VOLCANOES AND IGNEOUS ROCKS
Not only because of the great power and terrifying grandeur of violent
eruptions, but also because of their destruction of life and property,
volcanoes stand out in the popular mind as among the most real and
important of all geological phenomena. But great volcanic outbursts,
like violent earthquakes, are in truth only outward, sensible,
relatively minor manifestations of the tremendous earth-changing forces
below the surface. They are far less important as geological agencies
than the mighty interior forces which cause parts of continents to
be slowly upraised and the rocks folded, or even than the incessant
action of streams whereby the lands are cut down. Even as an igneous
agency, volcanoes are notably less important than the development and
shifting of molten materials within the earth's crust. Volcanic action
is, however, not only conspicuous, but it is also of real significance
as a means of changing the earth, such action having taken place since
very early known geologic time. After bringing out the main facts and
principles of volcanoes, aided by descriptions of specific eruptions,
we shall turn to a consideration of igneous activity within the earth's
crust.
Mount Vesuvius in Italy is perhaps the most famous active volcano in
the world. Its eruptions have been more or less carefully studied for
a longer time than any other. The eruption in the year 79 A. D. was
really a tremendous explosion causing a large part of the old crater to
be blown away, and sending immense volumes of rock fragments, mostly
finely divided (so-called "ashes") into the air which completely buried
the small city of Pompeii. Water from the great clouds of condensing
steam, mixed with "ashes," formed muddy floods which overwhelmed
Herculaneum. Little or no lava was erupted. Since that time the crater
has been more or less active and the present cone, 4,000 feet high, has
been built up. During the last fifty years the greatest eruptions took
place in 1872 and 1906, when, streams of molten rock flowed down the
sides of the mountain.
[Illustration: Fig. 16.--Map showing the distribution of active and
recently active volcanoes of the world.]
One of the greatest volcanic explosions ever recorded was that of the
island of Krakatoa, between Sumatra and Java, in 1883. The greater part
of the island was blown away and there was water 1,000 feet deep,
where just before the island stood hundreds of feet high. About a cubic
mile of rock material was sent into the air mostly in the form of fine
dust--some of it for seventeen miles--and completely hid the sun,
causing total darkness during the eruption. Dust fell over an area of
several hundred thousand square miles. Several days after the explosion
ships more than 1,000 miles away were dust covered. Such enormous
quantities of a light porous lava called "pumice" fell and floated
upon the sea that navigation was badly obstructed many miles from the
volcano. Extremely fine dust gradually spread through the whole earth's
atmosphere, causing the extraordinary red sunsets for several months.
The sound of the explosion was heard for hundreds of miles. Great
sea waves 50 to 100 feet high were stirred up and they swept inland
for several miles over the low-lying coast lands of neighboring Java
and Sumatra, overwhelming hundreds of villages and drowning tens of
thousands of people.
[Illustration: Fig. 17.--The great hole left after the top of Mt.
Katmai in southern Alaska was blown off in 1912 by one of the most
tremendous volcanic explosions in the annals of human history.
The water in the lake is hot. (After Griggs, National Geographic
Magazine.)]
One of the greatest explosions on record was that of Katmai volcano,
several thousand feet high, on the coast of Alaska, in June, 1912.
Not only was the top of the mountain completely blown off, but also a
great crater pit, three miles wide across the top and several thousand
feet deep, was developed in the stump of the former mountain. Volcanic
dust fell to a depth of several feet within twenty-five to fifty miles
of the mountain. Dust accumulated to a depth of nearly a foot in the
village of Kodiak, 100 miles east of the mountain, where total darkness
prevailed for more than two days. A lake of very hot water now occupies
the bottom of the great new crater. The noise of the explosion was
heard for at least 750 miles.
[Illustration: Fig. 18.--Diagrammatic vertical or structure section
through a portion of the earth illustrating the common modes of
occurrence of igneous rocks. P, deep-seated (plutonic) igneous rock; S,
strata; D, dikes; M, mass of igneous rock forced between strata bending
them upward; F, feeding channel of volcano; V, volcano; L, lava sheets.
(By the author.)]
One of the most frightful volcanic catastrophes of recent years was the
eruption of Mont Pelée, island of Martinique, West Indies, in 1902.
In this case, also, no lava was poured out, but violent explosions
sent great clouds of very highly heated gases and vapors, mingled with
incandescent dust, thousands of feet into the air. One of these great
clouds rushed down the mountain at hurricane speed and destroyed the
city of St. Pierre with its 30,000 inhabitants. After the main eruption
a spine or core of hard rock began to rise out of the crater and it
slowly grew to a height of 1,000 feet in several months, after which it
began to crumble away. This spine probably represented nearly frozen
lava which solidified as it was gradually forced out of the mountain.
Of special interest to us, though not of great importance is the only
active volcano in the United States. In May, 1914, Mount Lassen (or
Lassen Peak), a long inactive volcano in northern California, suddenly
burst forth explosively and during the next several years hundreds of
eruptions occurred. Little or no lava appeared, but great clouds of
steam and dust often shot into the air from one to three miles above
the top of the mountain, which lies over 10,000 feet above sea level.
(Plate 10.) Great quantities of dust have accumulated for miles around
the mountain. At this writing (October, 1920) the mountain is again
active.
It should not be presumed, however, that all, or nearly all, volcanoes
are of the explosive type. Others of the more quiet type are well
exemplified by the two great Hawaiian volcanoes, Mauna Loa and
Kilauea. Any but relatively very minor explosions rarely, if ever,
occur, the product of such volcanoes being almost wholly lava, which
flows down the mountainsides in molten streams. The Hawaiian Islands
have, in fact, been almost entirely built up by successive eruptions
of lava, the building-up process having begun well below sea level.
Mauna Loa rises to nearly 14,000 feet above the sea, but, due to the
fact that the streams of lava have spread so far, the mountain has an
exceptionally low angle of slope which makes it difficult to realize
that it is so high. Considering its submarine portion, Mauna Loa
really rises nearly 30,000 feet above the sea floor. Although Kilauea
lies nearly 4,000 feet above sea level on the flank of Mauna Loa, and
only twenty miles distant from it, the two volcanoes are singularly
independent in regard to their eruptions. Each mountain has a crater
irregularly oval in shape, nearly three miles long, bounded by almost
vertical walls of hard lava, in some cases arranged in terraces. The
floors of the great crater pits are relatively level, and consist of
black lava in which are lakes of molten and even boiling lava. The
black lava floor is, in each case, only a frozen or hardened crust upon
a great column of molten lava extending down into the mountain. Prior
to an eruption of Mauna Loa the lava column rises hundreds of feet in
the crater, but during recent years the lava seldom, if ever, flows out
over the crater rim. Instead, it breaks through the mountainsides at
various altitudes, the great flow of 1919 having started at an altitude
of about 8,000 feet. This stream of liquid rock, fully one-half of a
mile wide, flowed for weeks down the mountainside and into the ocean,
the waters of which, in contact with the highly heated lava, were
thrown into terrific commotion. In 1885 a stream of lava several miles
wide flowed forty-five miles. In one case, lava traveled the first
fifteen miles in two hours, but this is an unusually great rate of
speed. Lava streams in general seldom move faster than one or two miles
per hour, and as the liquid rock gradually cools and becomes more and
more viscous, the speed diminishes to zero. Almost incredible volumes
of steam emanate from streams of molten lava.
In 1840 an outflow of lava took place from the side of Kilauea Mountain
and ran into the sea. Since that time the floor of the great crater
pit (quoting Professor W. H. Hobbs) "has been essentially a movable
platform of frozen lava of unknown and doubtless variable thickness
which has risen and descended (hundreds of feet) like the floor of an
elevator car between its guiding ways. The floor has, however, never
been complete, for one or more open lakes are always to be seen, that
of Halemaumau, located near the southwestern margin, having been
much the most persistent. Within the open lakes the boiling lava is
apparently white hot at a depth of but a few inches below the surface,
and in the overturnings of the mass these hotter portions are brought
to the surface and appear as white streaks marking the redder surface
portions. From time to time the surface freezes over, the cracks open
and erupt at favored points along the fissures, sending up jets and
fountains of lava, the material of which falls in pasty fragments
that build up driblet cones. Small fluid clots are shot out, carrying
threadlike lines of lava glass behind them, the well-known 'Pelée's
hair.' Sometimes the open lakes build up congealed walls, rising above
the general level of the pit, and from their rim the lava spills over
in cascades to spread out upon the frozen floor."
In some regions, like the Columbian Plateau of the northwestern United
States and the Deccan of India, each covering about 200,000 square
miles, vast quantities of lava have been poured out layer upon layer to
depths of even thousands of feet. Distinct volcanic cones or mountains
in those regions are either absent or too scarce to look to as sources
of so much lava. Such lava floods were probably mostly erupted from
great fissures in the earth's crust, the fluidity to spread many miles.
Some idea of the quantitative geological importance of volcanism may be
conveyed to the reader when we assert that, according to a conservative
estimate, fully one-half of a million cubic miles of molten rocks have
been poured out upon the surface of the earth through volcanic action
in relatively recent geological time! The Cascade Range with its lofty
peaks, including Mount Shasta and Mount Rainier, each rising more than
14,000 feet above the sea, has been built up very largely by volcanic
action during the last era of geologic time. Many other mountain peaks
and various ranges have been similarly developed either wholly or in
part. The great chain of Aleutian Islands extending hundreds of miles
into the sea, is the scene of much volcanic activity where a great
mountain range is now literally being born out of the sea by the
processes of vulcanism.
Before this the reader has more than likely wondered about the source
of the heat, vapors (mainly water), and power involved in volcanic
action. Answers to these questions are closely tied up with the precise
cause (or causes) of volcanic action which remains one of the most
uncertain of the larger problems of geologic science. Before briefly
discussing the causes, a few additional facts should be stated. First,
in regard to the heat, a careful determination of the temperature
of the molten lava of Kilauea in 1911 showed it to be 1,260 degrees
Centigrade, or 2,300 degrees F. This is, however, a relatively low
temperature, because many lavas from other regions show melting points
all the way up to at least 2,000 degrees Centigrade (3,600 degrees
F.). Water in the form of steam is quantitatively one of the greatest
products of volcanoes. A fair idea of the tremendous volumes of water
involved may be gained from the statement that a careful estimate shows
that fully 460,000,000 gallons of water in the form of steam erupted
from a single secondary cone of Mount Etna during a period of 100
days. Among other gases which are given off in greater or less amounts
during volcanic activity are carbonic acid gas, sulphureted hydrogen,
sulphur dioxide, and hydrochloric acid. Some idea of the power back of
volcanoes may be gained not only from the tremendous explosions such
as those above described, but also from the fact that the pressure
necessary to raise the column of lava from sea level to the top of
Mauna Loa (nearly 14,000 feet) is about 1,150 atmospheres, or about
17,000 pounds per square inch. The actual pressure must there be much
greater because the lava is forced up from far below sea level.
A long-held idea that a relatively thin crust covers a molten interior,
and that downward pressure of this crust due to earth contraction
causes molten rocks to be forced out, has been too thoroughly disproved
to now be at all seriously entertained. The fact that near-by volcanoes
commonly erupt entirely independently, as in the case of Mauna Loa
and Kilauea, shows that there can be no universal liquid beneath a
relatively thin crust. Other arguments against liquidity of the earth's
interior are that the earth acts like a body nearly as rigid as steel
against the powerful tide-producing forces, and that earthquake waves
which pass through the earth to a depth of at least 2,000 miles are the
kind which require a solid medium for transmission.
Let us then briefly consider more plausible views regarding the cause
of volcanic action. First of all we may be sure that the earth is
highly heated inside. Measurements in many deep borings show that the
temperature increases at the rate of about 1 degree F. for each 50 to
60 feet downward, to depths greater than a mile. Accordingly, on the
basis of 1 degree rise in 50 feet, at depths of 20 to 35 miles, the
temperature must be great enough (2,120 degrees to 3,590 degrees F.),
to cause all ordinary rocks to melt _if they were at the surface_. At
such depths, however, the downward pressure upon the rocks is so great
that their melting points are notably raised, and there is every reason
to believe that under ordinary conditions the rocks 20 to 35 miles down
are not molten. If we adhere to the older (nebular) hypothesis of earth
origin, the interior heat of the earth is left over from the cooling,
once molten, earth. On the basis of another (planetesimal) hypothesis,
the earth's heat is due to the steady, powerful action of gravity
causing the earth to contract. In any case, the earth is hot inside as
proved by deep well records and igneous phenomena in general, and it is
a contracting or shrinking body as proved by the many large scale zones
of wrinkling or folding of rocks. If, then, highly heated solid rocks
at reasonable distances down in any part of the earth are subjected
to relief of pressure by an earth movement such as upward crumpling
of the crust, or by readjustment of large fault blocks, such heated
solid rocks would become molten. The very earth movement which brings
about relief of pressure and melting may very reasonably be regarded
as the power which forces some of the newly formed molten material
higher up into the earth's crust, and even out upon the surface. This
view harmonizes with the well-known fact, already mentioned, that the
main belts of active volcanoes are also the main belts of active earth
movements, such as earthquakes.
Another source of power behind volcanic action is steam pressure. We
have already mentioned the fact that vast amounts of water in the form
of steam escape from volcanoes or even from streams of molten lava. The
violent volcanic explosions are quite certainly all, or nearly all,
direct results of sudden giving way of volcanoes to steam pressure
which accumulates during greater or less periods of time, and with
little or no possibility of escape, without rupturing the mountain.
Steam alone, or combined with some of the other gases so common as
volcanic products, may also aid in forcing out molten rock. What is
the source of the steam and other gases or vapors? According to one
view they were originally in the earth, while according to another view
the water at least has been absorbed by the molten rocks from surface
waters which worked their way downward. At least two arguments oppose
the second hypothesis: first, that not a few volcanoes are really many
miles from the sea or other bodies of water, while downward percolation
of rain water would fall far short of supplying the tremendous
quantities of water ejected, and second, any water taken up by molten
rock must be absorbed within a very few miles of the surface because,
as we have learned, farther down there are no openings large enough to
permit the downward passage of water, but, as a matter of fact, the
very upper part of the earth's crust is just the place where molten
rocks begin to give up their water, often with terrific violence.
We may now turn to a consideration of the other very important kind of
igneous activity, namely, the rise and transfer of molten materials
within the earth's crust, but not to the surface. The quantity of such
deep-seated (so-called "plutonic") igneous rock material which has
been intruded into the earth's crust within known geologic time, is
far greater than that which has been forced to surface, that is the
so-called "volcanic" material. The plutonic rocks are always thoroughly
crystallized, and they are generally coarser grained than the volcanic
rocks.
Where molten materials have been forced into cracks or fissures in the
crust of the earth and there congealed, we have a very common mode of
occurrence called "dikes" (Plate 9). In many regions often one set of
dikes was formed, after which one or more succeeding injections from
the same or different deep-seated bodies of molten rock took place,
and some of the later dikes were forced to cut across earlier ones.
Dikes of all lengths up to at least thirty miles, and of all widths up
to many hundreds of feet, are known, but they are generally less than
a mile long and not more than a few feet or rods wide. They have been
intruded into all kinds of rock formations--igneous, sedimentary, and
metamorphic. Dikes are common in many parts of the world and they often
excite the interest of lay-men. They are wonderfully displayed along
the southern coast of Maine. Plate 9 shows small dikes where the molten
material was forced from a larger mass into a body of older dark
rock. The Palisades of the Hudson River, just north of New York City,
consists of a layer of igneous rock several hundred feet thick which,
in the molten condition, was forced nearly horizontally between layers
of sandstone millions of years ago, that is in the early Mesozoic era.
The palisade or columnar structure was caused by cracking of the rock
during the cooling and contraction. This is the explanation of most
columnar structures of igneous rocks, exceptionally fine exhibitions
being at the Giant's Causeway in Ireland, and Devil's Tower, Wyoming
(Plate 10).
A type of occurrence not so common, but of special interest, is where
a body of molten rock rising in nearly horizontal strata becomes
cooler and therefore stiffer or more viscous and, losing its power to
penetrate, forces its way between the layers causing the strata to be
arched or domed over it. Sufficient removal of overlying material by
erosion has revealed many fine examples of this type of occurrence.
Another type of interest is the volcanic neck, which is the core or
plug filling the feeding channel of a volcano. In certain regions, like
parts of Arizona and New Mexico, extinct volcanic mountains may be all
cut away by erosion, except the central cores or necks which, both
because they are more resistant and are last to be reached by erosion,
stand out conspicuously as great towers on the landscape (Plate 9).
Most important of all from the quantitative standpoint, however, are
the great bodies of igneous rocks, ranging up to many miles across,
which, in a molten condition, were forced irregularly into the earth's
crust from unknown depths.
The common rock called granite belongs in this category of rocks, which
are the best and most extensively developed of all igneous types.
The roots or cores of great mountain ranges often consist of such
rocks which are exposed to view only after removal of great thickness
of overlying material. Immense areas of granite and other plutonic
rocks of extra deep-seated origin are exposed, because of removal of
overlying material by erosion, in southeastern Canada, the Adirondack
Mountains, New England, the Piedmont Plateau of the Atlantic Coast, and
in the Sierra Nevada Mountains. All the rock forming the lofty walls of
Yosemite Valley is granite, which was forced into the earth's crust in
relatively late Mesozoic time, and which has since been laid bare by
erosion.