floor apart, but also contributed basalt magma and water to the
growing crust. Magma would be erupted at the ocean floor to form the

Drum Matthews (red

shirt), Tony Laughton

(blue hat), and Ron

Oxburgh work amid

basalt lavas at the Gulf

ofTadjura, Djibouti, in

Janitan/ 1967. The

Gulf ofTadjura is

where the Gulf of Aden

spreading center comes

ashore, and is splitting

apart slowly. The lavas

come from magma

chambers below the

seafloor.

Winter 1991/92

:-, - ./ •--:_ -- ,,, :.

, ... iA - ,

?*-.'

- jg- ; • -. ^ '-+- :-• ,

V^;> ^|^

--' - ^:'3:
-^'f. -.- - / "^^' ^X

.;-

' £-'''

Troodos

ophiolite complex in

Cyprus, a road cut

shows the top of a

seafloor magma

chamber formed 1 to 2

kilometers below the

ancient seafloor. The

pale rocks are gabbros

and trondhjemites
produced by crystalli-
zation of the top of the
magma chamber. The
gray stripes are dikes
intruded from another
chamber nearby. Hazel
Prichard is the figure,
once a student of the
author and now at the
Open University in the
United Kingdom.

seafloor lavas that were now being collected regularly, and water would
circulate down through the porous lavas to alter the uppermost mantle
to serpentinite, creating the lower part of the crust. The base of the crust
would thus mark the lowest level that serpentinite could form at the
ridge axis, representing the temperature at which serpentinite dehy-
drates back to peridotite.

The demonstration that Hess had been broadly right was a triumph
of the 1960s. From data gathered by towing a magnetometer across the

oceans, Fred Vine and Drum
Matthews explained that
magnetic anomalies were
created as the result of ocean
floor spreading, while Earth's
magnetic field periodically
reversed. Tuzo Wilson
invented the concept of
transform faults to account
for the great oceanic fracture
zones. A sequence of other
important papers trans-
formed ocean-floor spreading
to plate tectonics and con-
vinced all but the most
recalcitrant oil-company
geologist that the mobilist
view was correct. But that is
all part of a different story, and shed no further light on what is happen-
ing at mid-ocean ridges.

That revelation was already being achieved elsewhere, namely in the
Cyprus Geological Survey. In Cyprus (and in other places such as
Newfoundland, Oman, and Papua New Guinea, a combination that
accounts for some curious stamps in my passport), there is a thick slab of
rock, an ophiolite complex, made up of basalt, peridotite, and
serpentinite, containing seafloor lavas and deep-sea sediments. Smaller
fragments of similar rocks had long been known from mountain belts,
and had been studied by, among others, Harry Hess. Now the Cyprus
Survey, spurred by the discovery of iron, copper, and zinc sulphides,
and chrome ore, decided to map the Troodos ophiolite, which was 100
kilometers long by 50 kilometers wide. The first map was started by
R.A.M. Wilson. His work was a masterpiece of acute observation and
justified interpretation that is a pleasure to read, even today.

Within the ophiolite structure, which forms a gently warped and
eroded sheet several kilometers thick, he found a unit composed entirely
of dikes soon called the sheeted-dike complex. Dikes are thin, vertical
sheets of magma, relics from when magma intruded into vertical cracks
and became frozen there. They are common in the rock record, and show
that the rock has been stretched when magma was around. In some
places on the continents they make up perhaps five percent of the
terrain, which up to that time had been considered a large amount. In
Cyprus, Wilson showed that they make up 100 percent of one unit that is
1 kilometer thick and stretches for 70 kilometers across the mountains.

Ocean us

Here was ocean-floor spreading frozen into geology, 70 kilometers of it,
though Wilson did not make the connection at first.

By the mid-1960s the link had been made, and the oceanic and
ophiolitic strands of evidence became inextricably tangled. In the oceans
it is possible to observe active mid-ocean ridges, especially using geo-
physical methods, but very difficult to see what is happening below the
seafloor, except by inference. In ophiolites the processes ceased long ago,
but it is possible to wander over the countryside, passing deeper below
the ancient seafloor at will, and reconstruct past events using the stan-
dard tools of geology. The two approaches are complementary, but
communication between them presented problems. Certainly there
seemed to be a conflict between what Hess predicted for mid-ocean
ridges and what was observed in ophiolites.

When I first came into marine science, I trod warily, watching in
admiration as my geophysical colleagues manipulated mathematics,
patched instruments at sea, and set apparently arbitrary constraints on
what was and was not possible. After all, I was an impeccably
orthodoxly trained microscope man. Some moments were magical, as
when Tuzo Wilson first propounded his transform-fault theory, grinning
like a Cheshire cat that had swallowed the cream and snipping newspa-
pers with a large pair of scissors to show how his theory worked. Other
moments were more prosaic, and I gradually realized that geophysicists
did not hold a monopoly on truth — or perhaps I just learned some
geophysics.

Was it possible to make a simple model of ocean-crust construction
at mid-ocean ridges that drew on all of the evidence available, ophiolitic
and oceanic, geophysical and
geological? I had a false start:
My first model was undone
by a graduate student's
simple question, "What
determines the position of the
Moho, the boundary between
crust and mantle, in your
model?" I said something in
reply, floundering, hoping
that he wouldn't notice. I
expect he did.

Then, a month or so later,
digging the sandy soil of our
garden in Norwich, I sud-
denly saw what was wrong-
perhaps also what was right.
Suppose that magma rose up

from the mantle as the plates moved apart. Suppose it rose high in the
crust, not far below the seafloor, and collected there as a magma cham-
ber, stretching along the axis of the mid-ocean ridge at a shallow depth.
When the crust cracked above it, magma could rise along the crack to
make a dike and then feed seafloor lava flows. The dike would intrude
older dikes, and in turn cut yet older ones. When the magma chamber
froze it would make a layer of gabbro in the lower crust. Crust produced

Author Cann cooking

porridge over a steam

vent in northern

Iceland in August

1991. The vent is part

of the Theistareykir hot

springfield, lying in

the Theistareykir rift

zone, and heated

(almost certainly) by a

magma chamber deep

below the rift. Note

that Iceland marks

where the Mid-
Atlantic Ridge comes
ashore, though it is
anomalous in
many ways.

Winter 1991/92

Me\\

The " infinite onion"
model (above) for
magma chambers
beneath fast-spreading
ridge segments is
compared to the
"infinite leak" model
(below) for magma
storage beneath slow-
spreading ridge
segments.

this way would have the same
structure as the Cyprus (and now
the Oman) ophiolites, and a
shallow magma chamber might fit
the geophysical observations, too.
This outline evolved into the
infinite-onion model, since in its
ideal form it required a magma
chamber that was onion-shaped in
cross section and as long as that
part of the ridge. Soon I was
involved in stout defense of the
onion. It proved very difficult to
make the seismic observations that
would test it properly, and inconclu-
sive tests were regarded by skeptics
as negative evidence. Soon it
became clear that, in its simplest
form, the model did not hold at
slow-spreading ridges such as the
Mid-Atlantic Ridge. Euan Nisbet
and Mary Fowler devised an
alternative, punning infinite-leak
model to cope with that. Recently
Debbie Smith and I have come up
with observations that support
infinite leaks in the Atlantic.

But in the Pacific, where
spreading rates are faster, there
seemed every reason to expect the
infinite onion. People looked for
and found hot mantle, but no
magma; they were looking for
magma in the wrong place, it
turned out. John Orcutt said he

could see the magma. I liked his evidence, perhaps naturally, but many
others stonewalled. Eventually, Bob Derrick (I simplify — John and Bob will
have to stand for the teams they led) managed to image the top of the
magma chamber for tens of kilometers along the ridge, using seismic
reflection, just as the oil companies do to find oil — oil and magma look
surprisingly similar by seismics. The chamber was much thinner than I had
originally predicted, but the top was just at the right level (1 to 2 kilometers
below the seafloor), as could have been predicted from ophiolites.

And the onion? So far Bob and John stand out against the spike on
the top of the chamber that would make the onion complete, but one day
in the future...? Do I need to say that I still feel it is there? And what
then? There is no space to tell of the other successful models of mid-
ocean ridges, of George Constantinou in Cyprus showing that the ore
deposits there were formed from hot springs on the ocean floor, thus
leading the way toward black smokers; or the recent recognition that the
third dimension, the variation of ridges along the axis, has as much of a

Temporary High-level
Chamber

Gabbros and
Cumulates

Trapped
Melt

Relict
Crystals

Oceanus

story to tell as the across-ridge models we started with. But models are there
to be overthrown: Perhaps the second-best experience as a scientist is to see
a model elegantly destroyed. The best? To do it yourself by creating a new
one, of course. In spring 1992, Debbie Smith and I will be leading an expedi-
tion to the Mid- Atlantic Ridge, trying hard to do just that.

Joe Cann is Professor of Earth Sciences at the University of Leeds in the UK and
Adjunct Scientist at Woods Hole Oceanographic Institution. He took his Ph.D. in
1962 and is thus one of the old fogies of marine geology, but he is still trying hard
to destroy his and other people's models of mid-ocean ridges. For the last few
years he has been worrying more about black smokers than magmas, but recently
he has come back into seafloor volcanoes. He works happily at sea or on land (in
Greece or Cyprus) with a microscope, or a computer, or an X-ray set, especially
on figuring out how the different aspects of mid-ocean ridges knit together.

1970

1974

lavas
dikes

isotropic gabbro

(Rising
Asthenosphere

layered gabbro

M|i!!!!lharzburgite
tectonite

1982

Lava Thickness TL
Dike Thickness T.

Cumulate Thickness T

Magma chamber Blob of primitive melt

deflated volume V0 volume V

In the last 35 years,
geological "certainty"
has changed enor-
mously. Models are
created, proved, then
sometimes disproved—
with the end result,
ultimately, of better
understanding. Simple

diagrams of models
from the last 20 years
illustrate (in a punctu-
ated manner) this
evolution.

Winter 1991/92

The first
volcanic rocks

from a

mid-ocean

ridge were

accidentally

sampled during

cable-laying

operations

in the North

Atlantic

in 1874.

Sketches for this article
are by the author.

From Pillow Lava
to Sheet Flow

Evolution of Deep-Sea Volcanology

Wilfred B. Bryan

he black, fine-grained volcanic rock called basalt has long
been associated with ocean basins, though sometimes for the
wrong reasons. Today, basaltic lava is a familiar sight to
millions of tourists who have visited Hawaii, and millions
more have watched it flowing into the sea on television news
programs. But 200 years ago in western Europe, basalt was known
mostly by its association with sedimentary rocks containing marine
fossils, and so was widely regarded as a chemical precipitate from
seawater. A few practioners of the new science of geology at that time
recognized the similarity of basalt to the lavas of nearby volcanoes such
as Vesuvius. This led to one of the first major controversies in geology,
between the so-called "neptunists" and the "plutonists," who believed
that basalt was the product of volcanic eruptions. That issue was eventu-
ally solved when one of the supposed basalt precipitates was traced back
to its source at an obvious volcanic vent. But the nature and extent of
volcanic rock on the deep seafloor would not be known for many more
years. Prior to the mid-1960s, scientific papers on this subject still were
largely constrained to rocks observed and collected on land; their
association with sedimentary rocks typical of the deep seafloor contin-
ued to be the principal evidence for their deep-sea origin.

The first volcanic rocks from a mid-ocean ridge were accidentally
sampled during cable-laying operations in the North Atlantic in 1874,
about 200 nautical miles east of what we now know to be the Mid-
Atlantic Ridge (MAR). The dark basalt was dismissed as having been
dropped from a drifting iceberg, although the 27.5-ton tension required
to recover the cable would seem to suggest this was not a loose fragment.
In 1898 P. Termier described basaltic glass also recovered from the MAR
at about 47°N, during cable repairs. He correctly deduced that this
material indicated a volcanic origin for the seafloor at this location, but it
would require another 60 years of study before the true extent and
nature of the Mid-Atlantic Ridge would be known. Meanwhile, in the
Pacific, widely scattered dredges recovered by the Challenger Expedition
included samples of dark basaltic rock, also indicating a likely volcanic
origin for the deep seafloor.

Throughout the first half of the 20th century the seafloor was widely

Oceanus

75°-

30°-

60°-

120C

180C

120°

60C

assumed to be basaltic, but evidence for this assumption was still sketchy
and indirect. A "basaltic" and therefore "volcanic" seafloor was consis-
tent with the arguments based on isostasy and bathymetry that remain
valid today: The continents must stand high, because they are composed
of relatively thick, light granitic rock that literally floats higher on the
underlying mantle than does the thinner, heavier rock comprising the
oceanic crust. Also, petrologists generally assumed that basalts of
volcanic islands such as Hawaii or Iceland were representative of the
rocks to be found on the deep seafloor. Although there are often striking
differences between continental volcanic rocks and the deeper crustal
rocks on which they have been erupted, the shaky logic of this analogy
as applied to the seafloor does not ever appear to have been challenged.
Finally, with the recognition of the reality of seafloor spreading and
plate tectonics in the mid-1960s, mid-ocean ridge volcanism became a
logical geometric necessity for creating new seafloor. The spreading
model predicted that seafloor of similar basaltic composition but of
regularly increasing age should extend to the margins of the ocean
basins, a relation that was soon confirmed by basement samples recov-
ered during legs 2 and 3 of the Deep Sea Drilling Program. Attention
could now be redirected toward defining the nature of volcanic pro-
cesses on mid-ocean ridges and the nature and extent of compositional
variation in volcanic rocks erupted there.

Locations are plotted
from which oceanic
basalts were dredged

as early ridge

petrologists defined

compositional and

structural boundaries

between oceanic and

continental crust.

Marker's "Pacific"

boundary and Hobbs 's

"andesite line" bracket

the circum-Pacific

"Ring of Fire."

Winter 1991/92

Chemical Variations

Some of the

most intriguing

compositional

features of

ocean-ridge

basalts are

found in their

trace-element

and isotopic

signatures.

Because of their very fine-grained or glassy nature, volcanic rocks are
most easily studied quantitatively by their chemical composition.
Chemical analyses of volcanic rocks in and around the major ocean
basins began to appear in the latter half of the 19th century. By the
beginning of the 20th century there were already enough data to support
speculation on the global distribution of volcanic rock types; in these
schemes it was implicit that the volcanic rocks somehow reflected the
nature of the ocean floor with which they were associated. One of the
best-known global distributions was proposed by the British petrologist
Alfred Marker in 1909. He recognized three main groups, which were
named for the ocean basins in or adjacent to which they were first
identified. The "Atlantic" type was characterized by the dominance of
soda (sodium oxide), the "Mediterranean" type by potash (potassium
oxide), and the "Pacific" type by lime (calcium oxide). Marker's "Pacific"
type, however, was based entirely on data from volcanoes and volcanic
islands from the "Ring of Fire" around the Pacific margin. Almost
immediately, new data from various Pacific Islands proved similar to
those from the Atlantic, and Marker's scheme was discredited.

About 20 years later, W.H. Hobbs called attention to the composi-
tional differences between volcanic rocks from islands within the Pacific
Ocean basins and those of the volcanic-island arcs and continental
volcanoes along the Pacific margins of Asia and North and South
America. It is interesting to compare this boundary, which Hobbs called
the "andesite line," with the boundary drawn by Marker between the
"Pacific" and "Atlantic" rock groups. Following Hobbs, most geologists
and volcanologists quickly accepted the andesite line as the structural
and compositional boundary between oceanic and continental crust. It
was not until detailed mapping and sampling of some of the circum-
pacific volcanic-island arcs in the 1950s and 1960s that Harker's bound-
ary was rediscovered.

Chemical analyses of rocks specifically associated with mid-ocean
ridges were not published until the 1930s in papers by C.W. Correns and
J.D.H. Wiseman. Their samples came from the Mid-Atlantic Ridge and
the Carlsberg Ridge in the Indian Ocean. Both authors recognized the
unusually low potash contained in these rocks compared to both the
island basalts and continental rocks, and correctly deduced some of the
chemical effects of seawater alteration on basalt. Wiseman's paper
contained the first carefully detailed drawing of crystal forms observed
with a petrographic microscope; Correns recognized the similarity of his
sample to those collected in the Pacific by the Challenger Expedition, and
suggested that these might be typical of the seafloor as a whole.

Some of the most intriguing compositional features of ocean-ridge
basalts are found in their trace-element and isotopic signatures, but these
data had to await the mid-1960s development of more sophisticated
analytical technology. Analyses of basalts dredged both from the Mid-
Atlantic Ridge and the East Pacific Rise showed that, compared to typical
basalts of continents and oceanic islands, ocean-ridge basalts are highly
depleted not only in potash but in many trace elements chemically
similar in behavior to potash, such as lanthanum, rubidium, thorium,

Oceanus

and uranium. Because these elements are concentrated in typical volca-
nic rocks on Earth's surface and upper lithosphere, geochemists refer to
them as "large-ion-lithophile elements."

Based on these data, some geochemists emphasized the depleted and
homogeneous nature of ocean-floor basalt. This view was quickly
challenged when new analyses of basalts from the northern Mid-Atlantic
Ridge that were enriched in these same chemical elements were pre-
sented. However, the most extensive early collections of samples from a
mid-ocean ridge were recovered from the Mid-Atlantic Ridge between
22° and 30°N, and the "depleted" chemical character of these basalts
became established as the definitive signature of "normal MORB" (mid-
ocean ridge basalt).

At the University of Rhode Island, Jean-Guy Schilling published a
series of pioneering papers that first conclusively showed the gradational
nature of geochemical variability along ocean ridges. First demonstrated
in the North Atlantic, these along-ridge variations are now known to
continue through the equatorial region into the South Atlantic, and are
also present along the Galapagos Rift and southern East Pacific Rise.
Sections of ocean ridges enriched in potash, trace elements such as
lanthanum, thorium, and uranium, and with high strontium-87/stron-
tium-86 were shown to be associated with shallow bathymetry or island
platforms such as Iceland and the Azores. This appeared consistent with
the idea that these are "hot spots," characterized by extensive melting of
a mantle source enriched in these elements and in radiogenic isotopes
such as strontium-87.

Hot Spots: How Normal is Normal?

Although models remain sketchy and highly speculative, a popular view
is that hot spots are the locis of upwelling "mantle plumes" that bring
new, hot, and previously undepleted mantle from a deep, previously
untapped source to a sufficiently shallow level, permitting partial
melting and the escape of basaltic magma to the surface. On the other
hand, depleted, supposedly "normal" mid-ocean ridge basalt is pre-
sumed to be derived from relatively shallow mantle, perhaps the "low-
velocity zone" defined by seismic surveys, which may have been de-
pleted by partial extraction of magma in previous melting events. Mixing
between melts derived from these two sources may account for much of
the intermediate isotopic and trace-element variability. Major hot spots
are now recognized along the Mid-Atlantic Ridge at Iceland, near 45°N;
the Azores, near 15°N; and near Tristan da Cunha, at about 36°S in the
South Atlantic. The chemical signature of the larger plumes extends
hundreds of kilometers along-ridge, and it can be asked, at least in the
Atlantic, if "plume" MORB isn't actually more normal than "normal"
MORB!

In the Pacific, the Galapagos Rift crosses the best-documented hot
spot, but another must exist on the East Pacific Rise near Easter Island,
and there are several small ones along the Gorda-Juan de Fuca Ridge
systems. Ridges have been less systematically sampled in the Indian
Ocean, but the available dredges and Deep-Sea-Drilling-Program
basement samples indicate both "normal" and "plume" chemistry in

Major hot spots

are now
recognized
along the Mid-
Atlantic Ridge
at Iceland, the
Azores, and
near Tristan
da Cunha.

Winter 1991/92

Author Bryan (left)

and T.H. Pearce (riglit)

on an expedition in

Quebec stand in front

of classic Archean

pillow lavas.

basalts recovered there. There now is even a "cold spot" recognized
along the Pacific-Antarctic Ridge, characterized by extreme depletion in
large-ion-lithophile elements. Most recently, researchers at the Lamont-
Doherty Geological Observatory have shown that systematic along-ridge
variations can be demonstrated in major-element chemistry as well as in
trace elements and isotopes, and also can be correlated with bathymetry
and the location of hot spots.

Petrography and Mineralogy

Much of the early work on ocean-floor basalts was based on chemical
analyses and ignored mineralogical details of the rocks. In 1972, 1 described
in detail for the first time the sometimes-bizarre crystal morphology that
results from rapid underwater quenching of magma. An unexpected result
of this paper was the recognition of similar quench-crystal morphology in
Archean pillow lavas that are up to 3.5 billion years old. Previously these
morphologies were believed to have been caused by chemical changes over

time, accompanied by recrys-
tallization. Now it was obvious
that these basalts had changed
little since they originally
erupted on ancient seafloor,
and both their chemistry and
morphology could be used to
interpret volcanic processes in
some of the oldest seafloor
known, now uplifted and
exposed on land.

While many of these
ancient basalts resemble their
modern counterparts, others
do not, including some
unique varieties that are very
enriched in magnesium,
nickel, and chromium.
Mineralogically, these rocks,

known as komatiite, are unusually rich in olivine, the major mineral
component of the upper mantle. One possible interpretation of these
komatiites is that they were derived from an oceanic lithosphere much
thinner and with a much steeper thermal gradient than that observed
today, resulting in more complete melting of the mantle source.

Morphology and Volcanic Processes

The size, shape, and other morphologic details of ocean-ridge lava flows
and associated volcanic structures provide important clues to the loca-
tions of eruptive vents, rates of eruption, flow mechanisms, and lava
distribution on the seafloor. As for deep-sea basalt compositions, the
question whether deep-sea lavas had a unique morphologic character
was heatedly debated for nearly a century.

Exposures of these lavas in cliffs, road cuts, or on glacially eroded
and smoothed outcrops on land were largely two-dimensional, and left
much room for arguments about the lateral extent of individual flows,

Oceanus

their three-dimensional forms, and
the nature of the larger volcanic
structures they built. In cross section
many of these lava flows consist of
elliptical to circular masses, 2 meters
to over 1 .5 meters in diameter; these
classic "pillow lavas" have been
cited as proof of eruption underwa-
ter at least since the first half of the
19th century. However, whether
these pillows are spherical or
tubular in three dimensions, and
whether they uniquely indicate
underwater eruption, was argued
for many years.

Central to these debates was a

Tlie sizes and shapes of

lava flows reveal

information about the

mechanics of eruptions.

Sheet flows from the

East Pacific Rise (above)

and a layered lava tube

in subglacial pillow lava

in Iceland (left) are

vastly different

morphologically. A

typical flow from the

FAMOUS area (below,

left) is further illustrated

with a schematic cross

section that reveals the

draining of lava.

Winter 1991/92

This "elephant seal"

pillow formed at the

end of a Hawaiian

pahoehoe lava flow.

question: Should the term "pillow lava" be reserved only for circular or
spherical structures in lava erupted underwater (or at least in wet mud),
or should it also be applied to morphologically similar lavas formed on
land? The case for purely descriptive use of the term "pillow" was well
argued as long ago as 1938 by J. T. Stark, who pointed out that in his
even earlier 1914 review of the subject, J.V. Lewis had cited 98 descrip-
tions of "pillow lava" dating back to 1834, of which more than half were
probably formed on land. Nevertheless, questions continued to arise as

to whether similar morphologies
could be produced in different
ways. For example, in 1968, J.G.
Jones documented "pillow lava"
composed of interconnected and
elongated tubular lava fingers
analogous to the tubular
"pahoehoe" lava commonly
observed in Hawaiian lava flows
(as also advocated by Lewis in
1914!). This interpretation was
challenged, and the issue would
not be put to rest until the mid-
1970s, when scuba divers ob-
served pillows forming on the
submarine extension of an active
lava flow in Hawaii, and the first

direct observation of deep-sea lavas was made by diving scientists in the
Project FAMOUS (French- American Mid-Ocean Undersea Study) on the
Mid -Atlantic Ridge in 1974. These lavas were indeed composed of
elongated tubes, which grow downslope by budding, as Lewis long ago
deduced. Recent observations of new submarine flows in Hawaii also
confirm that they are fed by master feeder channels that are direct
extensions of the adjacent island's pahoehoe lavas.

Diving scientists have provided abundant photographic records,
direct observations, and descriptions of the great variety of morphologic
details in lavas of the mid-ocean ridges. These observations make it clear
that elongated, tubular lava units are common on steep-flow fronts, but
pillows take many forms: On the upper flow surfaces some are hollow
bubbles, but others are highly ornamented sculptures that resemble
animal or human forms. Many similar forms are also found on land on
pahoehoe lava flows

The first submersible dives on the East Pacific Rise showed that
many lavas are not pillowed at all, but are composed of slabby plates
ornamented with swirls and wrinkles suggestive of drapery or a
wrinkled tablecloth. These "sheet flows" form when lava is temporarily
ponded. The lava sheets are produced by quenching against the overly-
ing seawater. When lava pressure breaks the barrier and lava drains
away, successive layers form as the level of the pond drops. Hollow
columns of lava surrounded by "bathtub rings" form within these pits,
where trapped water vapor has risen through the lava and quenched it.
On land, analogous features are found in "shelly pahoehoe," where lava
has temporarily ponded around trees, and in collapse pits formed in lava

Oceanus

that erupted onto wet ground. It is now obvious that this diversity of
form can be related to a variety of factors, including the steepness of the
flow surface, the rate and volume of lava extrusion, and the influence of
the underlying seafloor morphology, and that the morphologic differ-
ences resulting from quenching in air or water are relatively minor.

Small conical or moundlike volcanic structures form over eruptive
vents on the seafloor as they do on land, but few have been described in
detail. Some appear to be typical extrusive lava mounds similar to those
that form over active lava tubes or along eruptive fissures in Iceland or
Hawaii. Larger cratered cones and mounds, common on the Mid-
Atlantic Ridge between 22° and 26°N, have been a special focus of study
by geologists at Woods Hole Oceanographic Institution. One of these,
named Serocki Volcano after one of the Ocean Drilling Program engi-
neers, has been mapped in detail, observed at close range from a sub-
mersible, and even penetrated by drilling. These studies indicate Serocki
has a flattish, pancakelike form and is probably not a true volcano but
rather a "rootless vent." Originally a thick lava delta, the north flank of
Serocki broke open, allowing lava trapped within to escape to a lower
level, where it again ponded temporarily to form another delta. This
delta in turn also broke open, and was drained; collapse of the unsup-
ported surface crust on both deltas created the central craters.

Looking Toward the Future

About 25 years ago, marine geologists and geophysicists first became
aware of the vast extent and importance of volcanic activity along mid-
ocean ridges. Following initial hopes that the resulting volcanic rocks

Author Bryan sketched
this cross section of the
Serocki volcano region
based on Sea Beam
bathymetry, Sea
MARC sidescan
images, and observa-
tions by diving
scientists in DSV
Alvin. The Serocki
volcano is the large
opening at left.

Winter 1991/92

Long-term
observatories
will be required
on selected parts
of the Mid-
Ocean Ridge
system to
document
eruptive events.

would prove to be unique and homogeneous, we are continually recog-
nizing the great geochemical and mineralogical variability in ocean-ridge
basalts. The morphological similarities of submarine lavas and other
volcanic structures to lava flows on well-studied land volcanoes indi-
cates that processes of magma generation and eruption are similar in
both environments; thus, lessons learned in the study of the more
accessible land-based volcanoes can be applied to volcanic processes on
the deep seafloor. Already, some consistent correlations are beginning to
appear between certain chemical parameters and first-order geophysical
and morphological seafloor properties such as depth, gravity field, and
spreading rate. The most profitable future work is likely to come from
geophysical and petrologic studies carefully designed to integrate both
compositional data and physical properties of the ocean crust into
comprehensive models for melt generation, ascent, and the "plumbing
system" beneath ocean ridges.

Just as has been true of land volcanoes, long-term observatories will
be required on selected parts of the Mid-Ocean Ridge system to docu-
ment eruptive events, associated seismic activity, and subsequent
hydrothermal processes. Although the long controversy about the
significance and mode of pillow-lava formation has ended, much re-
mains to be learned about the growth of submarine volcanoes and the
mechanisms of lava distribution on the deep seafloor. Individual lava
flows must be mapped and sampled in detail, and their morphologies
carefully documented. Many morphologically diverse small volcanoes
and seamounts associated with active ridges must be imaged, sampled,
and restudied as they evolve with successive eruptions.

Such long-term observations are being discussed and planned as
part of the National Science Foundation-sponsored RIDGE initiative (See
article, page 21), but the magnitude of the commitment required for
definitive results is sobering. For example, observations carried on for
over 50 years at Kilauea Volcano in Hawaii are only now beginning to
yield a meaningful understanding of the volcano's eruption mechanics
and deep plumbing system. Further, this length of time still has not been
long enough for all styles of activity, as deduced from older lava and ash
deposits, to have been repeated for recording and analysis using modern
instrumentation. Emulating this work on our largest terrestrial basaltic
volcano, the 60,000-kilometer-long Mid-Ocean Ridge system, remains a
major challenge. ""%

Wilfred B. Bryan is a Senior Scientist in the Department of Geology and Geo-
physics at the Woods Hole Oceanographic Institution. He was Chief Diving
Scientist in Project FAMOUS, and has participated in studies of volcanic activity
on other parts of the Mid-Atlantic Ridge and East Pacific Rise. He was a principal
investigator in lunar volcanic landform studies for the Apollo Program and has
documented volcanic morphology and processes in Hawaii, the Southwest
Pacific, Iceland, Italy, and the western US and Canada.

Oceanus

Tectonics of

Slow-Spreading

Ridges

Jeffrey A. Karson

s oceanic plates diverge at mid-ocean ridge spreading
centers, two major processes produce and modify oceanic
lithosphere. The most familiar of these is magmatic con-
struction in the form of volcanic extrusion onto the
seafloor, probably accompanied by the intrusion of dikes
and larger bodies of coarse-grained, igneous material beneath the
seafloor. Just as important, however, are the effects of mechanical
extension, faulting of brittle surface materials, and plastic flow of hotter
material in the lower crust and upper mantle. At fast-spreading ridges,
magmatism nearly keeps pace with plate separation, so each increment
of separation is accompanied by sufficient igneous activity to fill any
cracks and fissures in the seafloor and bury most of the minor fault
scarps created since previous eruptions. In general, the wound inflicted
along the ridge axis is regularly healed, resulting in the formation of what
geologists call an "axial summit graben atop a very elongated shield
volcano." At any instant in time, the plate boundary resembles a series of
linked cracks in brittle material similar to cracks in a pane of glass.

In contrast, slow-spreading ridges display a completely different
interplay of mechanical extension and magmatism. Here the magma
supply is insufficient to completely restore the faulted axial crust.
Magmatism is discontinuous and episodic along the ridge axis despite
the relentless separation of the plate edges. The result is that the axial
crust at those edges is stretched and faulted in a manner similar to that of
continental rifts. This article describes some new insights gained from
submersible studies and continental rift analogs.

Ridges and Rifts: A Morphologic Comparison

In the late 1950s Bruce Heezen and colleagues at Lamont-Doherty
Geological Observatory discovered a deep cleft in the crest of many parts
of the mid-ocean ridge system. Based on similarities to profiles of the
East African Rift, they considered this cleft to be a rift valley produced
by extensional faulting of the oceanic crust. Much that has been learned
in the past 30 years about the geologic architecture of land and the
seafloor has stimulated a cross-pollination of ideas that derive from the
different constraints and limitations of these environments.

At any instant in

time, the plate

boundary

resembles a

series of linked

cracks in brittle

material similar

to cracks in a

pane of glass.

Winter 1991/92

Accomodation Zones

40km

The generalized geologic structure of the MARK Area on the Mid-Atlantic Ridge (A) and the Turkana Rift

of northern Kenya (B) allow a comparison of slow-spreading oceanic and continental rifts. Both are

composed of a series of discrete rift segments several tens of kilometers in length linked by accommodation

zones (stippled). Some segments have neovolcanic ridges (A) or quaternary volcanic centers (B) (black).

Hatched areas are rift-shoulder uplifts; dashes are exposures ofplutonic rocks; squiggles are serpentinites;

bold lines with boxes are major normal faults; and lines with or without tick marks are normal faults and

fissures. Bars labeled A, B, C indicate cross sections referred to in the text.

Although continental crust is typically about 35 kilometers thick
compared to only about 6 kilometers for oceanic crust, rifts in these two
settings are very similar both in scale and form. This is a result of the
dominating effect of temperature, which determines the lithosphere's
strength and thickness. The lithosphere is hot, thin, and weak at rifts, but
becomes thicker and stronger as it cools or moves away from a rift. As it
cools, the mantle beneath the crust becomes strong and controls the
rifting process. Oceanic lithosphere has a greater proportion of this
strong mantle than does continental lithosphere of similar thermal
structure or lithospheric thickness, limiting rift development in old
oceanic lithosphere.

Oceajius

If the oceans were drained, Earth's slow-spreading ridge systems
would resemble the well-known continental rift valleys, for example, the
4,000-kilometer-long East African Rift. The Mid-Atlantic Ridge occupies
nearly one-third of the seafloor beneath the Atlantic Ocean. It is broad
and undulating, with crests at hot spots like the Azores and Iceland. This
large-scale morphology is similar to the 100-kilometer- wide topographic
domes of Kenya and Ethiopia, upon which the East African Rift is
superimposed. On a finer scale, the continental and oceanic rift valleys
are segmented, that is, they are made up of a series of discrete fault-
bounded rift valleys. Each segment is several tens of kilometers long and
is linked end-to-end with adjoining valleys to form a nearly continuous
structure thousands of kilometers in length. Minor offsets, misalign-
ments, and overlaps of the rift-valley segments are typical of both
oceanic and continental rifts. This segmentation is also evident in the
gravity, magnetics, and seismic characteristics of rifts in both settings.

Viewed in profile, opposing rift valleys are commonly asymmetrical;
one bounding wall is higher and steeper than its mate across the axis.
Major faults with hundreds to thousands of meters of displacement
occur on the steep sides, and smaller faults and smoothly bent layers
occur on the lower sides. Thus,
half-graben forms are more
common than the symmetrical full
grabens with equal-sized faults on
both sides of the valley. The valley
depths are comparable, generally
around 2,000 meters. Lavas
partially fill both types of rift
valley, and sediments deposited in
rivers, lakes, and deltas reach
several-kilometer thicknesses in
the continental rifts. Where the
faulted rift-valley walls overlap,
the roughly symmetrical fault-
bounded troughs called grabens
or uplifted blocks called horsts are
created. Other areas, where no

overlap occurs, may have no rift valley at all, just a rugged, faulted
terrane that occupies the ridge axis.

The Neovolcanic Zone

The neovolcanic zone of the mid-ocean ridge system is the fresh bead of
lava that welds the ridge axis together. Along slow-spreading ridges this
most-recent volcanic material forms an imperfect seam, with many large
globs and gaps. The globs are referred to as neovolcanic ridges; their
lustrous lavas and very thin sediment dusting indicate they are only a
few thousand years old. The gaps are filled with faulted and fissured
lavas that erupted tens to hundreds of thousands of years ago, an earlier
version of the neovolcanic zone. The discontinuous nature of the young
lavas reveals that these areas are fed by a sputtering magma supply and
that the temperature of the lithosphere along the ridge axis is highly
variable. The discontinuity of the neovolcanic zone, as well as seismic

10km

Symmetrical Graben

Asymmetrical Half-Graben

Symmetrical rift

segments produced by

graben structures are

common in many rifts,

for example areas
marked by bars labeled
"A" opposite. Asym-
metrical half-graben
rift segments are also
common, for example
nrens marked by bars
labeled "B" opposite.
Note that half-grabens
may overlap to create a
symmetrical graben
morphology.

Winter 1991/92

Various types of

normal faults occur in

rifts, including steeply

dipping planar faults

(top), rotated planar

faults creating a

domino fault-block

pattern (second), Hstric

(curved) faults merging

doumward into a

horizontal detachment

fault (third), and

detachment faults

cutting across the full

thickness of the crust

(bottom). Steep planar

faults dominate the

median valley walls of

the Mid-Atlantic Ridge

creating a stair-step

shape and cutting any

earlier faults of the

median valley floor.

Median Valley Wall

Median Valley Floor

studies of the rift valley, indicate that there is no continuous magma
chamber beneath the axis of slow-spreading ridges. A similar scenario
applies to continental rifts. Major differences exist in the eruptive styles
of typical oceanic and continental rifts. In continental (subaerial) rifts,
extensive basaltic outpourings may precede the formation of a rift valley,
and explosive volcanism that produces cinder and ash cones is common.
In contrast, submarine rifts are dominated by monotonous fields of
pillow-lava mounds and ponded sheet flows.

In both continental and oceanic rifts, it appears that young magmatic
centers are associated with the rift-valley segments. Whereas small
volcanoes appear to have reached the surface through conduits provided
by various faults, the largest neovolcanic ridges appear to be centered in
well-defined grabens or half-grabens; thus the spacing of volcanic
centers appears to be similar to that of rift-valley segments. The distribu-
tion of very young volcanic rocks along slow-spreading ridges is known
in only a few areas, but in the eastern branch of the East African Rift,
extending from the Afar Triangle to northern Tanzania, the young
volcanic centers have a remarkably regular spacing of about 3 to 5
kilometers along the rift axis. Future seafloor mapping will determine if
a similar volcanic chain exists along the mid-ocean ridge axis.

Fault Structure

Axial lithosphere stretching is accommodated by faulting in the upper
crust. A number of factors determine the depth of faulting; the most

important is the temperature of the lithosphere. In some
places, where the lithosphere along the ridge axis is very
cold, and where there has not been a recent (less than 1
million years) magmatic event, faulting marked by
earthquakes affects the entire crust and even the upper
mantle beneath the spreading center. Because the axial
lithosphere is relatively thick and strong in such cases,
faulting affects a wide area, including the rift-valley
walls. In ridge segments where magmatic events have
occurred recently (perhaps marked by neovolcanic
ridges), the lithosphere is relatively thin and hot and
faulting affects only the narrow axial region. It is likely
that some ridge segments vacillate between these two
situations as axial temperatures wax and wane. Almost
certainly a similar variation occurs in continental rifts, but
it is complicated by the breaking of thicker, less uniform
continental crust with its many preexisting zones of
weakness. Still, hot, weak areas are expected to develop a
rift architecture distinct from that of cooler areas.

Scientists participating in the first submersible
studies of the mid-ocean ridge in the early 1970s de-
scribed rift-valley walls with numerous closely spaced
faults separating narrow blocks of crust and displacing
them to form a stair-step structure. Subsequent studies
have shown that this is just one of a family of ridge fault
10 km structures that includes simple planar faults, curved

Oceanus

Inactive
Faults

Active
Faults

Dike injection

Periodic
Magma

"teth^olphere Chamber

Uplifted Rift
Shoulder Detachment Minor (lateral?)

Lavas & Dikes
in Upper Crust

Gabbroic
Lower Crust

Mantle

Ductile
Deformation

Hanging
Wall

Lithosphere
Asthenosphere

Old
Shear Zone

Major
Shear Zone

10 km

"listric" faults, and very large continuous fault zones. All of these have
been studied in great detail in continental environments, but links between
processes of plastic flow and magmatic intrusion in the lower crust and
upper mantle are poorly understood at present.

Although only a few examples of rift-valley faulting have been
studied to date, they appear to follow a relatively simple pattern. Areas
with relatively high magma supplies that do not display large amounts
of extension have simple planar fault structures, and tend to form
symmetrical rift valleys where only basaltic rocks are exposed. Good
examples are known from the FAMOUS and AMAR rift valleys of the
Mid-Atlantic Ridge. Areas with somewhat larger amounts of stretching
and less magmatism become asymmetric as faulting on one side of the
rift valley begins to dominate. Listric and low-angle detachment faults
occur in some limited areas such as the TAG site. More stretching results
in extreme extension along gently to moderately inclined fault zones,
and may result in the exposure of materials once deeply buried. In the
most extreme situation known, a chaotic faulted assemblage of mixed
upper-crustal and mantle materials occurs across a ridge axis in the
MARK area. Although some cross sections in this area lack a clearly
defined rift valley, the ridge axis is nevertheless very highly extended.
This spectrum of fault structures mimics that of continental rifts. In both
settings, the amount of crustal stretching and displacement that indi-
vidual faults have sustained can be read in the types of rocks exposed
just beneath the fault surfaces. In the oceans, small amounts of extension
result in exposure of only basaltic rocks of the upper crust, while large
amounts can expose once deeply buried lower crustal rocks. In oceanic
areas, where the crust is only 6 kilometers thick, even upper-mantle
rocks can be exposed.

Ductile stretching of
the crust and mantle is
punctuated by periodic

injection of basaltic

dikes beneath a
symmetrical rift valley

with a neovolcanic

ridge (A). A deeper,
broader, asymmetrical
rift valley is created by
concentrated slip on a
major detachment fault

in cooler lithosphere
(B), for example section
"C"on the left figure on

page 52. Episodes of

magmatic (A) and
amagmatic (B) spread-
ing may alternate over
periods of a few

hundred thousand
years in the same ridge

segment. Adjacent

spreading segments
may be as different as

these two extreme
examples or an

intermediate stage.

Winter 1991/92

In the past few
years, seismic

reflection
studies have

provided

remarkable

new images of

the oceanic

crust's internal

structure.

Highly Extended Terranes

In both oceanic and continental rifts, prolonged periods of plate separa-
tion with little or no magmatic activity result in extreme stretching of the
crust and upper mantle as described. Such highly extended terranes are
well known in continental areas such as the Basin and Range Province of
the western US, where faulting has been localized along individual fault
surfaces called "detachment faults." These gently inclined dislocation
surfaces cut across rock units and smaller faults of the upper crust that
are free to rotate in a fragmented upper plate. Beneath, more plastic flow
occurs in a lower plate.

There is continuing debate concerning the inclination of these faults
when they were actually slipping. One school of thought argues that
only steep faults are mechanically feasible, and that the detachments
were formed by the rotation of steep fault segments that coalesced into a
single longer segment. Others propose that low-angle detachments have
maintained their near-horizontal attitude as fault blocks rotated above
them, allowing the detachment fault and lower-plate rocks to come
closer to Earth's surface. Still another hypothesis holds that the detach-
ments are individual giant faults, along which lower-plate rocks have
been pulled from deep beneath the overlying upper plate. In this case,
the detachment surface would be warped by vertical movements driven
by gravitational adjustments, maintaining a gently inclined attitude.

Regions of significant stretching also appear to exist along mid-
ocean-ridge spreading centers. Like continental detachment faults, they
are marked by major fault surfaces that expose deep crustal or even
upper-mantle rocks at the surface, or juxtapose them with shallow-level
rocks. In these areas, faulting must have been localized along single-fault
surfaces for long periods of time as plate separation continued. As a
result, 2 to 5 kilometers of crustal rocks have been stripped of underlying
deep-crustal and upper-mantle materials.

Unfortunately, areas of such exceptional faulting are difficult to find.
At present, there is no unambiguous link between the morphology of the
rift valley and the type of fault structure and rocks exposed there. This is
because numerous steep faults often cut and break up the large detach-
ment surfaces, giving even highly extended rift valleys a form not unlike
their less-stretched cousins. Studies of continental rifts show that earth-
quakes detectable with conventional instruments occur only on steeply
inclined faults. Slip on buried, low-angle detachments is known to occur
in some areas from the study of surface structures, like upper-crustal
faulting. Although required to link upper-crustal faulting to flow in the
lower crust, slip on such surfaces appears to occur without major earth-
quakes. In the oceans, such seismically quiet displacement could be
taking place undetected, because we have no detailed seismic studies or
maps of surface faults.

In the past few years, seismic reflection studies have provided
remarkable new images of the oceanic crust's internal structure. One
surprising feature of these sonograms is that they reveal numerous low-
angle reflectors cutting across the entire crust and sometimes intersecting
the surface at probable fault scarps. These structures appear to be major
detachment faults in the oceanic crust, spaced at about 30-kilometer

Oceanns

intervals. This translates to a periodicity for major faulting and magmatic
events of about 300,000 years, some 30 times longer than estimates based on
the apparent ages of lavas in the median valley floor. At present, the
interpretation of these features (as well as other intracrustal reflectors ) is
still debated; however, considering the rift valley's presently known
geology, detachment faulting is the most logical explanation.

Detachment faulting is also the most likely means of exposing
coarse-grained gabbroic and peridotitic rocks of the deep crust and
upper mantle along oceanic rift walls. The conditions that result in
extension with little or no magmatism could conceivably occur in any
ridge segment. However, persistently cool spots along the spreading
centers would be very likely places for these conditions. The intersec-
tions between rift segments and oceanic transform faults where a ridge
axis abuts a cold transform-fault wall are thought to be lithospheric cool
spots that may have very limited magma supplies. They are, therefore,
likely places for this type of extreme faulting. If cool, stretched crust
were formed at a ridge-transform intersection, it would pass laterally
along a transform fault and become the wall of an oceanic fracture zone.
This may explain the common occurrence of deep-level rocks along
fracture zones.

Connecting Structures

Individual rift segments are connected end-to-end by various types of
linkages. Some are discrete crustal faults while others are more diffuse
regions of bending or shattering. These features can be considered
collectively as "transfer zones," a term first used to describe linkages in
compressed and folded rocks of mountain belts, and useful to describe
the geometry and kinematics of rift linkages as well. Transfer zones in
continental rifts take many different forms, depending upon the charac-
ter of the faults in the rift segments they join. The diversity of fault
structures along slow-spreading ridges suggests that transfer zones are
important components of rift valleys. For example, different types of
transfer zones link rift-segment pairs that differ in amount of crustal
stretching, amount and timing of magmatic events, rate of stretching,
and style of faulting. In many cases asymmetrical rift-wall faults overlap
to create a class of transfer structures referred to as "accommodation
zones" that are typical of continental rifts. If small amounts of extension
have occurred, simple ramps or folds may suffice to transfer the effects
of faulting from one rift segment to the next. This geometry appears to
be typical of many oceanic and continental rifts. However, major faults
with large horizontal displacements may be required if large amounts of
extension occur in even one segment. The well-known transform faults
that occur along spreading centers can be regarded as just the largest of
a family of these transfer structures.

The asymmetry of the median valley of the Mid-Atlantic Ridge and
the geometry of steep linear slopes that suggest major faults create a
pattern very similar to that of continental rifts. At present, however, the
details of fault geometry and slip directions for mid-ocean-ridge spread-
ing-center segments and possible linking transfer zones are almost
completely unknown. The morphologic similarity, however, suggests

Jlie diversity

of fault
structures
along slow-
spreading
ridges suggests
that transfer
zones are
important
components of
rift valleys.

Winter 1991/92

Fast-spread

crust is likely to

be uniform and

continuous;

slow-spread
crust is apt to
be much more
heterogeneous.

that the growing body of detailed information on continental-rift faulting
can be applied at least in a general way to slow-spreading ridges.

It is important to recognize that the morphology of oceanic and
continental rifts is not necessarily a reliable indicator of their fault
structures. In particular, low-angle faults and strike-slip faults that do
not produce significant topographic or bathymetric relief are difficult to
detect with remote mapping systems such as multi-beam echo sounders,
including Sea Beam, or sidescan sonar systems like Sea MARC. Thus, the
present perception of oceanic rift structure is strongly biased by steep
faults with significant vertical offsets. These are certainly important
components of oceanic rifts; however, in some cases they represent only
a small amount of the total extension revealed by fault structures and
lithologic associations documented by detailed near-bottom investiga-
tions. It is clear that mapping seafloor morphology with remote-sensing
systems will not be sufficient to evaluate the geometry and extent of
faulting for segments of the mid-ocean ridge system. Much more de-
tailed near-bottom sampling and mapping from submersibles such as
Alvin or remotely operated vehicles like Argo-Jason will be required.

Implications of Major Faulting on Slow-Spreading Ridges

The recognition of major faulting in the median valley of slow-spreading
ridges has some important implications for understanding seafloor-
spreading processes and oceanic-crust production in these environments.
First, the extreme faulting found in some places, like the MARK area and
some transform-valley walls, suggests that long periods (perhaps as long
as 1 million years) of plate separation occur with little or no magmatic
construction. This implies that long magmatic "droughts" occur at least
locally along the ridge axis, contrasting sharply with observations on
fast-spreading ridges that suggest a persistent, robust magma supply. It
follows that the end products of accretion of fast- and slow-spreading
ridges may be very different geologically. Whereas fast-spread crust is
likely to be characterized by a generally uniform and continuous geo-
logic structure, slow-spread crust is apt to be much more heterogeneous.
Swaths of slow-spread crust tens of kilometers across probably resemble
that created at fast-spreading ridges. However, if the geology of the
present-day median valley of the Mid-Atlantic Ridge is a valid guide to
slow-spreading processes in general, there must also be patches of highly
disrupted and broken crust that might be essentially a jumbled mass of
faulted oceanic crustal and upper-mantle blocks that are locally welded
together by intrusive dikes and lava flows. This inference must somehow
be reconciled with the well-documented fact that the seismic structures
of fast- and slow-spread crust are nearly identical. It is probable that the
seismic structure is dictated by fractures and rock porosity rather than
rock compositions, a possibility that would limit the usefulness of
seismic studies in oceanic geology investigations.

The extreme type of faulting described above also raises some
questions regarding the origin of lineated marine magnetic anomalies.
How can they persist if the basaltic layer of the crust, generally thought
to be the source of magnetization, is highly faulted and even discontinu-
ous? Typical lineations are found over several areas where basaltic rocks

Oceanus

have been faulted away to expose deep-crustal or even upper-mantle
rocks. These occurrences suggest that magnetic lineations can also be
produced by the magnetization of metamorphosed deep-crustal and
upper-mantle material. This seems feasible if the magnetization is
acquired during faulting close to the median valley, the same place the
basalts have their magnetization frozen-in during normal spreading.

Faulting produces the major fracture porosity in all parts of Earth's
crust. If the fault patterns of continental rifts can be used as a template
for slow-spreading ridges, it should be possible to predict the fault and
porosity patterns of the seafloor at least in a general way. Faulting and
fracture porosity are likely to control the locus of magmatism and
hydrothermal venting along spreading centers, just as they do in conti-
nental settings. This relation could help explain patterns of volcanoes
and black smoker vents that are just beginning to emerge from near-
bottom studies, and might even prove important as a prospecting tool
for spreading-center ore deposits.

There is a growing awareness that fast- and slow-spreading ridges
function in very different ways. The sputtering magma supply of slow-
spreading ridges results in substantial periods of plate separation that
involve stretching and faulting of relatively cool oceanic lithosphere with
little or no magmatism. The fault patterns of the median valley appear to
mimic those of continental rifts; however, at least locally, very highly
stretched and thinned masses of crust and upper mantle occur. The
median-valley geology and fault structure documented by near-bottom
studies predict a very heterogeneous geological structure in slow-spread
crust. This result is yet to be clearly defined or reconciled with the
geophysical expression of the crust away from spreading centers. Future
studies of the geometry and kinematics of faulting on slow-spreading
ridges will determine the nature of faulting over much larger areas than
have been studied to date, and will help contribute to the overall under-
standing of how the lithosphere is pulled apart to form rifts in both the
continents and the seafloor.

Jeffrey A. Karson is an Associate Professor in the Department of Geology at
Duke University. He is a field-oriented structural geologist who has studied the
nature of faulting on the seafloor at both fast- and slow-spreading ridges and
associated transform faults during seven diving programs using DSV Alvin and
other submersibles. He maintains a parallel research program in the East African
Rift and ophiolite terranes.

There is a
growing

awareness that

fast- and slow-
spreading

ridges function
in very

different ways.

Winter 1991 192

Seismologists

soon realized

that a relatively

narrow band of

earthquakes

could be traced

through many

of the world's

ocean basins.

Mid-Ocean Ridge
Seismicity

Eric A. Bergman

he earliest observations of earthquakes in deep-ocean basins
were reported by seamen whose ships were rocked by
undersea disturbances, and by residents of oceanic islands
such as the Azores and Iceland. Scholarly studies based on
eyewitness accounts of so-called "seaquakes" began in the
late 19th century, but systematic investigations of oceanic seismicity did
not begin until a global network of earthquake observatories was estab-
lished in the early decades of this century. By the 1920s the International
Seismological Summary (ISS) was routinely compiling data from cooper-
ating stations, and publishing earthquake locations. Seismologists soon
realized that a relatively narrow band of earthquakes could be traced
through many of the world's ocean basins. The earthquakes were
associated with the mid-ocean mountains that had been revealed by
early oceanographic surveys, but the global continuity of mid-ocean
ridge seismicity was not demonstrated until the mid-1950s.

For several decades after these initial discoveries, mid-oceanic
earthquakes attracted relatively little research interest. Most seismic
stations were in the northern hemisphere and most instruments recorded
at low gains (that is, the signals were not magnified). As a result, the
long-distance, or teleseismic, detection threshold varied dramatically in
different regions, and the accuracy for locations in remote oceanic areas
was poor. Magnitudes could be estimated only for the largest earth-
quakes. Limited knowledge of the geology of the mid-ocean ridge
system also inhibited seismological research.

This humble status began to change in the late 1950s. Seismology
was invigorated by the emphasis on global geophysical observations
during the 1957-58 International Geophysical Year (IGY) project. Also,
deployment of new seismic stations to monitor tests of nuclear explo-
sions resulted in improved detection and location capabilities for earth-
quakes as well. In addition, the introduction of computers for data
processing and earthquake location made global monitoring of smaller
earthquakes possible.

The importance of mid-ocean ridge seismology soared in the late
1960s, when it provided compelling evidence for the plate- tectonic
hypothesis. Growing catalogs of accurately located earthquakes brought
clear delineation of plate boundaries on seismicity maps, and first-
motion studies confirmed predictions regarding the geometry of faulting

Ocennus

at different types of plate boundaries. This success can largely be attrib-
uted to the establishment in 1963 of the World-Wide Standardized
Seismograph Network (WWSSN), a global network of about 100 seismic
stations equipped with well-calibrated and standardized seismometers.
The improved global distribution of these stations and their relatively
high magnification of seismic signals significantly lowered the detection
threshold in oceanic regions. A notable advance was establishment of a
central archive from which complete seismograms from all the WWSSN
stations could be obtained quickly. For the first time it became relatively
easy for a researcher to collect all the data needed for a detailed source
study of an earthquake almost anywhere in the world.

Analytical techniques for digitized wave-form seismic data (both
body and surface waves) were developed in the 1970s. Except that they
had to be hand-digitized, the long-period WWSSN data were well-suited
for studies on the moderate-sized earthquakes typical of mid-ocean
ridges. At the same time, the first digital seismometers were deployed.
These stations eliminated the unpleasant task of digitizing analog
records for analysis, but the early instruments were best suited to
earthquakes that were larger than is typical for mid-ocean ridges. Also,
there were simply too few stations deployed to use them exclusively for
source studies in most areas. This situation is changing rapidly: Broad-
banded digital stations are being deployed by institutions and countries,
with the goal of obtaining global coverage at least as good as what
WWSSN provided. Seismologists will soon have the luxury of working
exclusively with digital data for global earthquakes studies.

With the proliferation of digital seismic stations, digital analysis
techniques for earthquake source studies have become quite sophisti-
cated. It is now standard practice to perform a formal inversion for
source parameters such as depth, focal mechanism, and seismic moment
(a measure of earthquake size that has many advantages over magni-
tude). Systematic studies of mid-ocean ridge earthquakes have produced
many insights concerning the tectonics of accreting plate boundaries.
Application of these techniques is still limited to the largest mid-ocean
ridge earthquakes, however; further progress depends on improvements
in our ability to study smaller earthquakes.

Establishment in 1964 of the International Seismological Centre (ISC)
stimulated global seismology significantly. The ISC collects phase
readings from thousands of seismic stations, associates them with
particular earthquakes, locates the earthquakes, and publishes the data
and results in a widely circulated bulletin. A whole new class of seismo-
logical research is based on the availability of the entire ISC data set since
1964 in machine-readable format (magnetic tape and CD-ROM).

Techniques are now being applied to mid-ocean ridge earthquakes
that improve epicenter location for smaller earthquakes and help to
correlate epicenters with geological features. These techniques, which
simultaneously locate many earthquakes in a region, provide useful
information about the location of mid-ocean ridge earthquakes with
magnitudes as low as 4.5 on the Richter scale (a logarithmic scale of
earthquake magnitude from -3 to 9 — the highest recroding to date is 8.9).

In the 1970s we began to study smaller oceanic earthquakes using
temporary (up to a few weeks) deployments of sonobuoy arrays and

Systematic

studies of mid-
ocean ridge
earthquakes have

produced many
insights

concerning the
tectonics of

accreting plate
boundaries.

Winter 1991/92

Tlie maximum

magnitude of

earthquakes

occurring

in close

association

with spreading

segments is

strongly
correlated with
spreading rate.

ocean-bottom seismometer (OBS) and hydrophone arrays. Epicenters
and focal depths can be determined with great accuracy for microearth-
quakes occurring within the array, but the relatively low rate of occur-
rence of earthquakes on mid-ocean ridges and the technical difficulty of
deploying (and recovering) enough instruments to permit useful seismo-
logical analysis has doomed many such studies to disappointment.
Progress in battery and data-storage technology, however, is extending
deployment times. Technology that permits rapid deployment of seis-
mometers on targets of opportunity will play a significant role in the
future of mid-ocean ridge seismology.

A Note on the Importance of Thermal Structure: Before discussing the
characteristics of mid-ocean ridge earthquakes, it is worth emphasizing
that the depth distribution of earthquakes is closely linked to the thermal
structure of the lithosphere. Earthquake faulting is commonly considered
to be analogous, if not identical, to the phenomenon of brittle failure,
which is the usual mode of deformation of small samples of crustal and
upper-mantle rocks at low temperatures. At sufficiently high tempera-
tures, ductile deformation relieves an applied stress before the brittle-
failure limit is reached, and no earthquakes occur. In any given tectonic
environment, therefore, the depth distribution of seismicity provides
information about the thermal structure. Conversely, prior knowledge of
the thermal structure can be used to interpret the seismic data more fully.

Earthquakes on Spreading Ridges

Given the difficulty (so far) in observing an actual spreading episode on
any deep-ocean ridge segment, earthquakes are perhaps the most
dramatic indicators of the tectonic processes involved in the creation of
new oceanic crust. A major issue in mid-ocean ridge seismology has been
the extent to which earthquakes signal active magmatism at shallow
crustal levels. The observations summarized below are beginning to
reveal the answer to this question, but much work remains to be done.

The maximum magnitude of earthquakes occurring in close associa-
tion with spreading segments of the mid-ocean ridges, so-called "ridge-
axis" events even though few probably occur in zero-age (true ridge axis)
crust, is strongly correlated with spreading rate. At slow-spreading
ridges with well-developed rift mountains and a median valley, such as
the Mid-Atlantic Ridge (MAR), earthquakes reach a maximum magni-
tude of about 6.0, while at the fast-spreading ridges of the East Pacific
Rise (EPR), very few earthquakes reach even the teleseismic detection
threshold, about 4.7, and the ridge segments appear to be aseismic on
global seismicity maps. Microearthquake surveys show that these ridge
crests are seismically active at lower magnitudes, however.

Ridge-axis seismicity frequently occurs as swarms of earthquakes
that usually last a day or two, although a few sequences lasting weeks
are known. As much as 50 percent of the ridge-axis seismicity is esti-
mated to be associated with this sort of cluster, so understanding the
tectonic significance of this seismicity is an important research goal.
Because swarm seismicity is frequently observed in terrestrial volcanic
centers, ridge-axis swarms are sometimes thought to reveal ongoing

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The seismically
active portion
of the ridge is
about 10 to 20
kilometers wide,
comparable to
the median-
valley width.

volcanism. Many characteristics of swarm seismicity on the MAR are
more consistent, however, with the view that most such earthquakes are
expressions of extensional tectonics, in particular, formation of the steep
scarps bounding the median valley. With few exceptions, seismicity
directly related to ongoing volcanism probably occurs at magnitudes
below the teleseismic detection threshold; this is one of the reasons for
the current emphasis on monitoring mid-ocean ridge seismicity at
microearthquake magnitudes.

Ridge-axis seismicity is well correlated with the segmentation of
slow-spreading ridges, which is revealed by bathymetric, magnetic, and
gravity data. The larger swarms sometimes extend for several tens of
kilometers along the axis, but the length of individual fault scarps
seldom exceeds about 10 kilometers. Together with the limit on depth of
faulting discussed below, this fact largely explains the upper limit on
magnitude (about 6.0) for MAR ridge-axis earthquakes. The seismically
active portion of the ridge is about 10 to 20 kilometers wide, comparable
to the median-valley width. Few earthquakes are located beyond the rift-
mountain crests, except for relatively rare intraplate events farther from
the ridge.

Studies of the depth ranges of seismic activity show that earthquakes
of all magnitudes are generally confined to the upper 8 to 10 kilometers
of slow-spreading ridge segments, and the depth limit becomes shal-
lower with increasing spreading rate. The depth limit on ridge-axis
earthquakes reflects the depth of a limiting isotherm that is determined
by the balance between heat input from the upper mantle and heat
removal from the upper few kilometers of the crust by hydrothermal
circulation. At higher spreading rates, the increased heat input reduces
the volume of crust capable of supporting brittle failure to the point
where no earthquakes that can be detected from land-based seismic
stations occur.

To explain the seismological observations summarized here, as well
as other types of geophysical studies on mid-ocean ridges, a current
hypothesis holds that the median valley of slow-spreading ridges is
formed by the necking of a mechanically strong brittle lithosphere under
regional horizontal extension, and that the thickness of this necking zone
corresponds to the maximum depth of seismic activity. When mid-ocean
ridge seismicity is analyzed with this model, seismogenic extension in
the median valley is found to account for about 20 percent (at most) of
the plate-separation rate. The remainder must be taken up by
nonseismogenic processes, notably the creation of new ridge-axis crust
by volcanic activity.

Transform Fault Earthquakes

Transform faults are the most seismically active portions of the mid-
ocean ridge system. Even at high spreading rates, where ridge-axis
seismicity apparently vanishes, transforms are usually well defined by
earthquake epicenters. Transform faults also produce significantly larger
earthquakes than ridge segments, commonly up to about magnitude 7.
Perhaps the most important parameter controlling earthquake size on a
transform is the age offset, that is, the age of the lithosphere opposite
each ridge-transform intersection. Given two transforms with equal

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lengths but different age offsets, the transform with the greater age offset
should support larger earthquakes because the isotherm that limits the
maximum depth of faulting on the transform will be deeper.

Early first-motion studies of transform earthquakes revealed strike-
slip faulting on near-vertical fault planes, striking parallel to the direc-
tion of plate motion. Detailed source studies using wave-form inversion
methods generally confirm these results. All known strike-slip events
have a sense of slip that is consistent with the transform hypothesis, but
it appears that there are exceptions to the "near-vertical fault plane"
observations: two large events on the western end of the Vema Trans-
form (10°N on the MAR) both have fault planes that dip into the north,
toward younger lithosphere, at angles of 50° to 60° from the horizontal.
Also, several cases of transform earthquakes with largely compressed or
extensional focal mechanisms are known, and complex rupture histories
are rather common in the larger strike-slip earthquakes. The main pulse
of rupture is often preceded or followed closely by smaller pulses that
sometimes appear to have a faulting geometry different from that of the
main rupture. Dip-slip faulting, whether expressed as a subevent during
a dominantly strike-slip transform event, or as an individual earthquake,
is thought to be caused by heterogeneities in the fault system on which
transform motion occurs. These narrow (less than about 5 kilometers)
zones apparently migrate within the larger transform valley in response
to variations in the regional crustal structure and stress system.

The observed segmentation of many transforms raises the issue of
whether an entire transform ever slips in a single earthquake. Such
events appear to be rare. Shorter transforms with relatively linear fault
zones would be the most likely candidates for a "home run" of this sort,
but the largest earthquakes generally occur on the longest transforms.
Long transforms are likely to develop heterogeneities that would tend to
prevent a rupture, once initiated, from propagating to both ends of the
fault zone. For example, the largest transform earthquake in the last
three decades on the northern MAR, a magnitude-7 event on the Vema
Transform in 1962, has a rupture length of about 40 kilometers, com-
pared to the 300 kilometer length of the transform. It is not even clear
that the entire length of all transforms produces large earthquakes. Most
transforms contain sections that have not slipped during the few decades
for which reliable records are available.

The depths of large, shallow, strike-slip earthquakes are especially
difficult to determine using wave-form analysis techniques. The trans-
form events that have been studied with these methods all appear to
rupture through to the seafloor, but for some events only an upper
bound on the maximum faulting depth can be assigned. Large transform
events on the MAR north of the equator appear to involve rupture no
deeper than 15 to 20 kilometers. The depth-limiting isotherm appears to
be 900° plus or minus 100°C for most of these transforms. Depths of
transform earthquakes in the Gulf of California are consistent with a
limiting isotherm of about 800°C. Recent large earthquakes on the 15°20'
Transform appear to be an exception to this pattern, however; they have
maximum depths of faulting of about 10 kilometers, corresponding to a
nominal isotherm of about 600°C. One explanation for this observation is
that the limiting isotherm for this transform may be at shallower depths
than calculated from a standard thermal model; this could reflect recent

The largest

transform

earthquake in

the last three

decades on

the northern

Mid-Atlantic

Ridge ivas a

magnitude-7

event on the

Vema Transform

in 1962.

Winter 1991/92

RECORDING SYSTEM •

RECOVERABLE BY TRANSPONDER

AND SERVICEABLE BY

SUBMERSIBLE

SURFACE BROAD BAND
SENSOR PACKAGE

BOREHOLE

BROADBAND

PACKAGE

BURIED BROAD BAND
SENSOR PACKAGE

Several approaches are
being investigated for

deploying high-
dynamic range, broad-
band seismometers in
deep ocean basins for

the Ocean Seismic
Network. These types
of instruments would

greatly enhance the
ability to monitor mid-
ocean ridge seismicity.

changes thought to have occurred in
the geometry and location of the
North America-South America-
Africa triple-plate junction.

Few microearthquake surveys
are available for transforms, but a re-
cent study on the Kane Transform
produced two surprising results: the
microearthquake activity was con-
centrated off the expected zone of
transform motion, both to the north
and south; and the focal mechanisms
of the microearthquakes indicated
normal faulting and strike-slip fault-
ing (inconsistent with transform mo-
tion) with the axis of horizontal ex-
tension oriented across the transform.
Proposed geodynamic models pre-
dict stress fields in the vicinity of
oceanic transforms, but the underly-
ing theory is controversial and there
is little evidence available to test the
hypothesis. Further microearthquake
studies are needed to determine
whether the observations on the
Kane Transform are representative of
oceanic transforms.
Transform faults are an end member of a spectrum of geologic
features associated with offsets of mid-ocean ridge spreading segments.
Little is known about the seismicity associated with very small offsets.
From a seismological point of view, it is natural to define a transform as
an offset capable of producing an earthquake with the characteristic
strike-slip focal mechanism. This definition may not be consistent with
one based on morphology; the issue has yet to be investigated. Obstacles
to such a study include obtaining sufficiently accurate epicenters to
unequivocally place earthquakes on small ridge offsets and the lack of a
reliable means to determine focal mechanisms for earthquakes with
magnitudes less than about 5.

The Future

The continued application of modern seismological analysis techniques
for improved location and source studies will undoubtedly help to
clarify some of these issues. The next significant pulse of activity in mid-
ocean ridge seismology is likely to be driven, however, by technologies
and observing programs that allow earthquakes to be studied at lower
magnitudes than is possible with any conceivable land-based seismo-
graph system, and for longer times and over wider areas than is possible
with the current ship-deployed OBS technology.

One current plan for monitoring the seismicity of selected mid-ocean
ridge segments at magnitude levels well below the teleseismic threshold
makes use of waterborne T-phase data recorded at permanent hydro-

Oceanus

phone arrays operated by the US Navy. These data would be especially
valuable for monitoring swarm seismicity that may indicate active
volcanism. Researchers are also discussing strategies for responding
rapidly and effectively to such events when they are observed. A recent
successful response to earthquake swarms on the Reykjanes Ridge
included dropping sonobuoys and other geophysical instruments from
long-range military patrol planes flying from Iceland, and diverting
oceanographic research vessels that happened to be in the area (see Box
on page 23).

Permanent seafloor geophysical observatories are also on the hori-
zon. In some plans, instruments would be deployed autonomously on
the seafloor or in boreholes, in others they would be attached to old
undersea telephone cables that have been converted for scientific use.
One such cable crosses the Mid-Atlantic Ridge in the FAMOUS region near
37°N. If these technologies are developed and deployed, they will undoubt-
edly spark a new round of interest in mid-ocean ridge seismology.

Eric A. Bergman is a Geophysicist at the National Earthquake Information Center
of the US Geological Survey. His research interests include the seismotectonics
of oceanic mid-ocean ridges and intraplate regions, analysis techniques for
improved determination of earthquake locations and source characteristics, and
the state of stress in the lithosphere. He is currently active in the International
Seismological Observing Period Project, a program to coordinate and enhance
the observational activities of seismic observatories worldwide.

Seabeam Maps of
the Mid-Atlantic Ridge Available

A limited number of copies are available

of a map series that covers

the crest of the Mid-Atlantic Ridge

between latitudes 24°-31°N.

The Seabeam data are presented at a contour interval of 50 meters and a scale of 30 inches per
degree of longitude in a series of eleven color plates each measuring approximately 36 by 42 inches.
These are reprints of the maps published in Marine Geophysical Researches, Volume 12, pages 247-
252, 1990. They are suitable both for original studies of the morphological characteristics of slow-
spreading ridges and for teaching practical classes in the understanding and interpretation of high
resolution multibeam bathymetry data.

Upon request, copies of these maps will be mailed at no cost to U.S. academic institutions or
Government agencies, providing a brief statement is supplied that describes their intended use.
Multiple copies are available for teaching purposes if a clear statement of the nature of the course is
provided. Requests from outside the U.S. will be honored only if resources permit. Please send
requests to:

Dr. G.M. Purdy, Department of Geology and Geophysics,
Woods Hole Oceanographic Institution, Woods Hole, MA, 02543.

Winter 1991/92

Hydrothermal

systems

transfer large

amounts of

heat and mass

from Earth's

interior to

the oceans.

Hydrothermal
Vent Systems

Margaret K. Tivey

t's difficult to imagine that just 15 years ago no one had ever seen
a "black smoker chimney;" now they seem to be found at mid-
ocean ridge crests whenever we take a close look. Black smoker
chimney is the term used to describe a smokestacklike structure
composed of sulfide and sulfate minerals. "Black smoke" refers
to the abundance of dark particulates that form when extremely hot
(350°C) hydrothermal fluid rapidly exits the chimney opening and mixes
with cold (2°C) seawater. These chimneys, which would draw attention
no matter what the setting, are all the more spectacular since they cap
seafloor hydrothermal vent sites that are oases of activity on the other-
wise rather barren terrain of mid-ocean ridge crests.

Hydrothermal systems transfer large amounts of heat and mass from
Earth's interior to the oceans. Fluids exiting the chimneys are metal-rich,
hot, and acidic, and vent at velocities on the order of meters per second.
A striking feature of black smoker chimneys is how remarkably thin
their walls are: They vary in thickness from about 5 inches to as little as
.25 of an inch. Across this thin layer is a temperature difference of 300°C
or greater, and similar steep elemental composition gradients also exist.
Chimney structures are thus fascinating subjects for scientific study.
Many questions come to mind when first seeing these chimneys in action
such as,

Where is all the fluid coming from?
Why is it flowing so fast?
How did it get so hot?
Where did all the particulates come from?
How do the chimneys form? And equally puzzling,
Why did it take so long to find them?

The existence of large-scale hydrothermal convection (fluid circula-
tion) within oceanic crust near mid-ocean ridges was predicted in the
mid-1960s, more than a decade before the first discoveries of active
vents. It was recognized that oceanic crust could act as a porous me-
dium, a magma chamber or newly solidified rock as a heat source, and
seawater as a convecting fluid. But at this time, ridge crests were not well
explored on the scale of tens of meters, the size of most vent fields. In
1977, active hydrothermal vents on mid-ocean ridge crests were first
discovered on the Galapagos Rift, venting warm (25°C) fluid. The first
discovery of high-temperature fluids actively forming chimneylike

Oceamis

mineral deposits occurred in 1979 on the East Pacific Rise at 21 °N. Since
then numerous additional seafloor vent sites have been discovered in both
the Pacific and Atlantic oceans. All detailed studies of vent sites have
employed submersibles to photograph and map vent fields, measure
temperatures of fluids, collect fluids, and recover fragile chimney samples.

Where Is All the Fluid Coming From?
Why Does It Circulate?
How Does It Get So Hot?

At all of these locations, the general processes
of porous media convection, interaction
between fluid and rock, and mineral deposi-
tion are similar. The schematic cross-sectional
view across a ridge axis shows the ridge axis
underlain by a heat source, either a magma
chamber or newly solidified hot rock. The
overlying crust, formed by volcanic activity, is
permeable, owing to contraction and cracking
as it cools. Seawater percolates down into these
cracks, and circulates through hot basalt. Heat
is transferred from the hot rock to the fluid.

As water is heated, its physical properties
change. It expands, becoming less dense, and
its viscosity decreases, so that it flows more
easily. If this circulation occurred on land,
drastic changes would occur when the tem-
perature of the water reached 100°C, the
boiling point of water. But at the depth of mid-
ocean ridge crests, 2,000 to 4,000 meters below
sea level, at pressures of 200 to 400 bars, the
boiling point of seawater is much higher. Fluid
can reach temperatures as high as 350°C
without boiling. (The boiling point of seawater
is 370°C at a pressure of 200 bars, and 404°C at

300 bars.) Fluid of this temperature is extremely buoyant, with a density
less than seven-tenths that of seawater. If this fluid finds an open path to
the seafloor, for instance a large open crack, or a series of interconnected
cracks and void spaces, it will rise rapidly to the surface.

How Do the Fluids Become Metal-Rich? Where Do the
Particulates Come From? How Do Chimneys Form?

As the fluid circulates within the crust, it interacts with basaltic rock at
high temperatures. Clay and sulfate minerals precipitate from seawater
as it is initially heated, resulting in a modified fluid with little to no
magnesium or sulfate, ions that are abundant in seawater. At higher
temperatures, metals, silica, and sulfide are leached from the rock. The
result is a hot, acidic (low pH) fluid with abundant silica, hydrogen
sulfide, and metals, relative to seawater.

The hot, buoyant, metal-rich fluid exits the seafloor at velocities on
the order of meters per second. When hydrothermal fluid mixes with

A black smoker

chimney from the East

Pacific Rise at 21° N

vents 350° fluid at

velocities on the order

of 1 to 5 meters per

second. Tlie plume of

black particulates

(smoke) forms when the

hot, low pH vent fluid

mixes turbulent!}/ with

the surrounding cold,

higher-pH water.

Winter 1991/92

Permeable
Ocean
Crust

Fluid/ Rock
Interaction

Impermeable

Heat

Source

A schematic cross

section of a sen floor

hydrothermal system

shows an impermeable

heat source (magma

chamber or hot rocks)

overlain by permeable

ocean crust at an

unsedimented ridge

crest. Fluid circulates

within the crust, driven

by temperature

differences. During this

circulation seawater is

modified by fluid /rock

interaction to hot,

metal-rich fluid that is

buoyant, and vents on

the seafloor.

seawater, changes in pH and
temperature result in the precipi-
tation of minerals, the formation
of black smoke, and black
smoker chimneys. Black smoke is
composed dominantly of fine-
grained sulfide and oxide
minerals (pyrrhotite, chalcopy-
rite, sphalerite, and amorphous
iron oxides). Black smoker
chimneys are concentric hollow
spires up to 20 feet high, with
inner channels .5 to 4 inches in
diameter, that vent fluid in
excess of 300°C. Early stages of
black smoker chimney growth
involve emplacement of an
anhydrite-rich wall around the
vent opening. Anhydrite (cal-
cium sulfate) precipitates when
seawater, rich in calcium and
sulfate, and hydrothermal fluid,
rich in calcium but depleted with
respect to sulfate, mix. Anhydrite
is an unusual mineral that is
more soluble at low tempera-
tures than at high temperatures. In seawater, it is saturated (and there-
fore should precipitate) at temperatures above approximately 150°C.
Once a wall is formed around the vent opening, mixing between hydro-
thermal fluid and seawater is restricted. The wall gradually becomes less
permeable as hydrothermal fluid and seawater mix through the wall,
and sulfide and sulfate minerals precipitate. The inner side of the wall is
in contact with hydrothermal fluid and chalcopyrite is deposited on this
surface. The result is a concentrically zoned structure with an inner
channel lined with chalcopyrite, and outer layers composed of varying
amounts of anhydrite, and iron, copper-iron, and zinc sulfide minerals (such
as pyrite and marcasite, chalcopyrite, bornite, sphalerite, and wurtzite).

Variations Among Vent Sites

Black smoker chimneys, and fluids with temperatures in excess of 300°C,
are found at most active vent sites, reflecting the similarities in the
general processes of fluid circulation and mineral deposition occurring at
unsedimented mid-ocean ridge crests. Details of these processes, how-
ever, vary, resulting in distinct fluid compositions, and differences in the
mineralogy, size, and gross morphology of the hydrothermal deposits.
Sizes of vent deposits range from relatively small (fields about 10 meters
in diameter) to those that resemble ore deposits exposed on land (up to
200 meters in diameter). Variations also exist in fluid composition,
maximum fluid temperature, mineralogy, shape of deposits, and geo-
logic setting. While the past decade of research focused on sampling the
highest temperature fluids present at each site and the associated min-

Oceanus

eral precipitates, the focus is now shifting toward understanding the
causes of variations and differences among vent sites.

Fluid Composition, All of the solutions sampled at vent sites on
unsedimented ridge crests are acidic, sulfide-rich, and capable of carry-
ing large amounts of ore-forming elements. Fluid composition differs
from site to site with respect to concentrations of chloride, metals,
hydrogen sulfide, silica, and carbon dioxide, as well as pH and tempera-
ture. These variations reflect differences in fluid/rock interactions,
including the amount of fluid being seen by each piece of rock during
fluid circulation, depth of circulation and reaction, mineral assemblages
present at each depth, and temperatures of reaction. Fluid composition
can also be affected by processes occurring near the surface: Fluid can be
cooled and minerals deposited directly beneath the seafloor, either by
conduction (heat loss, with no addition of cold seawater) or from mixing
with cold seawater.

Within each vent site there is a range of exiting fluid temperatures,
compositions, and velocities. Scientists hypothesize that at each vent site
there is one highest temperature, or end-member solution, and that
ranges in temperature and composition within the vent field can be
accounted for either by conductive cooling of the end-member solution,
or mixing of the solution with seawater.

Size and Shape of Deposits. Vent sites on the East Pacific Rise at 21 °N
were the first ones analyzed for both fluid chemistry and mineralogy,
and are (to some extent) the type of vent system that all other systems
are compared to. At 21 °N, chimney structures are up to 6 meters high,
and have open channels 1 to 10
centimeters in diameter that are
lined with chalcopyrite. Maximum
fluid temperatures are 350°C, and
flow velocities range from 1 to 5
meters per second. The chimneys
sit on top of low-lying basal
mounds. The surfaces of these
mounds are comprised of fine-
grained sulfide-rich mud and
partially oxidized sulfide-rich
fragments, some of which appear
to be pieces of fallen chimneys. The
interiors of the mounds have not
been sampled or studied in detail. When they are ruptured, small black
smokers form, suggesting that the temperature of fluid circulating within
the mounds is high. The vent deposits are spaced along the center of a
narrow (5-kilometer wide) axial valley at 100- to 1,000-meter intervals,
and are located on fresh lava flows. At each of these sites the amount of
heat being transported from Earth's interior to the ocean is very large,
yet the amount of metal-rich minerals deposited is small relative to ore
deposits exposed on land. It is not clear whether these deposits will ever
grow to a large size; whether they are truly analogous to ore deposits is
thus open to question.

The vent sites with fluid chemistry most different from 21 °N are
those on the southern Juan de Fuca Ridge. These vent sites are both

Location of known
seafloor Jn/drothermal

vent sites (dosed
triangles) are shown
below. Solid lines
indicate ridges. The
lack of known sites on
ridge crests in the
South Pacific and
Indian oceans, and
along much of the Mid-
Atlantic Ridge, is
indicative of areas that

have not been ad-
equately explored. SJFR
indicates Southern
Juan de Fuca Ridge,
EPR indicates East
Pacific Rise.

30'

120

Winter 1991/92

Tlie vent site

most analogous

to ore deposits

exposed

on land is the

active TAG

mound located

at 26°N on the

Mid-Atlantic

Ridge.

similar and different when compared to those on the East Pacific Rise.
On the southern Juan de Fuca Ridge, chimneys and spires are the
dominant form of mineral deposition, and vent sites are located in the
center of the axial valley on fresh basalt. The morphology and mineral-
ogy of the chimneys, however, differ from those at 21 °N. In general the
chimneys are small (2 to 6 feet tall), and instead of exhibiting strong
concentric zonation around a large open channel, they are texturally
more complex and contain multiple small (1- to 10-millimeter-diameter)
fluid channels. Flow rates are less than at 21 °N. Mineralogy is dominated by
zinc sulfide (wurtzite and sphalerite) instead of copper-iron sulfide, and the
innermost copper-rich layer that is common in East Pacific Rise chimneys is
absent. Some of these differences are accounted for by the lower tempera-
ture (less than 300°C) and different composition of the venting fluid that is
forming the deposits, the most striking of which is the chlorinity: Chloride
concentration is up to twice that of sea water, and metal concentrations
(except copper) are also high since metals are present in solution as chloride
complexes (for example, iron chloride, lead chloride, and zinc chloride).
Low copper content could reflect that either the temperature of the fluids
never got high enough to leach copper from basaltic rock during fluid
circulation, or that copper-iron sulfides had been deposited in the subsur-
face directly beneath the vent sites. Again, as with the deposits at 21 °N, the
amount and distribution of material deposited on the southern Juan de Fuca
Ridge is not currently analogous to ore deposits.

The vent site most analogous to ore deposits exposed on land is the
active TAG (Trans-Atlantic Geotraverse) mound located at 26°N on the
Mid- Atlantic Ridge. The irony of this is that in the early 1980s research-
ers felt that the slow-spreading Mid-Atlantic Ridge could not sustain
high-temperature hydrothermal activity. Hydrothermal systems transfer
large amounts of heat from magma or newly solidified rock; on slow-
spreading ridges (spreading at a half-rate of 13 millimeters per year as
opposed to 30 millimeters per year at 21 °N), such heat was not thought
to be available. The TAG mound, however, is not only active, but larger
in diameter by an order of magnitude than mounds at sites in the faster-
spreading Pacific Ocean.

The active TAG mound is 200 to 250 meters in diameter and is
located at the east side of a wide axial valley that is coated with carbon-
ate sediment. The outer low-lying portion of the mound is composed of
carbonate ooze, metalliferous sediment, sulfide blocks, and basalt talus.
The inner portion of the mound is 150 to 170 meters in diameter, and is
covered entirely with hydrothermal precipitates. The edges of the inner
mound are steep talus slopes of sulfide and iron-oxide material that rise
20 meters above the outer mound. The center of the mound is dominated
by a cluster of black smoker chimneys venting fluid at temperatures up
to 363°C.The composition of this fluid is similar to fluids from the East
Pacific Rise vent sites. The chimneys are chalcopyrite and anhydrite rich,
and sit atop a 10- to 20-meter high, 40- to 50-meter-diameter cone of
sulfide and sulfate. The surface of the cone is platelike. It is composed of
chalcopyrite and pyrite with interspersed blocks of corroded massive
anhydrite. Black smoke seeps from small, fingerlike protrusions and
cracks in the cone surface and flows upward along the surface into the
plume of black smoke above.

Oceanus

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Winter 1991/92

How the
active TAG
mound grew

to such
large size is

a current
study topic.

At a lower elevation in the southeast quadrant of the mound, lower
temperature fluids (300°C and lower) exit "white smoker" chimneys.
These chimneys, which are mineralogically and structurally very similar
to spires from the southern Juan de Fuca Ridge site, are composed
dominantly of sphalerite (zinc sulfide) and exhibit numerous millimeter-
diameter flow conduits. The fluid venting from this portion of the
mound is not only cooler than fluid venting from the cluster of black
smokers, but is copper poor and has a low pH relative to the higher
temperature fluids. The lower temperature fluid may form as a result of
conductive cooling of the 363°C end-member solution. Diffuse, low-
temperature fluids emanate from the remainder of the top of the mound,
which is composed of fragile amorphous iron-oxide and silica crusts and
blocky to bulbous lobes of mixed zinc, iron, and copper-iron sulfides.
The overall size and concentric zonation of the mound, with highest
temperatures in the center, and lower temperature sulfides and iron-
oxides distributed near the outside, are similar to ore deposits exposed
on land. How the active TAG mound grew to such large size is a current
study topic, and whether or not deposits in the faster spreading Pacific
will ever attain this size is unknown. ""%

A New Set of Questions

Studies done in the last decade on seafloor hydrothermal systems have
led to an understanding of the basic processes involved in hydrothermal
circulation and mineral deposition — and have also shown how little we
know. The next decade of research will address such questions as,

• What is the extent of hydrothermal venting at mid-ocean spread-
ing centers and back-arc basins?

• What is the significance of variations in fluid composition, tem-
perature, flow rate, and composition of solid precipitates among
hydrothermal sites?

• How long are vent sites active?

• Does fluid composition change with time, and if so, on what time
scale?

• What proportion of minerals is deposited at the vent site versus
dispersed into the water column as black smoke?

All of these questions must be answered to estimate the contribution
from hydrothermal processes to global heat budgets and geochemical
cycles. While we now have a general understanding of hydrothermal
vent systems, we still have much to learn, and large sections of the mid-
ocean ridge system have yet to be explored.

Margaret K. Tivey is an Assistant Scientist in the Chemistry Department at
Woods Hole Oceanographic Institution. Her research focus is mineral precipita-
tion at seafloor hydrothermal vents.

Ocennus

The Biology of

Deep-Sea Vents

and Seeps

Alvin's Magical Mystery Tour

Richard A. Lutz

ver the past 12 years, the biology of deep-sea hydro thermal
vents and cold-water seeps has been discussed in Oceanus
several times. (For example, see Summer 1979, Fall 1984,
and Winter 1988/89.) Here I provide an overview of a
number of recent DSV Alvin expeditions to the East Pacific
and the Gulf of Mexico that have expanded our knowledge of biological
communities present at deep-sea vents and seeps in these
oceanic regions. From May 1990 through August 1991, we (Bob
Vrijenhoek and I, both of Rutgers University) organized several
biological expeditions, collectively known as the "Magical
Mystery Tour," as part of an ongoing National Science Founda-
tion-funded project to study the genetics and dispersal mecha-
nisms of organisms inhabiting vent environments. During this
"tour," Alvin visited 14 deep-sea hydrothermal vent fields and 4
cold-water-seep areas. In April 1991 an additional expedition,
known as the ADVENTURE (Alvin Diving in the VENTURE
hydrothermal fields) cruise, led by Rachel Haymon (University
of California at Santa Barbara) and Dan Fornari (Lamont-
Doherty Geological Observatory), visited an extensive series of
hydrothermal vents located between 9° and 10°N along the East
Pacific Rise (EPR). A number of observations, ranging from
faunal changes that had occurred at sites previously visited by
Alvin to the nature of communities encountered at new areas,
are summarized below.

Hydrothermal Vents

Galapagos Rift

Rose Garden. Scientists diving in Alvin visited the Rose Garden hydro-
thermal vent site (named for the abundance of red-plumed tube worms
found there) in 1979, 1985, and 1988. Bob Hessler (Scripps Institution of
Oceanography), who dove extensively at this site during each of these

HYDROTHERMAL VENT AND COLD
SEEP CRUISE SERIES • 199O/1991

An oceanographic

expedition without a

proper T-shirt is just

another cruise — this is

the logo for the Magical

Mystery Tour T-shirt.

Winter 1991192

Deep-sea hydrathermal

vents and cold seeps

visited by Alvin
between March 1990
and August 1991 on
the Magical Mystery
Tour are plotted here.

The Rose Garden vent

site along the

Galapagos Rift as it

appeared in 1979 (left)

and 1985 (right).

60C

30'

30°

West Florida
Escarpment

Galapagos Rift
(Rose Garden
& Mussel Bed)

Juan de Fuca Ridge

North Endeavor (vent)

Axial Volcano (vent)

South Cleft

Monterey Canyon (seep)
Guaymas Transform Ridge (seep)

Guaymas Basin (vent)

East Pacific Rise 21 °N
(Clam Acres, Hanging
Gardens, Nat'l Geographic
Site, DBS Site, Northern
Vent)

13°N

11°24.9'N
9- 10°N

180

150

120

30°

previous visits, returned to the site in May 1990. From Bob's perspective,
while significant faunal changes had occurred between 1979 and 1985
(notably a decrease in tube worm abundance, an increased dominance of
mussels and clams, a crash in anemone and serpulid populations, and
increased numbers of galatheid crabs and whelks), the community
structure had not changed significantly between 1985 and 1990.

Mussel Bed. During May 1990 the vent area known as Mussel Bed was
also revisited. In 1979 this area was inhabited by an extensive population
of large, living mussels (Bathymodiolus thermopliilus), numerous speci-
mens of the giant clam Cah/ptogena niagnifica (as well as significant
numbers of empty clam shells), brachyuran crabs (Bi/thograea
thermi/dron), galatheid crabs (Munidopsis sp.), whelks (Phymorhynchus
sp.), pink bythitid vent fish very close to vent openings, two or three
small tube worms (Riftia padn/ptila) in the narrow vent openings, and

Oceanus

many species of limpets. The 1990 visit found a vent environment that
had changed remarkably little since 1979; mussels remained the domi-
nant megafaunal constituent, live and dead clams were present in
significant numbers, bythitid vent fish were still present around vent
openings, and two or three small tube worms were seen in narrow vent
openings. In contrast to the marked changes that had occurred at Rose
Garden since 1979, time appeared to have stood remarkably still at the
Mussel Bed site over the same 12-year period.

9° to 10°N Along the East Pacific Rise

Rachel Haymon, Dan Fornari, and their co-workers have recently
described a series of established and newly formed hydrothermal vents
between 9°16' and 9°54'N along the East Pacific Rise. Sampled vent
organisms include:

• three species of tube worms (Riftia packyptila, Tevnia sp., Oasisia sp.),

• the clam Catyptogena magnified,

• the mussel Bathymodiolus thermophilus (with an associated com-

mensal polychaete living in the mantle cavity of over 75 percent of
the collected specimens),

• nine species of limpets
(Eulepetopsis vitren, Lepetodrilus
cristatus, L. elevatus, L. ovalis,
L. pustulosus, Neolepetopsis
densata, Peltospira delicata,

P. opcrcnlata, and Sutilizona theca),

• six coiled archaeogastropods
(Batln/margarites symplector,
Cyathermia naticoides,
Melanodn/mia n.sp., and three
unidentified species),

• the mesogastropod Provannasp.,

• one unidentified turrid gastropod,

• one or possibly two species of
galatheid crabs within the genus
Munidopsis,

• the brachyuran crabs Bytliograea thermydron and Cyanograea pracdator
(and possibly a third new undescribed brachyuran species),

• at least two species of barnacles (one stalked),

• several species of bacteria occurring in thick mats and thin
coatings on basalt and sulfide substrates,

• the polychaete Amphisamytha galapagensis,

• the Pompei worm, Ahrinella pompejana,

• one or possibly two species of tubiculous polychaetes within the
genus Paralvinella ,

• an unidentified serpulid polychaete,

• the commensal polychaete Branchipolynoe symmytiUda (which
inhabits the mantle cavity of the mussel Bathymodiolus
tliermophilus), and

• numerous other unidentified polychaetes, amphipods, brittle stars
(ophiuroids), sea stars (asteroids), leptostracans, anemones,
sponges, copepods, and benthic foraminifera.

Tube worms, mussels

and a zoarcid vent fish

at the 9° to 10°N

In/drothermal vent

fields along the East

Pacific Rise.

Winter 1991/92

The Genesis hydrother-

mal vent at 1 3° N along

the East Pacific Rise as

it appeared in

June 1990.

Zoarcid vent fish were commonly observed, although not sampled,
in several vent areas throughout this stretch of the EPR ridge axis.

11° 24.9'N Along the East Pacific Rise

Biologists dove to this site for the first time in June 1990 to find a vent
environment characterized by one active black smoker and a few areas
with low-temperature venting. Dominant members of the vent
megafauna included mussels (Bathymodiolus thermophilus), tube worms

(Riftia pachyptila), galatheid crabs
(Munidopsis sp.), and brachyuran
crabs (Bythograea thermydron).
Many empty shell valves of the
clam Cdlyptogena magnified were
present, but only one living
specimen was observed through
the submersible's viewport. Other
sampled characteristic vent
organisms include:

• four species of limpets within
the genus Lepetodrilus

(L. cristatus, L. elevatus, L. ovalis,
and L. pustulosus),

• the "transparent limpet"
Eulepetopsis vitrea,

• the slit limpet Ch/peosectus
delectus,

• two coiled archaeogastropods (Bathymargarites symplector and
Melanodrymia aurantiaca),

• the brachyuran crab Cyanograea praedator,

• the tube worm Tevnia sp.,

• two polychaetes within the genus Paralvinella (P. grasslei
and P. pandorae),

• the ampharetid polychaete Amphisamytha galapagensis,

• an unidentified serpulid polychaete,

• the commensal polynoid Branchipolynoe symmytilida, present in the
mantle cavity of over 75 percent of the mussels sampled,

• amphipods, leptostracans, and several unidentified species of
polychaetes, which were also abundant in sieve washings and
appeared to be associated with clumps of Riftia and Tevnia
tube worms,

• numerous anenomes and brittle stars, which were abundant
throughout the hydrothermally active areas, and

• one specimen of an unidentified turrid gastropod.

Numerous specimens of a stalked (goose-necked) barnacle, presently
unidentified, were attached to basaltic rocks throughout the vent field.
Colonial siphonophores ("dandelions") were observed, but not sampled,
in peripheral areas of the vent field, and zoarcid vent fish were relatively
common among tube worms attached to the side of the active black smoker.

Oceanus

13°N Along the East Pacific Rise

Three major expeditions in 1982, 1984, and 1987 explored a variety of
vent fields in the vicinity of 13°N along the EPR. During this five-year
period, marked changes in vent activity and associated faunal composi-
tion, ranging from total cessation of vent flow and mass mortality of
constituent vent organisms to the "rebirth" of an inactive field, have
been reported by Daniel Desbruyeres (IFREMER, Institute Franchise
pour Recherche et Exploitation de la Mar) and his co-workers. In 1990,
three vent fields in the 13°N area (Totem, Genesis, and Parigo) were
revisited and sampled. Noteworthy observations made during this
cruise include:

• a vigorous level of vent activity and lush biological community
present at the Genesis site, which was once known as
"Pogomort," a vent field that had previously shut down and was
named for the large number of associated dead tube worms (the
vent tube worms were originally considered members of the
phylum Pogonophora but were later placed in the recently erected
phylum Vestimentifera),

• an increased dominance in 1990 (in contrast to 1987) of the tube
worm Rift in relative to the tube worm Tevnin at the Genesis site;

• newly formed smokers heavily colonized by alvinellid
polychaetes in the Genesis hydrothermal field; and

• a few isolated living mussels with no associated vent megafauna at
the Parigo vent field, where no heat anomalies were encountered.

21 °N Along the East Pacific Rise

A number of hydrothermal vent
fields at 21 °N along the EPR were
visited during major geological
and biological expeditions in 1979
(RISE — Rivera Submersible
Experiments Expedition), 1981,
1982 (Oasis Expedition), and 1985.
In 1990, Alvin visited five separate
21 °N vent areas, four of which had
been previously visited.

Clam Acres. Nineteen dives were
devoted in 1982 to a variety of
biological studies, most at an
extensive vent field known as
Clam Acres. At the beginning of
this dive sequence, the area was dominated by large populations of
Catyptogena magnified and occasional isolated clumps of the tube worm
Riftia pachyptila. As a result of the extensive sampling required by the
multidisciplinary Oasis program, virtually every clump of tube worms
had been "harvested" by the final dive of the series. When this area was
revisited in June 1990, biologists were struck by the dramatic rejuvena-
tion of the tube worm population; considerably larger and more numer-

Clam Acres, at 21°N

along the East
Pacific Rise.

Winter 1991/92

These tube worms were

attached to a sulfide

edifice in Guaymas

basin.

Photo by Richard A. Lutz

ous Riftia clumps were present than had been encountered even during
the beginning of the Oasis Expedition, and many of the tube worms
within the clumps were more than a meter long. The clam population at
this site remained extensive, and associated organisms collected were
similar to those sampled in 1982.

Hanging Gardens. Visits in 1979, 1981, and 1985 revealed a lush biological
community and one active black smoker at this site. During the return
visit in 1990, no dramatic changes in community structure were appar-
ent. The black smoker was still active and the vent field was dominated
by two species of tube worms, Riftia pacln/ptila and Oasisia alvinae, and
numerous clams, crabs, and limpets, all of which had been encountered
during previous dives to the site.

National Geographic Smoker (NGS) Site. This vent area, named after a
photograph of the site that appeared in the November 1979 issue of
National Geographic magazine, appeared to have changed little over a 10-
year period. Notes in Alvin pilots' records from 1981 described dead
clam shells, inactive sulfide deposits, a tall, warm vent with white and
dark smoke, and a few living clams, tube worms, and crabs. Numerous
inactive sulfide deposits were found during the 1990 return visit along
with a small (less than 2-meter high), Alvinella-covered smoker with
temperatures exceeding 300°C. Other biological and geological observa-
tions were consistent with the conclusion that little had changed at this
vent site since 1979.

OBS (Ocean Bottom Seismometer) Site. In 1981 this site was characterized
by three tall chimneys, several dead clam shells, and a few large
galatheid crabs (Mnnidopsis sp.), but no other specific vent megafauna.
During both the Oasis Expedition in 1982 and the return visit in 1990, at
least one of the three chimneys was vigorously active and the only
indication of vent-associated organisms was again the presence of dead
clam shells and occasional large galatheid crabs.

Northern Vent. Approximately 2 kilometers northeast of Clam Acres, this
previously undescribed vent field was encountered by Rich Lutz and
Daniel Desbruyeres. While few characteristic vent organisms were
observed, tremendous numbers of an attached jellyfishlike organism

(within the coelenterate order Stauromedusae) were
concentrated around low-temperature vents and
were also present in reduced numbers on
adjacent basalt surfaces.

Guaymas Basin

Unlike each of the other vent sites, the
hydrothermal fields of Guaymas Basin
are characterized by several hundred
meters of soft sediment (with occasional
outcropping sulfide edifices) through which
vent fluids percolate. This region was exten-
sively studied using Alvin in 1982 (10 dives), 1985
(40 dives), and 1988 (24 dives). In June 1990 the Magi-
cal Mystery Tour returned to find the region had not under-

Oceanns

gone substantial changes over an eight-year period. Bacterial mats,
infaunal vesicomyid clams, and tube worms (Riftia padn/ntila) on sulfide
edifices remained the most conspicuous organisms associated with the
vent fields. Empty shells of dead clams were scattered in localized
regions throughout the areas of active (or previously active) hydrother-
mal venting, and black corals with associated terebellid polychaetes were
retrieved from box core samples.

Juan de Fuca Ridge

South Cleft Segment. Organisms previously associated with vent fields
along this ridge segment were described by Verena Tunnicliffe and A.R.
Fontaine (University of Victoria) from photographs and limited samples
taken during a 1984 Alvin cruise. During August 1991 two of the de-
scribed vent areas, Vent 1A and IB, were revisited, and associated vent
organisms were sampled or photographed. While many tubes of the tube
worm Ridgein sp. were seen at Vent 1A (as they had been during the 1984
cruise) none appeared to contain living organisms and there was no
evidence of active hydrothermal venting at the site. Similarly, there was
no evidence of living vesicomyid clams at this site, despite the presence
of many empty clam shells. Occasional spider crabs (Macroregonia
macrochirn) were the only living vent-associated organisms observed at
the site. Approximately 100 meters north of this inactive vent area, a
small amount of low-temperature venting was seen percolating through
sulfide deposits along the west wall of the axial summit graben. Collec-
tions at this site included:

• a few living tube worms (Ridgein sp.),

• two species of limpets, Lepetodrilus fucensis and Ctypeosectus citrvus,

• one coiled archaeogastropod species, Depressigi/ra globulns,

• one species of mesogastropod, Provanna variabilis,

• one mussel species, Idasola sp.,

• palm worms, Paralvinella palmifonnis, and

• several unidentified polychaetes, a pycnogonid, and one specimen
of a living vesicomyid clam.

Several crabs (Macroregonia sp.) were seen, though not sampled, and
relatively sparse bacterial mats coated the surrounding basalt and sulfide
rock surfaces. Vent IB, which was approximately 300 to 400 meters
north, was characterized by numerous, tall sulfide chimneys, several of
which were vigorously active. Temperatures as high as 334°C were
measured at one of the smoker orifices. Tube worms, other unidentified
polychaetes, and sponges were common on the sides of active smokers,
and numerous sponges were also seen around the base.

Axial Volcano. The Ashes Vent field within the caldera of Axial Volcano
(Axial Seamount) was visited in 1984, 1986, 1987, and 1988. Biological
community changes occurring between 1984 and 1988, particularly at an
active sulfide mound known as "Mushroom Vent," have been described
by Verena Tunnicliffe and are attributed largely to effects of sampling
efforts and submersible maneuvering. In August 1991, this vent field was
revisited; with the exception of an undescribed limpet species that
appeared restricted to previously discharged submersible dive weights,
all species sampled had been encountered during previous expeditions

Tube worms,

other

unidentified

polychaetes,

and sponges

were common

on the sides

of active

smokers.

Winter 1991 192

Tube worms (upper

left), mussels (center)

and poh/chaetes (lower

right) at the West

Florida Escarpment

cold seep.

to this hydrothermally active region. Observations from the 1991 dive
revealed a previously unreported substantial quantity of bacteria on
basaltic and sulfide surfaces that may have reflected a recent increase in
hydrothermal activity or a decrease in the rate of bacterial consumption
by a variety of benthic invertebrates in the area.

North Endeavor Segment. A smoker (nicknamed "Godzilla"), the size of a
16-story building (50 meters high), numerous smaller smokers (one
affectionately called "Bambi"), and isolated pockets of sediment in low-
lying areas along the ridge axis characterized the North Endeavor
Segment in August 1991. Sampling efforts on the sides and at the base of
Godzilla yielded:

• three species of limpets (Clypeosectus curvus, Lepetodrilus fucensis,
and Temnocindis euripes),

• one coiled archaeogastropod species,
Depressigi/ra globulus,

• the mesogastropod Provanna variabilis,

• two neogastropod species, Buccinna viridum
and an unidentified cancellarid,

• one aplacophoran, Helicoradomenia juani,

• tube worms, Ridgeia sp.,

• numerous polychaetes, including three
species of Paralvinella and the ampharetid
Ampliisann/tha galapagensis,

• soft corals,

• hexactinellid sponges,

• anemones,

• a pycnogonid, and

• crabs (Macrooregonia sp.) with caprellid

amphipods attached to their legs.

Many specimens of an unidentified vesicomyid clam were also
collected from low-lying, sedimented regions of the axial graben just
south of Godzilla.

Cold Seeps

West Florida Escarpment. Alvin visited this cold-water sulfide/methane
seep site during geological and biological expeditions in 1984 and 1986.
Barbara Hecker (Lamont-Doherty Geological Observatory), the sole
biologist to dive at the site in 1984, returned to the seep area in 1990 to
find little change in the biological community structure over the six-year
period. Sampled or observed organisms included two unidentified
mussel species (one of which was collected during both of the previous
expeditions; the other was represented in the extensive 1990 samples by
only a single individual), vesicomyid clams, the limpet Paralepetopsis
floridensis, an undescribed coiled trochid gastropod, a turrid gastropod,
numerous tube worms (Escarpia laminata), ophiuroids, and commensal
polychaetes found within the mantle cavities of sampled mussels.

Louisiana Slope. While the first Alvin dives to the hydrocarbon seeps of
the Louisiana Slope took place in April 1990, these methane-rich areas
had previously been studied extensively by Jim Brooks (Texas A&M

Oceanus

University) and co-workers using Johnson Sea-Link, Pisces II, and NR-1.
Sampling efforts during the Magical Mystery Tour portion of the 1990
expedition were restricted to collecting two species of vesicomyid clams
(Vesiconn/a cordata and Calyptogena ponderosa) and several new species of
mussels, which are being described and systematically classified as part
of ongoing genetic and taxonomic studies.

Guaymas Transform Ridge. Approximately 30 kilometers north of the
active hydrothermal fields visited in Guaymas Basin, a transform ridge
rises above the seafloor and crests at a depth of approximately 1,600
meters. In 1985, chemist John Edmond (Massachusetts Institute of
Technology) and geologist Peter Lonsdale (Scripps Institution of Ocean-
ography) explored the region and found buoyant hydrocarbon plumes
and associated assemblages of biological organisms. In March 1991 Luis
Soto (Universidad National Autonoma de Mexico) and I returned to the
area and sampled several seep-associated organisms from large, depressed
"pochmark" regions along the ridge crest. Retrieved specimens included:

• two species of vesicomyid clams,

• numerous specimens of a protobranch bivalve Nuculana sp.,

• two limpet species, Lepetodrilus guaymasensis and an unidentified
species,

• two species of mesogastropods, Provanna goniata and Provanna laevis,

• several specimens of a heterobranch gastropod "Melanella" lomana,

• two unidentified species of tube worms,

• galatheid crabs Munidopsis sp.,

• ophiuroids, and

• a variety of miscellaneous polychaetes.

Monterey Cam/on. Alvin first visited the Monterey Canyon cold-seep area
(located at a depth of approximately 3,400 meters) in October 1988, and
returned two years later in September 1990. During both expeditions, the
restricted areas of hydrocarbon seepage were characterized by dense
populations of large vesicomyid clams with shells more than 20 centime-
ters long. While few other organisms appeared to be attached to
or living among the clams, several empty shells of the
protobranch bivalve Soleyma sp. were present in adjacent sedi-
ments, as were numerous small pogonophorans (phylum
Pogonophora, former subphylum Perviata) that were likened by
observers within the submersible to "fields of grass." "^N

Acknowledgments: I ivish to express my sincere gratitude to the pilots and
entire crew of the Atlantis II/ Alvin whose untiring dedication and
competence made the Magical Mystery Tour a tremendous success. This is
publication number D-32402-6-91 of the New Jersey Agricidtural Experi-
ment Station and contribution number 91-52 of the Institute of Marine and
Coastal Sciences, Rutgers University, and is supported by state funds and
NSF grants OCE-8716591 and OCE-8943896.

Richard A. Lutz is a Professor in the Institute of Marine and Coastal
Sciences of Rutgers University. He has been involved in a variety of
ecological studies of deep-sea hydrothermal vent communities since
the initial discovery of the Galapagos Rift vent fields in 1977. Pres-
ently he is Project Coordinator of a large interdisciplinary study of
temporal changes in biological community structure at newly formed
hydrothermal vents at 9P to 10°N along the East Pacific Rise.

Note: The author has

prepared an informative

chart listing the various

vent and seep regions

and their known
resident fauna. If you
would like a copy, free

of charge, write to

Oceanus at the address

on page 4.

The author (right) and

Howard Sanders

(center) prepare to

enter Alvin during an

early dive to the

Mussel Bed vent along

the Galapagos Rift.

Winter 1991/92

The

megaplume
story has

all the

plot devices

of a good

detective

yarn.

Megaplumes

Edward T. Baker

he megaplume story, like other engaging scientific puzzles,
has all the plot devices of a good detective yarn: a continuing
investigation cracked by a provocative and unanticipated
event, a patient assembling of evidence from new clues, and
a logical trail that leads to the suspected but long-elusive
perpetrator. But since this is science, not Sam Spade, the puzzle solved
leads not to a case closed but to newer, more intriguing puzzles.

The question at the heart of the megaplume story is one central to
marine science: How does Earth's mantle evolve into rigid crust, and
how does this evolution affect the deep oceans' heat and chemical
budgets? The investigation began during the plate-tectonics revolution
of the 1960s, when oceanographers first recognized the Mid-Ocean Ridge
(MOR) as the birthplace of new ocean crust. Filtered by the broad time-
and-space scales of geologic history, this creation process appears
continuous, driven by the relentlessly separating plates of Earth's crust.
On a human scale, however, the actual production of new ocean crust
along plate boundaries is highly intermittent, and has been observed
only at those few places, such as Iceland, where the MOR emerges above
sea level. Less than 3 square kilometers of new crust is added yearly
along the 70,000-kilometer length of the MOR. This increase is equivalent
to only 5 percent of the MOR widening by just 1 meter every year.

Snippets of new crust are added to the axial crest of the MOR as the
broad upwelling of mantle-derived magma is focused into a narrow
ribbon of volcanic activity that is usually no wider than a few hundred
meters. As the new crust cools, it shrinks and cracks. Seawater percolates
downward through the cracks and porous new crust to where magmatic
heat can raise its temperature to over 400°C. The transformed seawater
gushes upward as geysers of "black smoker" hydrothermal fluids,
building chimneys of precipitated metal sulfides and supporting a
unique ecosystem of animals totally dependent on chemosynthetic
bacteria. Oceanographers now realize that hydrothermal venting,
unknown just 15 years ago, largely mediates the exchange of heat and
chemicals between the Earth's crust and the ocean.

Marine scientists knew that studying a very recently active piece of
the MOR would promote their investigation, but how could such a spot
be found along its largely unexplored length? Since hydrothermal
venting is powered by magmatic heat, some oceanographers reasoned
that mapping the active-vent-field distribution on selected portions of
the MOR might speed the search for sites of active spreading. Over the
last few years the Vents Program of the National Oceanic and Atmo-
spheric Administration (NOAA) has searched for vent fields along more

Oceanus

than 80 percent of the Juan de Fuca Ridge axial crest, a 500-kilometer-
long spreading center in the northeast Pacific that consists of six separate
tectonic segments. It is now one of the few lengthy portions of the MOR
where we know with confidence the distribution of vent fields.

We locate vent fields by slowly towing a conductivity/temperature/
depth (CTD) sensor in a sawtooth pattern through the deep waters above
the ridge crest. Hot hydrothermal fluids, diluted and cooled as they rise
and mix with the surrounding seawater, form tenuous clouds that hang
100 to 300 meters above the vent fields, like chimney smoke from a 19th-
century steel town. Investigators exploring small pieces of the East
Pacific Rise and the Mid-Atlantic Ridge in the early 1980s hypothesized
that vent fields should preferentially develop above the shallowest part
of each tectonic segment, because injections of hot, low density magma
would cause the crust to inflate. The Juan de Fuca Ridge results provide
the most comprehensive support yet for this prediction.

In the course of these hydrothermal explorations of the Juan de Fuca,
we serendipitiously discovered a plume so remarkable in its size, shape,
and distance above the seafloor that it could only have been the product
of fluid discharge far greater than any yet witnessed or anticipated. This
plume, quickly dubbed the "megaplume," was found during an explor-
atory CTD tow along the northern end of the Cleft segment in August
1986. Although baffling at first, its discovery was quickly recognized as
an opportunistic break in an investigation that was of increasing interest
to a variety of scientific detectives.

The first and most startling new clue in the case was the unprec-
edented rise height of the megaplume. The rise height of a buoyant
plume increases with the discharge rate of its source fluid, and all
previously observed hydrothermal plumes had been found no more than
a few hundred meters above their sources. The megaplume reached a
stunning 1,000 meters above the ridge axis depth of 2,300 meters.

Abandoning a meticulously planned cruise agenda, we devoted
several days to sampling this unexpected phenomenon. Subsequent data
processing and laboratory analyses — forensic oceanography — estab-

CLEFT

VANCE

1300

1800-

§" 2300-

ENDEAVOUR

Hydrothermal plumes
along three tectonic
segments of the junn
de Fucn Ridge in the

northeast Pacific

Ocean. Mixing of hot

hydrothermal fluids

with ambient seawater

produces tempera t lire

anomalies that identify

the plumes. Normal,

steadily discharging,

plumes rise about 300

meters above the
seafloor and are most

intense above seg-
ments (such as Cleft)
or parts of segments
(such as the midpoint
of Endeavor) that are

bathymetrically

elevated. Megaplumes,

shown in yellow, were

found above the Cleft

segment in 1986 and

above the Vance

segment in 1987.

2800-1 1 1 1 1 1 TJLT 1 1 1 1 1 r

44.5 44.7 44.9 45.1 45.0 45.2 45.4 45.6 47.6 47.8 48.0 48.2

A0 (°C)

ABOVE 0.050

0.040 - 0.050

0.030 - 0.040

0.020 - 0.030

0.010 - 0.020

BELOW 0.010

Latitude (°N)

lished three important facts. First, a hydrothermal origin for the plume
was confirmed. The plume waters were rich in several elements that are
also in hydrothermal fluids, including manganese, iron, silicon, and
helium-3, a rare isotope of the much more common helium-4. Second,
abundant anhydrite crystals in the plume indicated a very recent, and
thus local, origin. Anhydrite, which can crystallize only during the initial

Winter 1991/92

1200

1400

Proms

Megaplume 1
Temperature Anomaly

2400

8 10 12 14
Distance (km)

20 22

A mug shot of a
megaplume in cross-
section shows density
surfaces (dotted lines)

superimposed on

temperature anomaly

contours. The zig-zag

line is the path of the

CTD tow-i/o.

Temperature Anomaly °C
— -12-.20
.04-. 12

>.20

mixing of hot hydrothermal fluids
and ambient seawater (before the
temperature of this mixture falls
below 125°C) subsequently
dissolves within a matter of days
in cold seawater.

Third, and most exciting, the
plume was the residue of a brief
but very massive discharge event,
quite unlike the familiar steady
flow from small chimneys. The
megaplume's history was gleaned
from detailed mapping that
revealed an almost perfect three-
dimensional symmetry. In geo-
metrical jargon, the megaplume
formed an oblate spheroid — a
Frisbee — with a diameter of more
than 20 kilometers and a thickness
of 700 meters at its center. "Nor-
mal" plumes issuing steadily from
a collection of chimneys in a vent
field are imperfectly mixed and
habitually asymmetric, growing

erratically more dilute as deep currents sweep them away from their

sources, like wood smoke from a lazy campfire.

The megaplume's precise symmetry implied a bomblike event,

lasting only several hours to, at most, a few days.

Searching for a Motive: Two Possibilities

With these clues in hand, the researchers working on the case next
sought to construct a picture of the physical processes that created the
megaplume: the suspect's modus operandi. Because the megaplume had
known, symmetrical boundaries, we could confidently calculate its
burden of hydrothermal heat and chemicals. Even though its average
temperature increase above the surrounding seawater was only about
0.1 °C, its volume of over 130 cubic kilometers contained about 2 x 1016
calories of hydrothermal heat, or enough energy to electrify New York
City for almost a year. By knowing the hydrothermal heat content, it was
possible to calculate the original volume of hydrothermal fluids: 100
million cubic meters at a temperature of 350° C.

The megaplume event unleashed a staggering volume of hydrother-
mal fluid. By comparison, it would take 100 to 200 years for 100 million
cubic meters to escape from a single familiar "black smoker" chimney.
New ideas about fluid discharge from the seafloor were thus needed. A
geologically attractive alternative proposed that the megaplume fluids
erupted not from a forest of standard chimneys, nor from one yawning
megachimney, but rather from a long but narrow fissure cleaving the
vent field. Fissuring is common along the crest of the MOR, and pictures
returned by a deep-towed camera sled revealed a prominent band of
fissures cutting through the megaplume area.

Oceanus

45 N

50'

40'-

44 30'

/ i^//r \ l

Megapiume

130 30'

The fissures imaged by the camera sled were, of course, only the
exit ways of the kilometers-deep and tortuous paths through which
hydrothermal fluids must slowly percolate to reach the seafloor. The
normal concentration of pores and microcracks deep in the MOR's vol-
canic rocks is far too low to permit the fluid flow rate required by the
megaplume. Ideas about what triggered the megaplume, therefore, had
to have a common thread: the catastrophic rupturing of the crust beneath
an ordinary vent field.

To speculate how crustal permeability
can catastrophically increase, consider the
megaplume fluids from two points of view:
active and passive. An active model views
hydrothermal fluids, circulating deep in the
crust, as soup simmering in a pressure cook-
er afflicted with a stubborn safety valve.
Crustal fluids fracture the imprisoning vol-
canic rocks when fluid pressure increases
faster than normal venting can release it.
Pressure can be raised past the critical point
by a pulse of magmatic heat that raises the
temperature of the fluids, or by a sudden
release of magmatic gas.

The alternate view holds that a
megaplume is the passive result of an
impatient chef cracking open the pressure
cooker while the soup is still
boiling. An earthquake on the
MOR fractures the crust, tem-
porarily opening deep and
spacious fluid pathways, re-
leasing the trapped fluids as a
gushing megaplume. This
view evolves from a long his-
tory of observations at terres-
trial volcanic sites. Most inter-
esting are those from Iceland,
a segment of MOR obligingly
lifted above the concealing ocean. Records kept almost since the time of
the Vikings show that every 100 to 150 years a years-long episode of
crustal rifting and volcanic outpouring assaults Iceland. Careful mea-
surements collected during the 1975 to 1982 episode indicate that inter-
mittent crustal stretching was accompanied by subcrustal movements of
magma and subaerial eruptions of molten lava.

Expanding the Investigation

Several pieces of the puzzle were now in place, enough for tantalizing
speculations, but too few to convince a sober jury of scientific peers. To
find the crucial pieces needed for an airtight case, several investigators
decided on a long-term stakeout of the megaplume area. After several
years they hit pay dirt, in the form of two first-of-their-kind discoveries
on the MOR.

Vance / i y

Segmenl / ',

'•••! Cleft

Segment'

134C 130 126 122-

130 OO'W

DSVAIvm/Robert Embley. NOAA

A plan view of the

comparative extent of

the 1986 megaplume

mid the underlying

"normal" plume along

the axis of Cleft
Segment is shown
above. The size,
symmetry, and
hydrothermal tempera-
ture anomaly of the
megaplume are all in
sharp contrast to the
steadily emitted normal
plume. At left is the
sea floor fissure the
megaplume may have
erupted from. Taken

from DSV Alvin

(whose instruments are

in tlie foreground), the

fissure is several meters

wide and perhaps 10

meters deep.

Winter 1991/92

Within an hour of the

beginning of two

Kmfla eruptions, a

photographer in an

airplane captured the

image at right and the

one on page 2. The

eruptions began on

October 18 and
November 18, 1990,
and lasted five days

each. Both were

similar, occurring on

an 8-kilometer-long

fissure extending from

the center of the Krafla

caldera, northwrd

along the rift zone, and

were part of a series of

rifting and magmatic

events at the divergent

plate boundary in
nortlieast Iceland that

lasted from 1975 to

1984. Sudden decreases

in the elevation of the

Krafla caldera often

corresponded to

instances ofcrustal

widening and volcanic

eruptions (below).

Volcanologists believe

magma flows out of the

caldera of the Krafla

volcano, filling and

sometimes overflowing

the newly widened rift

zone as the crustal

plates separate. (After

Bjornsson, 1985, and

Tryggvason, 1984.)

The first solid evidence that the megaplume was a signal flare for
magmatic activity appeared in annual analyses of water samples from
the "normal" plume that always blankets the megaplume site. In August
of 1986, immediately after the megaplume release, the ratio of helium-3
to hydrothermal heat in the normal plume exceeded by several times any
value previously measured anywhere on the global MOR. The ratio then
decreased every year until 1988, when it reached a level typical of other
MOR vent sites, and of the megaplume itself. Such variability was no

more
antici-
pated
than the
megaplume
itself.

Upwelling
magma
from the
mantle is
the sole
source of
both
helium-3
and heat,
and work
elsewhere

on the MOR had found no evidence of a changing ratio. These new
observations had an exciting explanation: a sudden change in a pocket of
fluid magma had injected a surge of helium-3 into the hydrothermal
fluids feeding the vent field. Laboratory experiments have demonstrated
that magma can be rapidly stripped of helium-3 and other volatile gases
both by bubble formation as rising magma depressurizes and by crystal-
lization as magma cools.

The second discovery provided definitive evidence of the magmatic
activity needed to produce a burst of helium-3. Magma can suddenly
depressurize and cool when it intrudes into deep cracks opened in the
upper crust by the retreating plates. Occasionally these dikes of intrud-
ing magma overflow their cracks, spreading in shimmering lakes or
piling up in blocky mounds of fresh basalt on the seafloor. We know this
happens: The evidence is the MOR itself. But not until 1989 at the
megaplume site had oceanographers been able to identify and sample a
specific lava flow newly emplaced upon the seafloor. Careful bathymet-
ric mapping in 1987 and 1989, repeating survey lines originally run in

Cumulative Rift
Widening (m)

Ocean us

1981 and 1983, revealed a series of new lava mounds stretching for 16
kilometers along the trail of fissures that runs through the center of the
megaplume area. The volume of the mounds is roughly 0.5 cubic kilome-
ters, somewhat less than half the volume extruded during the Icelandic
eruption episode of 1975 to 1982.

Revealing the Crime — and the Collaborators

The discovery that the cataclysmic megaplume event of 1986 was closely
associated with both an escape of magmatic gas and an eruption of fresh
lava supplied enough pieces of evidence to make the case decipherable.
A closing argument might sound like this: The separating tectonic plates
on either side of the Juan de Fuca Ridge had been raising the tension
along the axis of the Cleft segment for some time, perhaps decades. In
mid-August of 1986, its crust failed and the width of the axial crest
increased by a few meters. A huge mass of hydrothermal broth impris-
oned in pores and crevices was released almost instantly. A several-
kilometers-long line of hydrothermal heat and chemicals surged 1,000
meters above the seafloor, turbulently mixing seawater over a 300-
square-kilometer area. The megaplume discharge ended abruptly as cold
seawater filled the emptying fissures.

On the heels of this release, a slab of molten magma pushed upward
through the cracked crust, lumping up mounds of fresh lava wherever it
breached the seafloor. Helium-3 and other volatile gases dissolved in the
magma were liberated as the magma rose and cooled. Newly forming
hydrothermal fluids absorbed these gases, and soon afterwards the flow
from seafloor chimneys contained extraordinary concentrations of
helium-3. The magmatic dike reaching up from the base of the crust was
only a few meters wide and stretched perhaps 20 kilometers along the
fissure line running through the north end of the Cleft segment. By 1988
it had completely solidified, and extraction rates of helium-3 and heat
were once again similar. New communities of vent creatures colonized
the lava mounds, attracted by warm, chemical-rich water leaking up
through the lava from the buried fissure system. A new sliver of ocean
crust had been added to the seafloor.

This hypothesis of the events surrounding the megaplume release
satisfied our puzzlement, while stimulating our curiosity with new
questions. How common are megaplume events, and how important are
they in the global budget of hydrothermal venting? Does volcano-
tectonic activity on segments of the MOR follow the Icelandic pattern of
several years of concentrated vigor separated by long quiescent periods?
And, most basically, how can we test our hypothesis?

The importance of episodic events relative to the more familiar
steady venting can only be surmised until we know more about the
frequency of megaplumes. Megaplume observations are still under-
standably scarce. A second megaplume, somewhat smaller than the first,
was found over the Juan de Fuca Ridge in 1987. The precise origin of that
plume is unknown; when found, the plume was several weeks to months
old and may have drifted far from its source. Researchers have also
claimed evidence of a megaplume in the Fiji Basin just southwest of
Samoa. More extensive evidence may actually exist in the geologic
record, where the prevalence of hydrothermal breccias, deposits of

A huge

mass of

hydrothermal

broth
imprisoned

in pores
and crevices
was released

almost
instantly.

Winter 1991/92

Cartoon of the hydro-
thermal and geological
observation program of
the Vents Program at
the megaplume site.
The geometry of the
magma chamber and
magma dike are
conjectural.

fractured and fragmented debris, suggests that cataclysmic releases of
hydrothermal fluids have not been uncommon along the MOR.

A few simple calculations can yield a rough estimate of the relative
importance of large hydrothermal events. In terms of the power released
by hydrothermal discharge on various scales, the megaplume is an
awesome force. During its short lifetime, it releases heat at a rate equal to
perhaps 10 percent of steady venting along the entire global MOR axis.
In terms of global budgets, however, we should more appropriately
compare the supply of hydrothermal emissions over a longer time
period. On an annual basis, the contribution of hydrothermal elements
such as iron and silicon from a single megaplume is about equal to that
of an entire vent field, or about 0.1 percent to 0.01 percent of the global
total. As a first estimate, then, the global supply of hydrothermal fluid
from megaplumes would equal that from steady venting if every vent
field produced a megaplume each year. What little experience we have
suggests that this schedule is too ambitious, so megaplume events are
unlikely to dominate the global budget. They may, however, be a
principal contributor to the hydrothermal supply from MOR segments
undergoing spreading episodes.

Megaplume

Plume Monitoring

Steady Venting

Acoustic
Extensometer

Volcanic System
Monitors

Magma Dike

Magma Chamber

Oceanus

Vents to Remain Vigilant

The progression from simple detection of megaplumes to a test of the
complex hypothesis in which we have embedded them requires the
establishment of a long-term observational program. The NOAA Vents
Program is currently developing such a program at the site of the
original megaplume. The seafloor observational system will initially
include three components. Sensors on moorings along the ridge axis will
monitor temperature and current velocity above the vent field, looking
for perturbations that signal a sudden change in the distribution or
intensity of steady venting, or the occurrence of hydrothermal events
such as megaplumes. Volcanic system monitors resting on the seafloor
will contain seismic recorders tuned to detect tremors indicative of
magma rumbling through the crust, and sensitive inclinometers and
pressure gauges to measure seafloor deformation caused by swelling or
contraction of the magma chamber. Pilot deployments of these first two
components began in 1991. The third component, an acoustic extensom-
eter network, will be added in 1992. Acoustical beacons on either side of
the rift zone will search for evidence of crustal spreading events by
continuously monitoring the distance between themselves, much as a
land-based laser-ranging system currently monitors movement along the
San Andreas Fault system in southern California. In addition to these
seafloor monitors, the Vents Program will tap into an existing array of
military hydrophones throughout the Pacific to listen for the telltale
underwater sounds, called T-phases, made by the cracking of earth-
quakes and the rumbling of magma.

In the world of pulp detective novels, the criminal always returns to
the scene of the crime. Nature, however, may not so obligingly furnish
another megaplume to the scientific Sam Spades now on a stakeout at
the Cleft segment. But even without one, a long-term study of its birth-
place will increase our understanding of the interrelationships of crustal
rifting, magma movement, and hydrothermal activity, providing new
puzzles and clues for the next detectives.

The NOAA
Vents Program

is currently
developing a

long-term
observational

program at

the site
of the original

megaplume.

Edward T. Baker is a Research Oceanographer at the National Oceanic and
Atmospheric Administration's Pacific Marine Environmental Laboratory, Seattle,
and an Affiliate Associate Professor in the School of Oceanography, University of
Washington. He presently serves on the RIDGE Program Steering Committee
and has been mapping hydrothermal plumes since 1984.

Winter 1991 /92

Seismic
tomography
will help us
understand

plate-
boundary
mechanics.

Tomographic

Imaging of

Spreading Centers

Douglas R. Toomey

ince we cannot observe the vast regions beneath the seafloor
directly, we must use remote-sensing methods such as sound
waves to help us answer fundamental questions about the
structures that make up mid-ocean ridges. We want to know
the size, shape, and location of spreading-center magma-
storage zones and the physical properties of the solid and molten rocks
beneath the axis of accretion. We need to know the strength of ridge-
building materials and the spatial and temporal relationships of various
ridge components. Seismic tomography, a powerful method for mapping
three dimensional physical properties within Earth's interior, promises
to help us gain this knowledge, which in turn will help us understand
plate-boundary mechanics. By analyzing the propagation of vibrational
waves generated by earthquakes or man-made sources such as explo-
sions, we can map seismic structure, that is, the travel speeds of different
types of vibrational or seismic waves and their attenuation with distance
due to frictional energy loss. One of the most frequently studied seismic
waves, the P-wave, is similar to the acoustic waves we hear. A shock
wave generated by a large explosion is a graphic analogue of a P-wave.
By measuring the transit time of P-waves passing around and through
an active spreading center, we can develop a three-dimensional image of
the seismic structure beneath the plate boundary.

Seismic tomography can be used to image three-dimensional physi-
cal properties over a wide spectrum of length scales, including, for
example, images at a scale of hundreds of meters of the upper-oceanic
crust near regions of hydrothermal venting, reconstruction of kilometer-
sized velocity anomalies characterizing axial magma chambers, and
maps of physical properties within the zone of mantle upwelling and
melt generation, located deep (10 to 100 kilometers) beneath seafloor-
spreading centers. Several fundamental improvements in the descriptive
and theoretical models of oceanic ridges await detailed, three-dimen-
sional mapping of velocity structure.

Oceanus

The Importance of Seismic Imaging

Within a spreading center, the geology and morphology of the sea floor
and the thermal and mechanical structure of the newly formed oceanic
plate are controlled by the complex interplay of magmatic injection,
tectonic rifting, and hydrothermal cooling. These dynamic processes all
exhibit pronounced spatial and temporal dependencies. Moreover, while
an individual process may express itself on the seafloor as a volcano, an
uplifted mountain range, or a hydrothermal vent field, the majority of
the dynamic activity invariably occurs at some depth beneath the
seafloor. Understanding the nature of oceanic spreading centers requires
knowledge of the behavior of these dynamic processes as they evolve
directly beneath the axis of accretion.

In recent years, working models of oceanic spreading centers have
evolved from two-dimensional, steady-state idealizations to more
realistic three-dimensional, time-dependent systems (see the segmented
Mid- Atlantic Ridge, page 11). The new dimension added to the working
models is the pervasive along-strike variability of mid-ocean ridge
processes, notably in the production of melt beneath the spreading
center. Current hypotheses suggest that ascending melt within the
mantle is focused into magmatic centers separated on the order of tens to
a hundred kilometers. Each magmatic center supplies the greater portion
of melt and heat to a single ridge segment. Within an individual ridge
segment, processes such as faulting, hydrothermal circulation, and
magmatic accretion vary systematically as a function of distance from the
magmatic center. The hypothetical structural unit, consisting of a local
maximum of magmatism bounded by along-axis minima, became
known as a spreading-center segment or cell. This simple model of
cellular segmentation provides an improved, but controversial, working
hypothesis for mid-ocean ridge studies.

What follows is a review of results of seismic tomography studies
used to investigate the spatial variability of physical properties and
processes deep within spreading centers. Divergent plate boundaries
display several different structural forms, including the classic rift valley
of the Mid-Atlantic Ridge, the pronounced en echelon or steplike structure
of the Reykjanes Peninsula within southwest Iceland, or the more
morphologically subdued East Pacific Rise. Each of these different
spreading centers is the topic of a tomographic study. These investiga-
tions are unified by a common purpose: furthering our knowledge of
physical structure beneath ridge axes, and using these observational
constraints to improve working hypotheses of the mechanics of diver-
gent plate boundaries.

Tomographic Images of Rifts and Rises

Mid-Atlantic Ridge

The cellular model of focused magmatic accretion is particularly apt for
characterizing the rugged, slow-spreading Mid-Atlantic Ridge. Undula-
tions in the ridge-parallel profile of axial seafloor depth are thought to
result from variations in melt production. Magmatic centers presumably
coincide with the shallower portions of the ridge axis, while the far ends
of magmatic cells (regions of low melt production) are thought to

The majority
of the dynamic

activity

invariably

occurs at some

depth beneath

the seafloor.

Winter 1991 192

Tliis cross-section of the

Mid-Atlantic Ridge

median valley along-axis

deep near 23°N, shows

microearthquake
hypocenters (circles) and

contours of P-wave

velocity (in kilometers

per second) obtained

from two-dimensional

tomographic imaging.

The seafloor bathymetry

reveals the rift's

relatively flat inner floor

and rugged mountains

to the east. A conjectural

fault plane for recent

large earthquakes is

shown (see Mid-Ocean

Ridge Seismicity, page

60). At the far end of

th is 40-kilometer-long

ridge segment, tomogra-

phy data shows nearly

normal oceanic crustal

structure.

correlate with the deeper parts of the ridge profile. Two recent studies
conducted on the Mid-Atlantic Ridge by researchers from the Massachu-
setts Institute of Technology (MIT) and the Woods Hole Oceanographic
Institution (WHOI), one within an axial low near 23°N and the other
astride an axial high near 26°N, demonstrate the utility of tomographic
imaging for characterizing crustal seismic structure throughout spread-
ing-center cells.

The first application of tomographic methods to the study of mid-
ocean ridge crustal structure
occurred during an investigation
of the seismicity and seismic
structure of the Mid-Atlantic
Ridge near 23°N, south of the
Kane Fracture zone. The micro-
earthquake study was located in
an along-axis deep at the far end
of a ridge segment approximately
40 kilometers long. During a two-
week deployment within the rift
valley, hundreds of microearth-
quakes were recorded by ocean-
bottom receivers. In addition to
locating the earthquakes, a
tomographic analysis of travel-
time data was conducted; the P-wave data comprised transit times from
earthquakes and several man-made explosions (seismic refraction data)
to the ocean-bottom receivers. By analyzing variations in the transit
times among many different paths, images of anomalous volumes of
seismic velocity were obtained. The two-dimensional seismic structure
across the rift-valley inner floor and transecting the axial deep was
similar to normal off-axis oceanic crustal structure, excepting a small
decrease in mid-crustal (1 to 4 kilometers beneath the seafloor) velocities
at zero-age crust. These low velocities quickly evolved with age (or with
off-axis distance) within the first few hundred thousand years of crustal
formation. The similarity in structure between axial crust within an
along-axis deep and normal off-axis oceanic crust that had undergone
extensive cooling as a result of aging was remarkable. From these and
other observations we hypothesized that at this far end of a ridge seg-
ment, considerable time (about 10,000 years) had elapsed since an
episode of significant magmatic accretion. Without the addition of new
crustal material that results from magmatic injection, we also inferred that
this rift-valley deep had undergone horizontal extension; in effect, the
spreading of oceanic plates at the end of a ridge segment was accommo-
dated by stretching and thinning of the axial crust.

A second study near the Transatlantic Geophysical Profile (TAG)
hydrothermal field at 26°N was located close to an along-axis high near a
ridge-segment center also approximately 40 kilometers in length. In
contrast to the magmatically quiescent, cool crust beneath the along-axis
deep at 23°N, the axial high of the ridge segment at 26°N is characterized
by high-temperature, black smoker chimneys. At 26°N the two-dimen-
sional seismic structure along the rift valley, including the region of the
axial high, was remarkably heterogeneous in comparison with the

Oceanus

structure near 23°N. Anomalously high velocities in the upper crust were
detected near the axial high, and a low velocity anomaly was detected
beneath an axial volcano. Both of these crustal velocity anomalies were
interpreted to be the result of recent magmatic intrusion. In contrast,
toward the axial deep at the southern end of the TAG ridge segment, the
seismic structure is comparable to normal oceanic crustal structure,
similar to the axial deep near 23°N. The axis-parallel velocity patterns near
26°N, including complex structures near the ridge segment center and a
transition to more homogeneous, almost normal crustal structure near the
segment's far end, appear consistent with the hypothesis that crustal
accretion is focused centrally beneath a slow-spreading ridge segment.

We don't yet know whether or not the observed variations in axial
seismic structure are fundamentally related to axial segmentation and
focusing of magmatic accretion. Current models of mid-ocean ridge
processes suggest that slow-spreading Mid-Atlantic Ridge segments of
equal length, such as the 23°N and 26°N ridge segments, are
magmatically fed by either upwelling plumes of similar size or volu-
metrically similar amounts of melt. If the model predictions hold true,
the tomographic images resulting from these two microearthquake
experiments may begin to characterize the seismic structure near the
axial high and axial low of a 40-kilometer-long ridge segment.

Reykjanes Peninsula, Iceland

A tomographic study of the Hengill-Grensdalur volcanic field in south-
western Iceland provides further indication of the power of seismic
methods for imaging the interior of active spreading centers. Working
with Gillian Foulger of the UK, we resolved the anomalous three-
dimensional crustal structure underlying the magmatic center of a slow-
spreading Icelandic rift segment. Geologic maps of the area show that
the magmatic center or central volcanic region incorporates the recently
active Hengill Volcano, the inactive Grensdalur Volcano, and the high-
temperature geothermal field associated with these features. To either
side of the Hengill central volcano, and extending to the far ends of the
rift segment, are a set of fissure swarms indicating the locus of past
eruptions. Our scientific objective was to tomographically image the
seismic structure of the center of this magmatic cell. These results also
aided the Icelandic Energy Authority in their search for volumes of
anomalously hot rock beneath the spreading center and to evaluate this
volcano's geothermal energy potential.

A significant advantage of land-based surveys is the ease of record-
ing seismic data for a longer period of time than is typically possible for
marine seismic experiments. A longer recording period provides a larger
data set, and thus more extensive sampling of the study volume; as
expected, higher resolution tomographic images are obtained when
larger quantities of data are available. Using P-wave travel times re-
corded during a four-month period by over 20 seismometers, we
tomographically imaged seismic velocities within a 14-by-15-by-6-cubic-
kilometer volume that underlies the high-temperature Hengill-
Grensdalur geothermal field. A dense distribution of sources and
receivers permits structural resolution to within approximately 1 and 2
kilometers in the vertical and horizontal directions, respectively. The

A dense

distribution

of sources

and receivers

permits

structural

resolution to

within
approximately

1 and 2
kilometers.

Winter 1991/92

Three-dimensional
tomographic image of

P-wave velocity
beneath the Hengill-
Grensdalur volcanic
complex, Iceland. The

color scale denotes
percentage difference in

velocity from the

regional structure. For

display purposes, the

model is represented by

constant-velocity cubic

blocks of dimension
0.25 km. Two views of
the tomographic image
are shown; both views
are from the northeast.
Positions of the surface

expressions of the
Grensdalur and Hen-
gill volcanoes (red
circles) and the axis of
crustal accretion (solid
bar) are shown .

final model of the area's structure is charac-
terized by distinct bodies of anomalously
high velocities: Two of these bodies are
continuous from the surface to about 3
kilometers depth, and each is associated
with a site of past volcanic eruption; the
third body of high velocity lies beneath the
center of the active geothermal field at a 3-
to 4-kilometer depth.

The volcanic features we directly ob-
serve on the surface are clearly the expres-
sion of igneous processes occurring at great
depths. They include the crustal-level stor-
age of molten magma and the cooling of
such bodies to form magmatic intrusions or
plutons. For crustal-level rocks, the P-wave
velocity varies little at temperatures below
500°C and decreases rapidly at tempera-
tures in excess of 500° to 800°C (basalt be-
gins to melt at about 800°C). We thus infer
that neither molten magma nor rock hotter
than about 500°C exist presently in large
volumes beneath the Hengill-Grensdalur
volcanic and geothermal field. The presence
of hot springs and fumaroles at the surface
with water temperatures between 300°and
370°C, however, indicates the presence of

intrusive rock at similar temperatures. We interpret the tomographic images
of anomalously high velocity to be the result of recently solidified magmatic
intrusions into the upper crust. Furthermore, those intrusions beneath active
geothermal fields, while solid, are most likely hot enough (about 400°C) to
provide a usable source of thermal energy.

The tomographic image of the Hengill-Grensdalur volcanic field may
provide an analog to the type of three-dimensional seismic structures
possibly present beneath the TAG area of the Mid-Atlantic Ridge. Both sites
are coincident with the center of a ridge segment, and both are characterized
by profound structural heterogeneity suggestive of recent crustal-level
intrusion of magma.

East Pacific Rise

Long-standing fundamental questions surround models of the size,
shape, and physical properties of mid-ocean-ridge axial magma cham-
bers. Mid-ocean ridge magmatism is a significant function of spreading
rate, and at faster spreading rises, such as the East Pacific Rise, the
volume of melt and the amount of heat delivered to the crust greatly
exceeds that of the slower-spreading Mid-Atlantic Ridge. Consequently,
thermal models for mid-ocean ridges predict that shallow crustal tem-
peratures are generally higher, and axial magma chambers are generally
larger and more long-lived along faster spreading rises. To test these
models, a seismic tomography experiment was recently conducted on
the East Pacific Rise (EPR) near 9°30'N by MIT and WHOI. It employed

Oceanus

A mn}] and his

charge: Beecher

Wooding of WHO/

prepares to launch tin

explosive charge

(in the cardboard box)

during the 1988

East Pacific Rise

seismic tomography

experiment aboard

R/V Washington.

2 km South

-8

-6

9°28W
deval

-4

-202
West-East, km

1 km West

15 ocean-bottom receivers and over 450 shots to image for the first time
the three-dimensional seismic structure of an axial magma chamber. The
15 receivers included ocean-bottom
hydrophones and seismometers,
designed and built by engineers and
technicians at WHOI and MIT.
Unlike the so-called passive tomog-
raphy studies of the Mid- Atlantic
and Icelandic rifts that used P-waves
generated by local earthquakes, the
EPR experiment was an active
seismic-imaging experiment that
used P-wave energy generated by
explosives. We deployed the
individual explosive shots in a dense
grid to ensure good sampling of the
crustal volume beneath an 18-by-16-
square kilometer area centered on
the EPR axis. Over 7,000 seismo-
grams were recorded, each provid-
ing some measure of the crustal
seismic structure along a different
path connecting a source to a
receiver.

A vertical section (at right) of
the EPR tomographic reconstruction
shows the anomalous P-wave
structure across the rise axis and
cutting through the axial magmatic
system, which appears primarily as
the anomalously low seismic
velocities (orange and red areas)
about 2 to 4 kilometers beneath the
seafloor. From laboratory studies of

deval

-8

-202
South— North, km

Tliese vertical cross sections through the P-wave velocity structure
were obtained by tomographic imaging of the East Pacific Rise. The
top and bottom sections are transverse and parallel to the rise
summit, respectively. The colors show departures of the three-
dimensional model from an average one-dimensional, depth-
dependent velocity structure: blues are faster than average and
greens to reds are slower tlian average. Tlie contour interval is 0.2
kilometers per second. Two deviations in the along-axis trend of the
rise summit (devals) are shown on the rise parallel section. Both
images pass through the axial mag)natic system.

Winter 1991/92

Juxtaposing the EPR
seismic tomography
results and the seafloor
bathymetry permits a
perspective view. Two
map-view sections
through the three-
dimensional model are
shown, one near the
seafloor and the deeper

one at a depth of 2
kilometers beneath the
rise summit; the deeper
section passes through

the lowest seismic
velocities of the axial
magrnatic system. The
colors show variations
in seismic velocity
structure; blues are
faster than average,
greens to reds are
slower than average.
The three-dimensional
mesh depicts undula-
tions of seafloor
bathymetry; the axial
summit is depicted by

shallowing of the
seafloor. The location of
two devals are noted, as
is the vertical projec-
tion of these seafloor
features down to the
depth of the axial low-
velocity volume.

P-wave velocity with increasing temperature we infer that the subaxial
crustal region comprising low seismic velocities is extremely hot, with
temperatures well over 500°C We think the concentration of lower seismic
velocities near a 2-kilometer depth results from the accumulation and
storage of molten magma within a thin melt-filled sill; this magma lens is
frequently observed by other types of seismic experiments and its maxi-
mum cross-axis width and thickness are inferred to be 1 to 2 kilometers and
less than a few hundred meters, respectively. The estimated volume of melt

9°35'N
deval

9°28'N
de

stored within this sill is comparable to that of a typical seafloor lava flow,
suggesting that a volcanic eruption along a fast-spreading ridge draws melt
from this region. The tomographic images also show a large region of low
seismic velocities that presumably envelope the much smaller magma lens
near a 2-kilometer depth. In general, the seismic velocities throughout the
larger volume encompassing the melt lens are consistent with elevated
temperatures, but not necessarily with molten rock. The size and shape of
the seismic anomalies across the rise axis strongly constrain the size and
shape of the axial magrnatic system.

A vertical section parallel to the EPR axis shows a variation in
seismic velocity suggestive of an along-axis segmentation of the crustal
magma chamber and the axial thermal structure. Again, the elevated
temperatures associated with crustal-level magmatism are effectively
mapped as the regions of lower seismic velocity. Along this section of the
EPR, the observed seismic structure is noticeably segmented on a scale of
about 10 kilometers, with the lowest velocities observed immediately
south of the experiment center; from this we infer that thermal structure
is segmented in a similar manner.

The along-axis segmentation of the axial magrnatic system gives rise
to an observed segmentation of seafloor morphology. During the experi-

Oceanus

ment, we mapped seafloor bathymetry over a 3,600-square-kilometer
area. Inspecting these maps we found that along axis the trend of the
EPR axial summit was variable; within the aperture of our seismic
experiment, the rise summit was easily divided into three adjacent linear
segments. Our seismic tomography images included one complete 12-
kilometer-long linear segment and parts of the bordering rise sections. At
either end of this linear segment, the axial summit deviates from linear-
ity, a seafloor morphologic feature referred to as a deval. The along-axis
tomographic section shows that the axial devals coincide with a relative
increase in along-axis seismic velocities near a depth of 2 kilometers. The
interpretation is that at mid-crustal depths the temperature is highest in the
center of the morphologically defined linear-rise segment, and lowest at the
segment ends. The correlation of seafloor bathymetry with subseafloor
thermal structure shows that magmatic processes occurring at great depths
strongly affect surface geology.

A perspective plot (opposite page) shows a different view of the
seismic tomography results including anomalous seismic velocities near
the seafloor and 2 kilometers beneath the seafloor; variations in seismic
velocity are indicated with color. As in the other figures, anomalously
low and high seismic velocities are indicated by warmer and cooler
colors, respectively. Seafloor bathymetry undulations are represented by
a three-dimensional mesh, clearly showing the shallowing of the seafloor
that demarcates the axis of seafloor spreading. Seismic velocities near the
axial summit seafloor are notably high (shown as blue colors). Two
bathymetrically defined devals are shown as vertical lines penetrating
the seafloor and continuing downward to the horizontal section at a 2-
kilometer depth. The deeper section lies near the depth of the melt-filled
sill and through the core of the axial magmatic system. Our interpreta-
tion is that melt generated in the mantle tens of kilometers beneath the
seafloor is injected into the shallow crust at intervals of about 10 kilome-
ters along the rise axis, giving rise to magmatically defined rise segments
of similar length. The segmentation of crustal-level axial magmatism and
its relationship to segmentation of seafloor morphology is an important
new observation made possible by seismic imaging.

Tomographic studies of seismic velocity structure beneath local
segments of the East Pacific Rise, the Mid-Atlantic Ridge, and the
Icelandic rift represent a new and powerful approach to the seismologi-
cal study of divergent plate boundaries. Future seismic tomography
experiments will continue to provide images of the physical properties
deep within spreading centers, and the study of these images, in con-
junction with other geological and geophysical data, will greatly im-
prove models of the tectonic, magmatic, and hydrothermal processes
responsible for the formation of oceanic regions. "*\

Acknowledgements: Much of the research reported here was done in collaboration
with G. Michael Purdy (Woods Hole Oceanographic Institution) and Sean C.
Solomon (Massachusetts Institute of Technology). The results from the TAG area
of the Mid-Atlantic Ridge are from the Ph.D. thesis of Laura Kong (MIT/WHOI
Joint Program in Oceanography /Oceanographic Engineering).

Douglas Ft. Toomey is an Assistant Professor in the Department of Geological
Sciences at the University of Oregon, and a graduate of the MIT/WHOI Joint
Program in Oceanography/Oceanopgraphic Engineering.

Correlation

of seafloor

bathymetry with

subseafloor

thermal

structure shows
that magmatic

processes

occurring at

great depths

strongly affect

surface geology.

Winter 1991/92

Bruce Heezen, aboard
Vema, /// the 1970s.

Bruce C. Heezen

A Profile

Paul J. Fox

nice Heezen died prematurely at the age of 54 in June of 1977,
as he was preparing to dive aboard the Navy research subma-
rine NR-1 . His intended destination was the Mid-Atlantic-
Ridge axis, a limb of the world-encircling ridge system where
oceanic crust is created. Bruce had been fascinated with the
Mid -Atlantic Ridge since he first studied and explored it 30 years before,
as an undergraduate research assistant for Maurice Ewing at Woods
Hole Oceanographic Institution (WHOI). Bruce's passing was untimely,
and marine geology and geophysics lost one of the great visionary minds
of the science, but the way he died was in a sense heroic: He was at sea,
poised to enter the abyss in his never-ceasing quest to better understand
how Earth works. If asked, I cannot imagine that he would have scripted
his death any differently.

It is a daunting task to adequately profile the depth of character of a
man who contributed more than 300 publications and two books to
marine geological literature, who was the mentor and colleague of 13

100

Oceanus

Ph.D. students, who spent more than eight years at sea pursuing knowl-
edge about the seafloor, and who, through these achievements, changed
the way we think about the processes that create and modify the sea-
floor. I will try, however, to focus on a few highlights of his early years,
his science, the particular gifts that allowed him to see further than most,
and the generosity and wisdom that made him such a memorable teacher.

As the son of a successful turkey farmer in Iowa, Bruce Heezen spent
a great deal of his childhood outdoors,
rambling about the countryside attending
to agrarian chores and developing a keen
interest in the natural sciences, a focus en-
couraged by one of his grandfathers. He
entered the University of Iowa as World
War II drew to a close, with a desire to
study science and an intention to never
have anything to do with turkeys,
whether it be their care or their consump-
tion. I cannot help but believe that Bruce's
intense dislike for these creatures contrib-
uted to his desire to distance himself from
the Iowa turkey pens and seek the sea's
far shores and mysteries.

His path to the sea, however, was
indirect, and conditioned by serendipi-
tous twists. His initial focus while an
undergraduate geology major was
paleontology, the study of fossil plants
and animals. An outstanding under-
graduate, he was selected to spend a
summer in the western US helping a
graduate student collect specimens for his
Ph.D. thesis. Bruce later said that at this
stage he had no doubt he would go on to
graduate school seeking a Ph.D. in
paleontology and spending his summers
out west in search of fossils.

By the 1940s the study of the fossil record was a mature science with
a 100-year record of scholarship; advances came slowly and only after a
great deal of careful work. This characteristic was clear to Bruce during
the winter term of 1947, as he labored on a study of toothlike fossil
elements called conodonts, sampled from 300-million-year-old rocks
around Iowa. When Maurice Ewing, a geophysicist from Columbia
University, arrived on campus as a visiting lecturer and spoke about the
vast terra incognita that lay beneath the obscuring blanket of the world's
oceans and the exciting science to be done at sea, Bruce was intrigued at
the seemingly great opportunities for discovery in this young science. It
was no coincidence that Ewing emphasized the romantic qualities of
oceanographic research: He was looking for undergraduates to join him
for a summer of work. During a tour of the geology department after the
lecture, Bruce was introduced to Ewing over a tray of fossils. Out of this
came a seductive invitation to join a National Geographic-sponsored

Aboard Atlantis in the
North Atlantic, Bruce
Heezen arms a surplus
World War II bomb for

a seismic refraction
experiment. This photo

was taken in the late
1940s or early 1950s.

Winter 1991/92

101

Again on Atlantis in

the North Atlantic,

Bruce Heezen (right,

facing) with Maurice

Ewing (left, facing)

arms an explosive

charge.

cruise aboard WHOFs R/V Atlantis to explore a long linear swell, the
Mid-Atlantic Ridge, that lay along the North Atlantic's center line.

The opportunity to participate in an investigation of the first-order
properties of an unknown mountain range beneath the sea offered a
refreshing change in perspective and scale from the microscopic study of
subtle changes in conodonts. Bruce accepted the invitation and arrived in
Woods Hole in June to join Ewing's team preparing for the cruise. For
the first several weeks, they worked feverishly to fabricate equipment for

the voyage. Bruce constructed a
photographic laboratory on Atlantis,
helped to build several deep-sea cam-
eras, and searched Harvard libraries for
literature about the Mid-Atlantic Ridge.
At that time, almost nothing was known
about the Mid- Atlantic Ridge excepted
that it existed.

It came as a shock and a surprise
to Bruce when Ewing told him, during
a walk home late one evening in
Woods Hole, that he was not to go on
Atlantis to the Mid-Atlantic Ridge.
Instead, he was to be chief scientist
aboard a small Navy ship, Balamis, that
had unexpectedly become available.
He was to use one of the newly
constructed bottom cameras to take
photographs of the submerged conti-
nental margin off the east coast, an
environment that had never been
photographed. Even in those expan-
sive days of oceanographic science
following World War II, it was most
unusual to be chief scientist on one's
first cruise. Ewing must have sensed
that despite his inexperienced state,
Bruce could be counted on to do the job.

This change in plans proved providential: It set the stage for Bruce's
uncompromising love for the seafloor and the processes that shape it.
The stomach of a flatlander from Iowa was in no way prepared for the
lively nature of a small ship at sea, and Bruce experienced terrible
seasickness. He reflected later that had he gone to the ridge with Ewing
aboard Atlantis, he would have been the youngest of a large number of
students and, as just one of many under Ewing's tutelage, would have
lacked the incentive to rally against relentless seasickness. Instead, he
was put in charge of a ship and given the responsibility of carrying out a
program. He persevered that summer, successfully taking 200 bottom
photographs of the uncharted abyss. He took these photographs home
with him that fall, and spent his senior year at the University of Iowa
trying to understand and interpret them. Bruce found that he had more
questions than he had answers, and the photographs became a catalyst
for dedicating his professional career to the study of the seafloor. The

102

Oceanus

Bruce's disdain for all things turkey remained inviolate for

over 20 years until an oceanographic cruise in the late

1960s. The cook aboard the ship had his roots in southern

cuisine and all animal and most vegetable products were

fried in a cavernous deep-fat fryer that seemed never to be

turned off. After six days of nothing but fried foods, one's

interest in meals was low indeed. Finally, at lunch on the

sixth day we arrived in the mess to find a roasted turkey on

the menu; roasted, because the 20-pound bird was too big

for the fryer. Bruce, who had not eaten turkey for over 20

years, hesitated, stared, poked, and then descended

ravenously upon his portion.

following summer he returned to the sea with Ewing, this time as a
beginning graduate student, and spent two months aboard Atlantis
continuing the Mid- Atlantic Ridge investigation. In the fall of 1948 he
joined Ewing and his growing entourage of graduate students at Lamont
Geological Observatory at Columbia University to begin his formal
training. Marine geology was never to be the same.

Like many talented people, Bruce was blessed with a very quick,
multidimensional mind and an exceptional memory. He had an insatiable
appetite for books on all aspects of ^^^^^^^^^^^^^^^^^^^^^
Earth science. These characteristics
combined to form an ability to see
and think imaginatively about link-
ages both between and across dif-
ferent, but complementary, investi-
gative results.

One of the great break-
throughs in our understanding of
the Mid-Ocean Ridge system pro-
vides a good example. When Bruce
started working with investigators
at Lamont on the data from
Atlantis cruises to the Mid-Atlantic
Ridge, the existence of a world-
encircling ridge system was un-
known. However, a ridge of some kind was known to extend the length
of the Atlantic based largely on results from English and German
oceanographic studies carried out before World War II. These investiga-
tions showed the bottom water of the eastern and western basins to be
different and, therefore, separated by a barrier with unknown properties.
Also, investigators sailing under flags of a variety of countries before
World War II had documented, with widely spaced soundings, the
existence of the Albatross Plateau in the equatorial eastern Pacific
(known today as the East Pacific Rise) and the Carlsberg Ridge in the
northeastern Indian Ocean.

Bruce's initial project at Lamont was to compile all the available
sounding profiles across the ridge in the Atlantic, in an attempt to
characterize its spatial properties. He was assisted in this task by Marie
Tharp, a new research assistant, who had recently completed an M.S. in
geology at the University of Michigan. Marie tackled the tedious and
demanding task of creating coherent profiles from noisy sounding data.
She compiled six widely spaced profiles across the ridge in the North
Atlantic and made the interesting observation that on each profile, the crest
of the ridge appeared to be notched by a several-thousand-meter-deep
valley that was 40- to 60-kilometers wide. Bruce was skeptical about the
existence of such a valley at a regional scale, but intrigued by the notion.

Coincidentally, Bruce was working with Ewing on another project
evaluating the linkage between underwater avalanches of sediment,
called turbidity currents, and earthquakes. For this study, Bruce created
a plot of earthquake epicenters on a North Atlantic map drawn to the
same scale as Marie's sounding-profile map. This was no coincidence;
Bruce believed that plotting different kinds of data at the same scale facili-

Winter 1991/92

103

Marie Tharp (in about
1956) working with the

first-edition

physiographic map of

the North Atlantic.

The famous six profiles

across the Mid-
Atlantic Ridge are on
her right, two sound-
ing records on her left,
and an early globe that
she and Bruce created
of ridges in North and
South America is at
center. In collaboration
with National Geo-
graphic, Marie and
Bruce made a physi-
ographic globe of the
earth in the late 1960s
(below).

tated comparisons. Laying the epicenter map over the profiles revealed that
the earthquakes defined a broad belt of activity down the center of the

North Atlantic, and in the six profiles Marie plotted across the
ridge, the earthquake locations fell within the bounds of
the axial valley. The occurrence of earthquakes along
the Mid-Atlantic Ridge had been recognized
previously, but the association of the seismic
events and the axial valley Marie proposed was
startling. Bruce became convinced that Marie's
insightful observation about the existence of a
rift valley was correct.

The coincidence of the valley and
earthquake activity, which is an indicator of
rupturing of the earth's brittle yet elastic
outer shell in response to forces that exceed
the shell's strength, indicated to Bruce and
Marie that this feature was dynamic and
shaped by currently active processes. The
earthquake belt was continuous along the length
of the North Atlantic, leading them to speculate that
the earthquake-belt location could be used to predict
the axial-valley location in the absence of sounding data. As
they slowly accumulated sounding profiles across the North Atlantic,
their plots began to reveal a deep axial valley coincident with the seismic
belt along the crest of the Mid-Atlantic Ridge. Ewing and Heezen then
embarked on a project to plot the locations of earthquakes in ocean
basins throughout the world, and noted once again the coincidence
between belts of earthquakes and the crests of known, but seemingly
separate, ridge segments scattered about the ocean basins. In addition,

104

Oceanus

the earthquake locations defined a diffuse but continuous belt that linked
the known ridges in the Atlantic, Indian, Arctic, and Pacific oceans,
leading them to suggest that the world was encircled by a mid-ocean
system of ridges that by their very existence, scale, and continuity, were
central to the history of the ocean basins. They also observed that when
sounding profiles crossed a branch of the ridge system, earthquake
epicenters fell within the boundaries of the axial valley that was inter-
preted to be an active rift zone. In 1956, they published their idea for a
continuous ridge system, and were met with some skepticism. Cruises
were planned to test their predictions by surveying unexplored portions
of the southern Pacific and Indian oceans to see if, indeed, a ridge existed
where proposed — and a ridge was always found.

During this global synthesis, Bruce noticed that a limb of the mid-
ocean seismic belt could be traced across the northwestern Indian Ocean
into the Gulf of Aden, where it linked with a north-south trending zone
of continental seismicity in East Africa. This belt of continental seismicity
was associated with the network of East African Rift valleys that con-
tained the great lakes of Africa, such as Victoria and Rudolf, and where
fieldwork by British and German geologists had documented that the
earth's crust was being extended in an east-west sense to create a north-
south trending rift system. Bruce and Marie constructed topographic
profiles across the rift valleys of East Africa and compared them with
profiles across the Mid-Atlantic Ridge. The similarity of the profiles was
striking. This, along with the continuity of the seismic belt, indicated to
Bruce that the axial terrain of the ridge and the rift valleys of East Africa
are genetically related. He proposed that the crust along the axis of the
Mid-Ocean Ridge system is stretched at right angles to the axis. By the
late 1950s, evidence for large displacements of the continents (based on
paleomagnetic studies of rocks by British investigators) was compelling.
Bruce suggested that these displacements were accommodated by the
creation of crust at the ridge axes, and that the history of continental
displacements were recorded in the seafloor's structural fabric. With this,
a major pillar in our understanding of how the earth works was in place.

During this phase of exploration and insight, Bruce and Marie
realized that it was difficult to create improved maps of the seafloor
because of the vast scale of the underwater terrain and the slow rate of
sounding data acquisition. Following the techniques developed by
continental cartographers, they created physiographic diagrams of the
seafloor. Unlike a contour map that links points of similar depth with
lines, a physiographic diagram creates an interpretive three-dimensional
view of the seafloor. Such a presentation also allowed Bruce and Marie to
extrapolate the seafloor's textured variations between widely spaced
sounding lines. They finished their first physiographic diagram of the
North Atlantic in 1956, and it was followed over the next 20 years by a
series of physiographic maps that, in one form or another, covered all the
world's oceans. These maps are remarkable because Bruce and Marie
had an ability to visualize seafloor morphology in three dimensions and
intelligently extrapolate trends and relationships into areas of sparse
data to create depictions that have since been shown to be remarkably
accurate. This collection is probably the most widely distributed set of
seafloor maps. As such, it has provided a pictorial gateway to the earth's

Earthquake

locations

defined a

diffuse but

continuous

belt that

linked the

known ridges

in the Atlantic,

Indian, Arctic,

and Pacific

oceans.

Winter 1991 /92

105

last frontier and captured the imaginations of students and researchers
around the world.

Bruce was happiest when he was at sea learning something new. He
was indefatigable in this environment, where he seemed to derive the
strength he needed to work night and day by feeding off the realization
that a major discovery was in the making if he could collect the right
kinds of data in the right way. Given the great expanse of unexplored
^^_^_^_^^___^^^^_^^_ ocean, more was always better—

and Bruce worked himself, his
colleagues, the ship, and its crew to
the limit. Data were not mindlessly
accumulated; each new observation
of interest was studied and as-

sessed for telling clues about the

Bruce became an avid and enthusiastic user of
submersibles in the late 1960s when this technology
became available and permitted manual presence in the
deep sea. One day, during an explanation to a graduate
student about what to expect when this student made his
first dive, Bruce found his descriptions about the experi- seafloor. With each new insight
ence to be lacking. In mid-sentence, he jumped up from his came hypothesis testing and cruise-
desk, turned off the lights, grabbed a flashlight and crawled plan modification to create a more
under his desk. From his confined quarters under the desk, effective investigative strategy. To
Bruce held the flashlight above his head and pointed the be a student working with Bruce at
narrow beam out across the floor slowly sweeping the
shaft of light across one partially illuminated object after
another. About this time his secretary opened the door to a

darkened room to find Bruce squeezed under his desk,

shining a light about the room with a silent but bemused

graduate student standing off to one side. Her look was

incredulous, but before she could say a word, Bruce

announced that he was diving in a submersible

and was not to be disturbed.

sea under these conditions (which I
had the fortune to be during
several cruises) was at once exhila-
rating and fearful: exhilarating in
that I learned so much because the
arrival of new information in the
form of a sample, photograph, or
profile always precipitated lively
and intense discussions about all

the data's aspects and implications,

and fearful because Bruce asked

penetrating questions and expected intelligent answers. I knew when I
had run aground with an idea if Bruce likened me to one of his feathered
friends from his early days in Iowa.

Bruce was a marvelous advisor to his graduate students. He under-
stood and respected the sanctity of research and the freedom to follow
one's own ideas. He exposed incoming students to the broad venue of
research possibilities that lay between the shorelines of the world's
oceans, and gave them free reign to choose problems of interest. He
recognized the importance of exploring unknown and untested avenues
of research and the time-consuming nature of this process. He had as
many as 11 Ph.D. students at one time, all working on a broad range of
problems, and, given his many involvements, he only had time to
measure a student's research progress every few months. When the call
did come, however, his students knew they had better be prepared,
because Bruce would expect to be challenged by new observations and
ideas. The sessions were often lengthy, as Bruce explored every aspect of
the student's work. I always left these encounters exhausted but enlight-
ened, because his probing questions and great depth of knowledge had
provided new insights about my results, opening up new avenues of
exploration. No matter how thoroughly I had analyzed a problem, Bruce
always seemed to see farther.

106

Oceamis

The science of the seafloor was Bruce's life, and the boundary
between his work and everything else that constituted his being was
invisible. When your goal is to map the world-ocean floors and under-
stand how this great expanse was formed, you need a great deal of
space, more than Bruce found available at the Lament Observatory, so
certain mapping and writing projects were carried out at his or Marie's
homes. Over a period of years, both homes evolved into laboratories
with drafting tables, a multi-
tude of maps, and books piled
everywhere. Because these en-
vironments were quieter than
the chaos of his offices and la-
boratory space at Lament,
where technicians and graduate
students swirled about, he
would often work at one house
or the other, and this is where a
student might go to work with
him, especially when preparing
a manuscript. These gatherings
could go on for hours as Bruce
probed every sentence for clarity
and insight, oblivious of the time.
I remember that when projects
were under way, there was al-
ways a welcome place at the
dinner table. After dinner,
typically a very rare meat and an
excellent bottle of Bordeaux, the
manuscript honing would
continue long into the night.
Bruce would reluctantly loosen
his grip on our text when he
observed that I had fallen asleep.

During his career Bruce received awards from many scientific
societies for his fundamental contributions to marine geology and our
understanding of the earth. He was a man for his time, because his wide-
ranging interests and probing intellect were free and unconstrained by
the lack of disciplinary boundaries found only in new fields of science.
Today, marine geology and geophysics is a much more mature science
with rigorously defined investigative disciplines, and it is difficult, if not
impossible, to work on and contribute to the range of problems that
Bruce examined. We were lucky to have him when we did.

Paul J. (Jeff) Fox is Professor of Oceanography at the University of Rhode Island
Graduate School of Oceanography. He was one of Bruce's graduate students at
Lamont-Doherty Geological Observatory of Columbia University and worked with
him at sea and in the laboratory from 1964 until Bruce's death. Under Bruce's
tutelage, he was introduced to the intriguing mysteries and romance of the Mid-
Ocean Ridge system; it is a love affair that continues to this day.

On an expedition to the

Caribbean in the early

1970s, Bruce Heezen

discusses a dredged

limestone sample with

students aboard

R/V Eastward.

Winter 1991/92

107

BOOK & VIDEO REVIEWS

Fire Under The Sea

By Joseph Cone, 1991. William Morrow and
Company, Inc., New York, NY; 286 pp. - $25.

The discovery of hot springs on the seafloor is
one of the most dramatic findings in marine
science in the last 15 years. From the for-
mation of mineral deposits to the existence of
previously unknown biological communities,
studies of these hot springs have had profound
effects on our understanding of ocean floor
processes. In Fire Under The Sea, Joseph Cone
traces their exploration in an action-packed

story that not only
conveys the chal-
lenges and excite-
ment of exploring
the ocean bottom,
but also provides a
glimpse into the
lives and work of
sea-going scientists.
Addressing the
field in general,
Cone concentrates
on the exploration
of hot springs on the
northwest coast of
the United States.
The book opens

aboard the research vessel Atlantis II off the
Oregon coast on a typical morning as scientists
prepare to dive in the submersible Alvin to the
ocean floor for a day of observations and
sampling. This is the first of many "dives" the
reader makes during the book's course and,
through the thoughts and comments of the sci-
entists, the story captures the essence of being
part of an oceanographic expedition.

Cleverly interwoven into the story of hot
springs on the Gorda and Juan de Fuca Ridges
is an account of the development of modern
ideas of seafloor spreading and continental
drift. As in any good mystery story, a number
of unconnected pieces — in this case, studies
done independently by continental and marine
geologists and geophysicists — have been fitted
together to produce a model of the Earth's

surface plates created at mid-ocean ridges.
From the time when Alfred Wegener first
noted the "fit" of the continents of South
America and Africa early in this century, the
reader is led through scientists' work as they
developed new ideas, designed experiments,
and debated their results. Although the story is
full of information, Cone manages to keep the
reader's interest with anecdotes that illustrate
the personalities of those involved. By the
1960s, the plate tectonics paradigm had gained
general acceptance. Against this background,
scientists predicted the occurrence of hot
springs on the ocean floor and took on the
challenge of proving their existence.

Apart from the highly visible aspects of
scientific discovery, there are many other
important facets — from the technological
developments that frequently pave the way for
exploration, to the policy decisions necessary
when new, potential mineral resources are
found. Whether describing the development of
sophisticated echo-sounding techniques or
Law of the Sea negotiations, Cone's writing is
authoritative, easy to read, and without jargon.

In writing fire Under The Sea, Cone inter-
viewed an impressive list of scientists and
consulted a large number of references, both of
which are presented in the book. The inclusion
of a chronology of events at the beginning is
helpful, and eight pages of color photographs
illustrate the strange chimney structures and
unusual organisms associated with seafloor hot
springs, fire Under The Sea is an entertaining
and compelling account of discoveries in a
young and exciting field, and should be of
interest to anyone curious about the ocean.

— Susan Humphris

Dean, Sea Education Association

Woods Hole, Massachusetts

WE'D LIKE TO HEAR FROM YOU.

Oceanus welcomes and occasionally

publishes letters from readers
regarding editorial content or other ocean-
science issues. Please write to
the address on page 4.

108

Oceanus

BOOK & VIDEO REVIEWS

4000 Meters Under the Sea

By Films for the Humanities and Sciences,
Princeton, N.J. 1991. 28 minutes - purchase
$149/rent $75.

This 28-minute video, made by NHK (Japan
Broadcasting Corporation) and distributed by
Films for the Humanities and Sciences, con-
cerns a 1987 joint US-Japan expedition to study
the geology and biology of the Marianas
Trough spreading center. The film depicts the
findings of hot vents and associated fauna at
the accreting plate margin in the trough using
the deep submersible Alvin and R/ V Atlantis II.
It is aimed toward the lay audience.

The film nicely captures the essence of a
multidisciplinary expedition and the use of
Alvin as a research tool. A voice-over commen-
tary identifies the principal investigators, Bob
Hessler and Jim Hawkins of Scripps Institution
of Oceanography, and their Japanese col-
leagues, but, regrettably, with the exception of
a closing sentence from Hessler, we do not
hear first-hand from the scientists. The film
quality is good with some excellent close-ups
of the vents and the vent communities. The
development of the film's theme — the finding
of and questions regarding hot vents — is well
done (with some reservations that I will
discuss later), and draws the audience along
nicely. It makes for an interesting 28 minutes.

There are a few, minor irksome aspects:
Principally I found it most annoying to be
continually told in the introduction, and near
the end, that the expedition was to the
Marianas Trench. Presumably this assertion,
accompanied by continual references to the
deepest part of the world's oceans, is an
attempt to add glamour and excitement—
certainly not needed here. In actuality, some
excellent graphics clearly show the Marianas
Trough and its relative position to the Trench.
These graphics were so good and visually
striking, I regretted that they didn't spend a
couple of minutes more explaining the spread-
ing center and its relevance to the Trench, plate
motions, etc. The other irksome aspects were in
the commentary: Generally it was well done,

but a little better quality control could have
avoided small things like referring to the Alvin
manipulators as "magic hands" or, more
importantly, reference to the planktonic food
falling to the seafloor and being converted into
bacteria (rather than by).

On the whole, the scientific aspects were
well covered and well explained. A few errors
slipped through: References to "magma in the
venting solutions making them cloudy," rather
than products of rock-water interaction; the
chimneys or smokers being alluded to as
"cooled molten lava," rather than precipitates
of sulfides and sulfates; the observed minerals
being "crystals of iron, copper, and zinc,"
rather than sulfides of these elements. I think
the interaction of seawater with the rocks,
subsequent extraction of metals, gases, etc.,
and how they reach the sea surface had the
weakest coverage, but the main points did
come through.

While recognizing the educational and

EGRET
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urith (Burt Jones and !Maurine Shimiock^

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Secret Sea tours scheduled for 1992 include
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photography and travel with Secret Sea Visions,

write or call Burt Jones and Maurine Shimlock,

Secret Sea Visions, P.O. Box 162931, Austin,

Texas, 78716; (512) 328-1201.

Winter 1991/92

109

entertainment aspects needed for this kind of
video, I was most disturbed by the implicit
(and at times explicit) suggestion that this was
the first discovery of vents, and the expression
of great "surprise" to find life at this depth;
similar vents and life had been discovered and
investigated in many locales, and at similar
depths, before this expedition. The questions
being addressed (implicitly for the first time):
Where does the hot water come from? Could
the cloudy waters be the key to life? How does
it sustain life? How do the animals feed? All
these have been previously investigated at
other vents and are known to some degree. The
really important aspects of the Marianas
Trough vents were not made clear — their
setting in a back-arc basin as opposed to a mid-

ocean ridge, and their biological community
that is slightly different and dominated by new
species (hairy gastropods) compared to other
vents. This is never spelled out, and previous
work is not properly referenced, except for one
comment from Hessler about how exciting it
was to find "new friends" (animals) as well as
"old ones" (previously discovered vent fauna)
at the Marianas vents.

All in all, in spite of my minor misgivings,
I found it an interesting video. It should excite
the lay audience, and the shots of the vent
communities are certainly worth a look by
serious submarine-hot-spring researchers.

—Geoffrey Thompson

Senior Scientist, Chemistry Department

Woods Hole Oceanographic Institution

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110

Oceanus

continued from page 8

strike-slip fault — a fault showing predomi-
nantly horizontal movement parallel to the
strike; vertical displacement is absent

subduction zone — area of crustal plate colli-
sion where one crustal block descends
beneath another, marked by a deep ocean
trench caused by the bend in the submerg-
ing plate. The downward movement of the
subducting plate results in earthquakes,
volcanos, and intrusions on the far side of
the trench.

swath mapping tools — instruments installed
on a research vessel that use sound re-
flected from the seafloor to map the shape
of the seafloor along a band or swath that
extends as far as 1 to 5 kilometers on either
side of the vessel's track. Common instru-
ment names are GLORIA, Hydrosweep,
Sea Beam, and Sea MARK.

tectonics — the forces and movements that
create Earth's larger features

terrane — the area or surface over which a
particular rock or group of rocks prevails

transform fault — a strike-slip fault of a par-
ticular type where displacement stops
abruptly, especially associated with
offsetting of mid-ocean ridges

transverse feature — a geological feature whose
strike is generally perpendicular to the
general structural trend of the region

vent — place where water heated and altered
by circulation through porous volcanic
rock erupts from the seafloor, precipitating
minerals and supporting sulfide-depen-
dent biological communities

The Oceanus staff acknowledges the valuable aid in
assembly of this glossary of The Facts on File
Dictionary of Geology and Geophysics (© 1987 by
Dorothy Farris Lapidus, Facts On File Publications,
New York, New York, and Oxford, England).

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Winter 1991/92

111

••*A

CREATURE FEATURE: These photographs represent the many
interesting animals found in the colonies that thrive around mid-ocean
ridge hydrothermal vents. (For a broad look at recent work on vent communi-
ties, please see The Biology of Deep-Sea Vents and Seeps... by Rich Lutz on
page 75.) The large photo is a field of tube worms, Riftia pachyptila, on the
Galapagos Rift, and the upper right inset is a closeup of these animals. The

lower left inset is an Alvin camera view of giant clams, Calyptogena
magnified, and scavenging galatheid crabs, Munidopsis sp. At upper left, a line

of crabs marches along the edge of a bed of mussels, Bathymodiolus

thermophilus. At lower right, the Pompeii worm, Alvinella pompejana, was

photographed on the surface with the tube it calls home. (Large photo by

Kathleen Crane, small photos clockwise from lower left by Alvin exterior

camera, Robert Hessler, Dudley Foster, and John Porteous.)

MBL WHO! LIBRARY

COM

EXT

1992 in Oceanus: Ocean Sciences — Four Disciplines

• The four issues of Oceanus for 1992 will

make up a volume on four basic disciplines in

oceanography. Each will offer a summary of

the discipline and articles that expand on

several topics. • The Spring Issue features

Marine Chemistry, including an update on

carbon dioxide and climate, how tracers aid

the study of marine processes, a look at

natural marine chemicals and their uses, and

the chemistry of seafloor vents. • We will also

bring you several regular departments-
Focus on the Coast, Toolbox, Issues in Ocean
Law & Policy, and Creature Features. • Our
Summer Issue will revolve around Physical
Oceanography, Biological Oceanography will
follow in the fall, and Marine Geology &
Geophysics will wrap up the series next
winter. • Don't miss this interesting, educa-
tional review of oceanography in the '90s!

ORDER BACK ISSUES!

What's Still Available?

• Reproductive Adaptations

Vol. 34/3, Fall 1991

• Soviet- American Cooperation

Vol. 34/2, Summer 1991

• Naval Oceanography

Vol. 33/4, Winter 1990/91

• The Mediterranean

Vol. 33/1, Spring 1990

• Pacific Century, Dead Ahead!

Vol. 32/4, Winter 1989/90

• The Bismarck Saga and
Ports & Harbors

Vol. 32/3, Fall 1989

• The Oceans and Global Warming

Vol. 32/2, Summer 1989

• DSV Alvin: 25 Years of Discovery

Vol. 31/4, Winter 1988/89

• Sea Grant

Vol. 31/3, Fall 1988

• and many, many, more...

To place your order, send a check or money
order (payable to WHOI) to:

Oceanus Back Issues

WHOI
Woods Hole, MA 02543

Please enclose $8.00 plus $1.00 shipping and han-
dling for each magazine ordered, and include a street
address and daytime telephone number with your
order. Allow 3 to 4 weeks for delivery. All payments
must be made in US dollars drawn on a US bank. For
orders outside the US, please add an additional $1 .00
per item for shipping. Sorry, we cannot deliver to
Canadian addresses.

For information on other available back issues, con-
sult the Oceanus editorial offices at the address listed
above. Back issues of Oceanus are also available on
microfilm through University Microfilm International,
300 N. Zeeb Road, Ann Arbor, MI 48106.

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