VOLCANOES AND MOUNTAIN FORMATION
The great prominence which we have given in the preceding pages
to earthquakes is owing to the growing belief in the influence of
earthquakes on the appearance and structure of those portions of the
world's crust which are known to us. There are two views which we
can take of earthquakes. One is to regard the larger number of them
as being caused by slipping movements of the earth's crust. Looking
at things in this way we should say that whenever there was a sudden
break in the earth's strata, such as might occur (in accordance with
an illustration given in a previous chapter) if all the level strata
were broken up like a crumpled page of type--then that an earthquake
would result. So that whenever we saw what geologists call a "fault"
in strata we should know that an earthquake had occurred there. And
why did it occur? Well, if we had a massive column of steel or of
granite five miles high, the steel or granite at the bottom of the
column would have to sustain such an enormous weight of material
above it that it would begin to spread. If we had a pyramid of the
same materials five miles high, the tendency to spread would not be
so great, but still it would be there. Consequently, wherever there
are high mountains there is a tendency of the earth strata beneath
them to spread, perhaps slowly, but inevitably; and if there is any
weakness in the structure of the rocks near the base of the mountain,
then these will give way with a crash. A great "fault" will be
produced, and with it an earthquake.
People living on the earth will only see the results of the
earthquake on the ground just immediately below their feet; and there
these results are often very destructive to life and property; yet if
they were all that happened, we should expect them to be covered up
in time, and the "geological record" of an earthquake would not be a
very important or even discernible thing a million years after it had
happened. But are these things, which the eye of man can perceive,
the only things that are happening during an earthquake? Is nothing
happening underneath the earth which will leave its mark thousands of
years after man has left the spot where the earthquake took place?
May it not be that the earthquake is the outcome of some mighty
force deep down in the earth; and may not this force cause both the
earthquake and the geological "fault" which remains as the witness
of its occurrence? If this be the case then the earthquake may be of
enormous importance in geology.
We regard an earthquake, as we see it, as a destructive force.
That is because it destroys the works of man. But earthquakes are
doing constructive work as well; or, at any rate, they are usually
present when constructive work is being done. Destructive forces,
such as erosion, are wearing down the structure of the globe, while
earthquakes are the only known forces that are building it up. It
is true that when an earthquake occurs rocks often fall, loose
sediment is shaken down, and other settlements occur, but the real
constructive work consists in upheavals, little by little, as it may
be, of beaches, islands, coasts, plateaux, and perhaps larger areas.
These elevations are actually witnessed in certain earthquakes.
Many islands in the sea have been raised from time to time within
even living memory.
The south-western part of the island of Crete has been elevated
within the historical period.
The region about Pozzuoli and the Bay of Naples has suffered both
elevation and depression. There is the famous instance cited by
Sir Charles Lyell nearly eighty years ago of the Temple of Jupiter
Serapis. This temple had many columns; and they are now situated
on dry land. The pillars are forty-two feet in height, and for
twelve feet upwards they are of smooth undisfigured marble. Then for
another twelve feet they are pitted with the holes made by a marine
shell-fish called _Lithodomus_, the stone-dweller. What are we to
judge from this? The temple was first built on dry land. Then the
land sank taking the temple with it, and the columns were submerged
in sea sediment to a depth of some thirty feet above their pedestals.
The lower portions of these pedestals were preserved intact, but
the marine shell-fish found a home in the upper part of the marble
columns, and pierced them with the channels and grooves. After this
had gone on for a number of years the elevation of the land lifted
the temple and its columns clean out of the sea again, and the marine
shell-fish could no longer live in the columns. But the traces of
their habitation remain.
The elevation of this coast was actually witnessed at the time of the
eruption of Monte Nuovo in 1538. Moreover, the raising of the land
was perceived on a larger scale round the whole of the Bay of Naples
during the eruption of Vesuvius in April, 1706. Professor Lorenzo
found the elevation of the land at Pozzuoli to be six inches, and at
Portici one foot.[14] The foundations of both Etna and Vesuvius were
ages ago laid in the sea.
[Footnote 14: The coast about Pozzuoli is now sinking again.]
In almost every part of the world there are raised beaches, such as
we have already mentioned in the neighbourhood of Valparaiso, on the
Chilian coast. The idea has been put forward by Dr. T. J. See that
the same cause which produces earthquakes produces these elevations
of the land and produces also volcanoes. There are many circumstances
which favour this idea. Let us consider what is happening at the
bed of the sea. Some years ago, when certain officers of the United
States Navy were making ocean surveys, it was found that if hollow
balls of thick glass were sunk to great depths in the ocean, they
came up more and more completely filled with water in proportion as
the depth increased, though no breakage or cracking of the glass had
occurred, and no holes in it could be discovered even by the best
microscopes. In other words, it became evident that the water had
been slowly but bodily forced through the thick walls of the glass
(under a pressure of less than 15,000 lb. to the square inch) in less
than an hour's time. Evidently, then, even such a substance as glass
will be penetrated by water if the pressure is great enough.
To make a practical application of these principles, what shall
we now say with respect to the ocean bottoms? In deep places the
pressure of the sea-water on them is very great, sufficient to force
water through glass. Obviously most of these bottoms will leak, and
leak at a rapid rate under the enormous pressure operating in the
greatest depths of the sea. The bed of the ocean will not leak with
equal rapidity in all places; but almost universal leakage will
certainly develop, and the water will be driven back into the earth
at various rates. Where the rock is volcanic and badly fractured,
or sandy, the leakage will be most rapid; and where the bed is made
of clay or unbroken granite the leakage will be much more gradual.
It will also depend on the depth of the sea, and will be greatest
where the ocean is deepest, and quite insignificant in shallow
water. A rapid rate of leakage would mean that large quantities of
water quickly come in contact with the heated rock, and develop
correspondingly great steam pressure in the crust which underlies
that part of the ocean. One case in which we may suppose a rapid
leakage to be taking place is in the case of volcanoes near the
sea. In the case of lava pouring from a volcano, it is observed that
the molten rock emits vast quantities of vapour, of which, according
to Sir Archibald Geikie, 999 parts in 1000 are steam. The enormous
volume of these has been brought home to us in recent years by the
behaviour of the volcano Mount Pelée, from which for several years
after the great eruption which devastated Port au Prince the vapours
rose in clouds that were to be measured in cubic miles. Similar
observations about the quantities of vapour ejected by volcanoes have
been made in Japan.
While speaking of Mount Pelée we may recall another phenomenon
connected with it, which also appears to bear out the supposition
that in the volcano's activity the action of steam takes a very
large share. After its first outburst Mount Pelée continued to pour
out lava and great quantities of vapour, as if like some gigantic
cauldron it were being fed with fresh supplies of water; and there
in the early March of the following year a most amazing thing took
place, under the very eyes of a celebrated investigator of volcanoes,
now dead, Professor Angelo Heilprin, who was remaining on the island.
A great obelisk of andesite (a stone not unlike basalt) was forced
up from the crater. It rose rapidly, as much as five feet a day; and
it reached altogether a height of 840 feet above the crater's lip.
It was calculated to be about 300 feet in diameter at its base. It
continued to push itself up for some months, sometimes sinking a
little, sometimes rising like a colossal piston above a steam boiler.
Its greatest height was 1100 feet above the height of Mount Pelée,
and therefore at a height of 5143 feet above the sea-level.
[Illustration: The New Spine of Mont Pelée, showing Fissures and
Vertical Grooves
Photographed on March 15th, 1903. The spine was then 82 feet lower
than it became ten days later.]
[Illustration: The New Spine of Mont Pelée, viewed from the Basin of
the Lac des Palmistes
The apex, 1174 feet above the rim directly in front; the remains of
Morne la Croix on the edge of the crater at the right.]
A violent eruption would reduce its mass and its steeple-like
pinnacle; but after its losses it generally pushed up again.
Professor Heilprin at last got near enough to observe it, and the
obelisk was found to be not of pumice stone, as had at first been
suspected, but of the hard rock we have mentioned. It had, in fact,
been comparable to a Titanic cork of rock which had closed up some
vent far down in the crust of the earth, and which had at last been
lifted by the steam pressure beneath it. It finally sank back into
the crater, but it was replaced by a dome of rock which underwent
similar changes in height, though on a smaller scale, to those of
the obelisk. The dome of rock was, however, on a more massive scale
even than the obelisk, and at one period of its career a spine, 100
feet in height, like a smaller obelisk, was pushed up through its
middle. This dome was examined by the explorers, the Abbé Yvon and M.
Beaufroy, who found that the dome was a great mass of andesite, while
about it were fragments of the rock of which the obelisk had been
composed. They wrote at the time:--
"It is an error to suppose that there exists in the bottom of Mount
Pelée a hole from which lava and gases have come out. At present
there is a tremendous cork of andesite, which is called the 'Dome,'
and which must have as its dimensions a diameter half a mile across
at its base and a height of about 1200 feet. On all sides of the dome
there are fumaroles (small cone-like craters), some of which throw
out a reddish smoke, others of which discharge white smoke, and
others are still surrounded with a carpet of sulphur several yards in
depth."
After the great eruption of Mount Pelée in 1902 it was found by
measurement that a considerable portion of the adjacent sea bottom
had sunk down many fathoms. It is impossible to believe that this
sinking had been caused by the mere shaking of the earthquakes
accompanying that eruption. We must, therefore, suppose that after
the dreadful explosions which destroyed St. Pierre and devastated
Martinique a subsidence near the roots of the mountain (which is just
by the sea) took place. What we should judge to have happened is that
by some means an explosion took place below the sea bottom; that
parts of the molten rock, moved by the forces of the explosion, were
moved towards the mountain (Mount Pelée), which thereupon broke into
eruption, acting as an outlet for the imprisoned rocks. When these
molten rocks were thus removed a great cavity was formed in the bed
of the sea, which accordingly caved in.
A similar explanation would account for the raising of the Chilian
coast-line after the great earthquakes of 1835, of which we have
already spoken. The coast and, indeed, the whole country back to
the Andes was slightly raised. This could only be explained by the
pushing in or forcing in of a corresponding bulk of lava under the
land; and this lava could come from nowhere except from under the
bed of the great trough in the adjacent sea. After an explosion
(which is caused by the sea penetrating through to the molten rocks)
the trough, where the "accident" first took place, would naturally
deepen.
[Illustration:
_Stereo Copyright, Underwood & U._ _London and New York_
The Dead City of St. Pierre, Martinique
The town of St. Pierre was perhaps the most beautiful in the West
Indies. The volcano of Mont Pelée, which is seen in the background,
and which is five miles away, suddenly belched out deadly gases,
dust, steam, and boiling mud, which overwhelmed the town and
completely destroyed it. The houses were reduced to ruins, and the
people were killed by the wave of hot gases sweeping down from the
volcano.]
Moreover, if we suppose that after a time the water again penetrates,
and again comes into contact with rocks that have again become heated
up, there will be another explosion, and yet others. Each of these
explosions will push along the ejected streams of lava, step by step,
till they reach the land, and even till they reach the mountains
bordering the sea. The forces thus arising would cause upheavals,
even if they did not cause earthquakes. Such forces might bend
strata, contort the rocks, and cause "faults."
But why, the reader may ask, do you suppose that all these explosions
of lava are directed to the land? We do not suppose that they are.
The lava may be forced away from the land. Then if that occurs a
ridge may be upheaved, or possibly a submarine volcano.
"At the Hawaii Islands on 25th February, 1877," writes Sir Archibald
Geikie, "masses of pumice during a submarine volcanic explosion were
ejected to the surface, one of which struck the bottom of the boat
with considerable violence and then floated. At the same time, when
we reflect to what a considerable extent the bottom of the great
ocean basin is dotted over with volcanic cones, rising often solitary
from profound depths, we can understand how large a proportion of
the actual eruptions may take place under the sea. The foundations
of these volcanic islands doubtless consist of submarine lavas and
fragmentary materials, which in each case continue to accumulate to
a height of two or three miles, till the pile reaches the surface
of the water and appears above it. The immense abundance and wide
diffusion of volcanic ash, pumice, etc., over the bottom of the
Pacific and Atlantic oceans, even at distances remote from land, as
has been made known by the voyage of the _Challenger_, may indicate
the prevalence and persistence of submarine volcanic action."
It is fairly clear, therefore, that the sea bottom is leaky, and that
volcanoes which are a consequence of it are scattered freely over
the deep ocean floor. In some places, of course, very few eruptions
occur, either because the underlying rocks are less leaky or the sea
is too shallow for pressure.
We have pictured the water of the oceans thus sinking down into the
hot rocks. It will not always cause an explosion at once. The steam
may not immediately become free, but will become absorbed in the
hot rocks till the lava grows so fully saturated by the hot vapour
that it swells and requires more space. When the tension becomes
great enough the crust begins to shake and the paroxysm continues
till the steam-saturated lava moves along the nearest break or
"fault" or vent. When the underlying molten rock has thus obtained
more space the agitation ceases till the tension again becomes too
powerful for the crust to withstand, when another readjustment takes
place. A familiar illustration of this process is seen in the lid
of a tea-kettle when the steam pressure accumulates till it sets
the lid quivering. As the steam escapes at the sides the agitation
slowly dies down and the lid then remains quiet till the accumulating
pressure again requires relief, when the shaking is renewed. Thus the
process is periodic, and the period depends on the rapidity with
which the steam is developed. In the case of earthquakes, as already
remarked, the steam is not free, but absorbed in the molten rock,
and when the agitation begins this gives a similar quivering motion
to the block of the earth's crust overlying it, and ceases only when
readjustment occurs--usually by the neighbouring "fault" slipping in
some way so as to give more space to the swelling lava beneath. Of
course, many of the cracks caused by this swelling are never seen;
and the molten lava seldom reaches the surface except when through
volcano vents or cracks in mountains that are near the sea-shore; but
such outbreaks are probably more common in the deep sea.
To see how effective the pressure arising from the depths of the
ocean may be in driving water into the crust of the earth, we may
observe that the tendency to penetrate is everywhere proportional to
the depth of the sea. Now everybody knows that if a cistern be placed
at the top of a house and connected with a fountain in the garden the
fountain ought to throw a jet as high as the cistern because water,
as the saying goes, always rises to its own level. As a matter of
practice the water does not rise so high because of the resistance of
the air. But for theoretical purposes we may consider the proportion
true, and we might similarly say that the pressure in a sea one
mile deep would thus throw a stream a mile high; in a sea two miles
deep, two miles high; and so on. Now some of the ocean depths exceed
five miles, the greatest, near Guam, being 5269 fathoms, almost
exactly six miles. Is it therefore any wonder that the deeps east
of Japan, near the Aleutian Island, west of South America, near
Guam, between Samoa and New Zealand, give rise to enormous leakage of
the sea bottom, and consequently many world-shaking earthquakes? A
comparatively feeble pressure of water, such as hydraulic engineers
use in mining, rapidly cuts away hills and washes out all their gold;
in the same way the waters of Niagara, falling through only 160 feet,
slowly wear away the solid rock over which they pour. What, then, may
be expected of a constant water pressure which will throw a jet five
miles high? Such is the pressure all over the bed of the Tuscarora
Deep, and it continues from year to year, century to century. It is
this pressure which forces the water so rapidly into the earth, and
gives rise to all the great earthquakes and sea-waves with which
Japan is afflicted. No stone on earth, however thick its layers,
could withstand such a pressure; nay, under it the water would go
through the hardest metals, and sink down deeper and deeper into the
bowels of the earth. Thus subterranean steam would arise beneath the
crust and accumulate till relief was afforded by a shaking of the
earth.
Thus we see how immensely important the same causes that give rise
to earthquakes may be in moulding the outlines and contours of the
rocks and the "everlasting hills." In the present state of geological
knowledge we cannot say that these steam explosions are the sole
causes of mountain building, but it is evident that they must play
a great part in them. The action of the submarine explosions may
be compared to a man digging out a trench. As he digs along the
trench, the earth that he excavates he throws on to either side of
the trench, so that a ridge appears on each side of the excavation.
The result is the same in the case of the continuous lava explosions
in deep seas, especially in those deep seas like the waters off the
west coast of South America, where a great range of mountains runs
parallel to an ocean that is of great depth only a short distance
from the land. A trough or trench is cut downwards by successive
explosions and expulsions of lava. As the trough is arched downwards
like a broad letter =U=, the steam pressure from beneath cannot
easily force it upwards. What will therefore happen? Imagine what
would happen in a steam saucepan or kettle if the vapour could
not get out at the top or lid. It would tend to blow out at the
sides. Or if you think of a slab of dough rising under the effect
of yeast. Suppose the baker presses a flat board on the top of the
rising dough, and presses down on it so that it cannot force its way
upwards. It will then naturally spread out to the sides. Similarly
the rising yeasty lava under the curved ocean bed has to force its
way sideways under the crust. It forces its way partly towards the
land--where the mountains run along the coast as in the case of the
Andes--or farther out underneath the ocean. Generally the movement
of the lava will be towards the mountains till the trough gets broad
and deep and the mountains very far away, and so high that their
weight offers unexpectedly great resistance to the underground stream
of lava. Then the release will at length become easier towards the
ocean by the forcing up of ridges or volcanoes along the other margin
of the trough. Ridges with peaks in them will usually result, and
this is the beginning of a new range of mountains in the sea, which
are destined to rise slowly from it parallel to the great range
of mountains on the shore. There may thus be two parallel ridges,
perhaps hundreds of miles apart, with a valley between them. This
valley may be drained in the course of ages, or filled in by the
processes of erosion which we have described in the earliest chapters
of this volume.
It will be of interest to quote at this point what Pliny nearly two
thousand years ago said in his _Natural History_ (Book II) on islands
which have been uplifted from the Mediterranean, evidently as the
result of volcanic causes:--
"Land is sometimes formed in a different manner, rising suddenly out
of the sea, as if nature was compensating the earth for its losses,
restoring in one place what she had swallowed up in another. Delos
and Rhodes, islands which have now been long famous, are recorded
to have risen up in this way. More lately there have been some
smaller islands formed: Anapha, which is beyond Melos; Nea, between
Lemnos and the Hellespont; Halone, between Lebedos and Teos; Thera
and Therasia, among the Cyclades, in the fourth year of the 135th
Olympiad. And among the same islands, 130 years afterwards, Hiera,
also called Automate, made its appearance; also Thia, at the distance
of two stadia from the former, 110 years afterwards, in our own
times, when M. Junius Silanus and L. Balbus were consuls, on the 8th
of the Ides of July.
"Opposite to us, and near to Italy, among the Æolian Isles, an island
emerged from the sea; and likewise one near Crete, 2500 paces in
extent, and with warm springs in it; another made its appearance in
the third year of the 163rd Olympiad, in the Tuscan Gulf, burning
with a violent explosion. There is a tradition, too, that a great
number of fishes were floating about the spot, and that those who
employed them for food immediately expired. It is said that the
Pithecusan Isles rose up in the same way in the Bay of Campania, and
that shortly afterwards the mountain Epopos, from which flame had
suddenly burst forth, was reduced to the level of the neighbouring
plain. In the same island it is said that a town was sunk in the sea;
that, in consequence of another shock, a lake burst out, and that,
by a third, Prochytas was formed into an island, the neighbouring
mountains being rolled away from it."
There are, no doubt, other causes which warp and bend strata. We have
compared the earth to the core of a tightly wound golf ball--always
in a state of strain. The strain at great depths below the surface
might amount to several tons to the square inch, and it can easily
be understood that breaks might occur in consequence, especially if
some slight additional shock set the rocks into vibration. In the
deep copper mines of the northern peninsula of Michigan the behaviour
of the whole earth, with respect to earthquakes and stresses due
to other causes, is well illustrated on a small scale. At certain
times during each day blasts are set off in the solid rock at
various places in each mine. Each battery of blasts is a miniature
earthquake. In that particular spot, the earthquake centre, the rock
is fractured within a space limited by a radius of a few feet. Within
a large space, limited by a radius of a few hundred feet, elastic
vibrations are set up in the solid rock which are sufficiently
violent to be perceptible to the touch and to the hearing. Within
this larger space no fracture of the rock occurs. Feebler vibrations
doubtless extend out for miles from the point of fracture, just as
vibrations extend over the whole earth from an earthquake centre.
Now it also happens that in the lower levels of these deep mines,
at a mile below the surface of the earth, the solid rock is slowly
yielding, in a non-elastic manner, under the influence of the great
weight above it, so that the larger openings are gradually closing
up. This is so clearly recognised and progresses so rapidly that
it is proposed as routine practice, at the deep levels in these
mines, to take out the ore at the distant end of each drift first.
The miners will then work back slowly toward the shaft from which
the drift is entered, while the spaces in which they have recently
laboured gradually close up behind them. The gradual collapse known
to be in progress occurs apparently by imperceptible flow and by
minor fracturing, but not, as a rule, by catastrophes which close up
any opening suddenly. In this respect it is an epitome of what is
taking place every year in the failing earth as it yields under such
stresses.
There may be local tremors due to causes which are less immense and
world-wide. One such cause might be the collapse of cavities in the
earth. We are well acquainted with some such caves near the surface
of the earth. These caves, especially in limestone, are commonly
caused by the action of springs. Even pure water will dissolve a
minute quantity of the substance of many rocks, and rain water is far
from being chemically pure water. It takes oxygen and carbonic acid
out of the air as it falls, and it abstracts acids out of the soil
through which it sinks. The presence of this acid gives the water a
greatly increased power of attacking carbonate of lime. Now limestone
is a rock almost entirely composed of carbonate of lime. It occurs
in most parts of the world, covering sometimes tracts of hundreds
or thousands of square miles, and often rising into groups of hills
and ranges of mountains. The abundance of this rock offers ample
opportunity for the display of the dissolving action of subterranean
water. The water trickles down the vertical fissures along the planes
below the limestone beds. As it flows on it dissolves and removes the
stone till in the course of centuries these passages are gradually
enlarged into clefts, tunnels, and caverns. The ground becomes
honeycombed with dark subterranean chambers, and running streams fall
into these chambers and continue their course underground.
In England there are famous "pot-hole" caverns in Yorkshire and the
west of England. The Peak Cavern in Derbyshire is believed to be
1200 feet long, and in some places 120 feet high. The caverns of
Adelsberg near Trieste have been explored to a distance of several
miles. The River Poik has broken into one part of the labyrinth of
chambers through which it rushes before emerging again to the light.
"Narrow tunnels," says Sir Archibald Geikie, "expand into spacious
halls, beyond which egress is again afforded by low passages into
other lofty recesses. The most stupendous chamber measures 669 feet
in length, 630 feet in breadth, and 111 feet in height. From the roof
hang white stalactites which uniting with the floor form pillars
showing endless varieties of form and size." Still more gigantic is
the system of subterranean passages in the Mammoth Cave of Kentucky,
the accessible parts of which are believed to have a combined length
of about 150 miles. The caverns of Luray, in Virginia, are scarcely
less wonderful; and in their case American ingenuity has hit on the
idea of sucking the pure, dustless air out of these caverns in order
to ventilate a sanatorium. Indeed, a book might easily be written
on the wonders of the limestone caverns of the world, but our only
purpose in mentioning them in this chapter is to indicate how the
rocks of the earth may be made unstable, so that a slight shock may
precipitate a catastrophe in them--a kind of subterranean landslip
which in its turn may give rise to some of the symptoms of earth
tremors.
[Illustration: A Yorkshire Pot-Hole: showing the Effects which can be
Produced in Limestone by Underground Water
The immense depth may be better realised by comparing the pot-hole
with the Nelson Monument, which is 162 feet in height.]