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Full text of "Volcanoes and their eruptions / Milton A. Fischer"

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Milton A. Fischer 
September 8, 1943 



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) 

Lava flows. 
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