MARYLAND BETA CHAPTER
TAU BETA PI
VOLCANOES AND THEIR ERUPTIONS
Milton A. Fischer
September 8, 1943
VOLCANOSS AND THEIR ERUPTIONS
One of the more interesting studies of the geological
structure of the earth is the phenomenon that is connected with
the movements of molten rock matter and its subsequent formation
into igneous rock. Within the earth's core the high temperature
is sufficiently great to melt solid rocks. This molten matter,
dissolving and fusing its way, rises toward the surface along
fissures and fractured zones which were probably formed during
the periods of mountain growth.
The opening in the earth's surface through which these
hot rocks are forced out is called a volcano. The word being
derived from Vulcanus, the Roman god of fire, who was supposed
to dwell in the volcano Mount Etna in Sicily. When solid rock
fragments or lavas, which are rocks in a fluid state, accumulate
around the opening, a cone is built up which increases in size.
A cone so constructed is called a volcano. Thi3 formation is
only a result rather than a part of the phenomena of volcanic
movement. At the top of the cone there is generally a crater
that is usually funnel shaped and represents the vent through
which the volcanic matter is ejected, widened near the top by
explosions. (Fig- 1) The slope of the cone usually is as steep
as the erupted material will lie. If the eruptions consist of
ash and large solid fragments, the cone will have steep walls.
The volcanoes that throw out this type of matter are of the
explosive type and generally eject gases in considerable amounts
If the eruptions consist of flowing lavas, the volcano will
have gentler slopes. These volcanoes throw out little solid
matter. Gases are generally present, however, but they do not
accumulate under pressure, thereby escaping quietly.
A view into the crater of Vesuvius.
Volcanoes vary in size from the small conical hills
to some of the loftiest mountains on the earth's surface. The
Eawailan Islands contain the greatest volcanic mountains in the
world, rising nearly 14,000 feet above sea level, and 30,000
feet above the sea floor.
The eruption of a volcano often|i.s accompanied by earth-
quakes and by loud rumblings, like thunder. The rumblings prob-
ably are due to the movement of gases and lavas that are held
in under great pressure. During eruptions the condensation of
the steam causes heavy rains accompanied by vivid lightning.
The rain often washes vast quantities of loose ash down the steep
slopes In destructive mud flows. Preceding the eruptions fissures
often are opened. Hot springs appear at places, and flowing
springs are dried up. Cracks open, and gases issue from them.
The materials erupted from volcanoes include solids,
liquids, and gases. If the eruption is moderate, melted rock
usually flows out, and in cooling, forms lava flows. (Fig. 2)
In violent eruptions the expansion of the steam blows the lava
to pieces, forming scoria, pumice, and ash. These are so light
and porous that they float in water, and the fine ash even re-
mains suspended In the air. Lumps of lava thrown into the air,
cooling in oval, twisted masses, are known as volcanic bombs .
Gases seem to rise upward with the lava, although some may form
by reaction in the craters. Steam rises with the lava, but some
of it forms when water flows over the hot lavas. Some of the
most violent eruptions known are those near sea level, where it
appears probable that water has penetrated fissures to the hot
rocks and has been converted into steam. After a period of
dormancy a seal forms in the crater, and gases and lavas accum-
ulate, but when the seal is broken by the great pressure of the
gases, a violent eruption follows.
There are thousands of volcanic cones, only about 400
of which have been observed in activity. Wherever molten rock
rises to the surface, volcanoes may be built up. They may form
on mountains or plateaus, on low plains, and on the bottom of
the sea. Volcanoes are widely distributed both chronologically
and geographically. The great majority of these cones are in
or near the sea, A far the greatest number being in the mountains
and Islands which encircle the Pacific Ocean. A volcano in
eruption is active. One that has ceased to erupt is dormant.
If It is believed that the volcano will not erupt again, it is
said to be extinct. It is difficult to distinguish between an
extinct volcano and a dormant one, and many volcanoes supposed
to be extinct have erupted with great violence.
Gas Issues from certain volcanoes in large amounts.
Steam is by far the most abundant. Some of it is formed by
heating ground water and surface water that have come In eon-
tact with the hot products of the volcanoes. In some volcanoes,
the temperature of the steam has been measured as high as 650°
C. Other gases that have been found to issue from volcanoes In
usually small amounts are oxygen, nitrogen, argon, carbon di-
oxide, carbon monoxide, sulphur dioxide, hydrogen sulphide,
hydrochloric acid, hydrofluoric acid, ammonia, and sulphuric
There have been many great eruptions, each possessing
particular characteristics different from the others A few of
the largest and most famous will be discussed as a generaliza-
tion of all volcanoes, although the essentials may vary slightly.
Vesuvius, the best known volcano, is situated 7 miles
southeast of Naples in a densely populated region. The height^
of the mountain varies, but is about 4,000 feet above sea level.
(Fig. 3) A great ridge known as Mount Somma half encircles the
Vesuvius in eruption, 1872
present active cone. In 79 A.D. the ancient crater had been
quiet so long that trees were growing in it. That year an ex-
plosion blew off a large part of the cone of Vesuvius, burling
three surrounding cities, one being Pompeii. Before the erup-
tion there were frequent earthquakes, one of which partly des-
troyed Pompeii. Little or no lava was ejected, but much dust,
ash, and steam issued, forming a pasty mud that flowed down
the slope and overwhelmed dwellings. Pompeii was covered to
Vesuvius and excavated ruins of Pompeii
25 to 50 feet. (Fig. 4) The ashes rose thousands of feet in
the air causing the day to be changed into darkness of night
and accompanying all this there was fierce thunder and light-
ning. Gases were exhaled, and these either poisoned or suffo-
cated the citizens. Thousands of people were undoubtedly killed,
though there is no record of the number. Since 79 A.D. Vesuvius
has had many eruptions, some violent, some moderate, some of
ash, and soma of lava.
The most violent eruption recorded in historic time
is that of Krakatao, a small island between Java and Sumatra.
For a century the small Island was dormant, but in 1883 an
eruption began throwing out pumice and dust, and continued for
three days. Nearly all of the island was blown away, and a hole
1,000 feet below sea level was blasted out. Fragments were hurl-
ed 17 miles high, and in 15 days the dust from the volcano borne
by air currents had encircled the earth. Windows were broken
100 miles from the volcano, and the sound of the explosion was
heard more than 150 miles away. Sea waves 100 feet high were
generated, and these reaching the shore of neighboring islands
destroyed many towns killing about 35,000 people. It appears
probable that water has penetrated fissures to the hot rocks
and has been converted into steam. The dust from the explosion
wa3 carried into the upper atmosphere so as to cause brilliant
sunsets for a period of several months, at first near by and
about two weeks later entirely around the globe.
Many other great volcanic eruptions have occurred.
Those notable of mention are briefly described. During the
eruption of Tambora on Sumbawa Island, Netherlands East Indies,
In 1815, rumblings were heard over an area with a radius of
about 1,000 miles. Approximately 38 cubic miles of material
was thrown out during this eruption, which is the greatest one
recorded by man. Mount Etna on the island of Sicily rises to
an elevation of about 11,000 feet and covers 460 square miles.
(fount Pelee situated on the north end of Martinique, an island
of the West Indies, erupted violently in 1902 when hot gas and
dust rolled down the mountain side and destroyed Saint Pierre,
killing nearly 30,000 people in a few seconds; and only one
person escaped death. (Fig. 5) The cloud was so dense that it
m " '•'- '
Volcanic a3h from Mount Pelee
seemed to act like a liquid. Hawaii, the largest of the Hawaiian
chain, Is built of volcanic matter, which rlse^ high above the
level of the sea. Mauna Loa in the southern part of the island
has an elevation of about 14,000 feet. It has a crater 2 miles
wide and 1,000 feet deep. Other large eruptions have occurred
In Japan, Alaska, Siberia, Hew Zealand, and the Phillipines.
In the formation and evolution of volcanoes, there
are three main factors present. First the temperature of the
earth's core must be of such intensity that molten rock will
be created. The actual temperature of the Interior of" the
earth Is unknown; however, the material ejected from volcanic
eruptions possess temperatures ranging from 1000° to 2000° C. ,
thereby indicating the Inner temperatures somewhere in excess
of this figure. Second, there must be a means provided for
the flowing of this material to the earth's surface. Thi3 is
probably supplied by cracka and fissures in the rock formations
These occurred during periods of mountain growth, for when
mountains stop growing, volcanic activity died out. Third,
the molten rock must be forced out of the earth. This is done
by the pressures created by the steam of percolating water
which had found its way down to the molten material. Such are
the functions and wonders of a volcano.
Tarr, R. S., New Physical Geography, New York, Macmillan Co.,
Iddings, J. P., The Problem of Volcanism, New Haven, Yale
University Press, 1914