THE VOLCANO.
“And it bubbles, and seethes, and hisses, and roars,
As when fire is with water commixed and comblending,
And a hell-molten surf thunders wild on its shores,
While a red-tumbling flood from its caverns outpours,
Hurling hills from their place, and the mountains downrending,
So the chaos eternal
Born of fury infernal,
Boils and belches and rumbles, unreined and unending.”
It is an axiom that there are three misstatements in the popular
description of a crab: “A fish, of a red color, that runs backward.”
_Ques._ What is a volcano?
_Ans._ A volcano is a burning mountain, from the summit of which issue
smoke and flames. (_Old Geography._)
The writer remembers the surprise he felt when a lad of nine, full of
childish confidence in the infallibility of text-book misinformation, on
reading in Prescott’s “Conquest in Mexico” that Cortez obtained sulphur
to replenish his stock of powder by lowering one of his soldiers into
the crater of Popocatepetl. He wondered how so reputable a historian as
Prescott had been induced to credit such an extravagant “yarn” on the
part of the Spanish chronicler. To his youthful fancy, fired by the
teachings of primary geographies, a volcano was a sort of chimney to a
titanic iron furnace in full blast; indeed, he would have supposed it
safer to descend into an iron furnace than into the crater. He
speculated long on the matter, and wondered if fire-proof dresses were
known in those days.
No small part of the non-traveling public has similar misconceptions of
the character of volcanoes; and to obtain the truth it is not so
necessary to learn as to unlearn. The description quoted from the old
text-book is false in every particular. The mountain can not be said to
be burning any more than melted lead. Nor does anything that answers to
either smoke or flame issue from it. Be it known to all, that the
greatest portion of the surface of an active crater is usually covered
with a solid crust, in which there may be a small fiery lake, or inner
secondary cone or crater. Into many craters it is possible therefore to
descend: and into one volcano of the Mediterranean Sea an enterprising
Scotch firm have long had quite remunerative chemical works.
The name is taken from one of the Lipari Islands--a small group near
Sicily--which was known to the ancients as Vulcano. When the Romans
imported the Grecian god, Hephaistos, to be their chief blacksmith, they
assigned him Vulcano as his forge, and rechristened the lame old fellow
with the adjective appellation of Vulcanus.
Men much prefer the marvellous or mysterious to the true. And, while
their reverence is of a merely superstitious sort, the reverence of the
ignorant often surpasses that of the learned. A superstitious people
readily manufactured a myth to explain the awe-inspiring demonstrations
of volcanoes; and the myth itself, because of its religious character,
would discourage any attempt to closely investigate volcanic phenomena
as sacrilege and impiety. There are similar volcano myths in the island
and Asiatic world. So firm was the belief of the Sandwich Islanders in
the certainty of dire vengeance upon all who trespassed on the domain of
Pele, the goddess of Kilauea, that when a princess of the blood royal
safely defied the goddess, ate her sacred berries, and threw rocks into
her boiling lake, the people at once abandoned their whole race of gods.
If there was no Pele, they knew of no god. Such reasons prevented the
ancients and the barbarian world from obtaining any light on volcanic
action.
Similar causes operated with equal force to hinder investigation during
the middle and dark ages. Christian teachers seized upon them as
convenient openings to the abode of eternal torment. The Arian heretic,
the Emperor Theodosius, was assigned to Vulcano; while poor Anne Boleyn,
for whose sake the “Defender of the Faith” defied the Pope, was sent by
the latter to Mt. Ætna, as the shortest route to her destination.
Similar ideas are noticed among semi-barbarous races. The Aztecs deemed
Popocatepetl, the greatest of their volcanoes, to be the place of
punishment for wicked rulers. These gentry were supposed to cherish no
good will toward their subjects, whose complaints had brought them to
that place of torment, and to be always seeking opportunity for
vengeance. The people held them in great awe, and were wont to invoke
the aid of the gods when it became necessary to travel near the
volcanoes. It is related that the high priest, Tezozomoc, was wont to
give aloe-leaves inscribed with sacred characters, to such persons.
These leaves were amulets to preserve the wearer from harm. Southey uses
this story in _Madoc_:
“So ye may safely pass
Between the mountains, which in endless war,
Hurtle with horrible uproar, and frush
Of rocks, that meet in battle.”
This mountain was in eruption when Cortes reached Tlascala on his march
to Mexico. It was believed to bode evil to the people of Anahuac.
Learning the native superstition, Diego de Ordaz, captain of artillery,
determined to beard the demons in their den, and with some companions
ascended the mountain. Their safe return convinced the natives that the
Spaniards were in league with the spirits, and did much to dishearten
them. In memory of this feat, the Ordaz family has a volcano pictured on
its coat of arms.
The Javanese call their greatest volcano Maha-Meru. Meru, in the
Sanscrit mythology, was the home of Brahma, and the Malays, having
adopted the legend, consider their greatest volcano the fittest symbol
of his throne and power.
[Illustration: AT THE SUMMIT OF POPOCATEPETL.]
Virgil’s Æneid affords a passage containing the Roman myth concerning
Mt. Ætna, and showing that the people of Virgil’s day were acquainted
with the phenomena of that mountain. Thus Dryden has translated:
“The flagging wind forsook us with the sun,
And wearied, on Cyclopean shores we run.
The port, capacious and secure from wind,
Is to the fort of thundering Ætna joined,
By turns a pitchy cloud she rolls on high,
And flakes of mountain flames that arch the sky;
Oft from her bowels massy rocks are thrown,
And shivered by the force, come piece-meal down;
Oft liquid cakes of burning sulphur flow,
Fed from the fiery springs that burn below.
Enceladus, they say, transfixed by Jove,
With blasted limbs came trembling from above,
And where he fell, th’ avenging father drew
This flaming hell, and on his body threw.
As often as he turns his weary sides,
He shakes the solid hill, and smoke the heaven hides.”
This conception was borrowed from the Greeks, one of whose poets has
told us
“How shaggy-breasted Typhon lay,
From sea-girt Cuma to Trinacria’s bay.”
Yet, even among the ancients an occasional great mind disregarded
popular superstition, and enunciated just and rational views upon the
matter. The elder Pliny lost his life in an effort to observe closely an
eruption of Vesuvius. But the ideas advanced by these men were speedily
forgotten; and the exact scientific examination of volcanoes is of the
past hundred years, the great Italian, Spallanzani, being the first to
publish a series of valuable observations on the volcanoes of his own
land.
The ancients were acquainted only with the few active volcanoes
distributed about the Mediterranean Sea, and the casual thinker might
hence suppose their opportunities for observation were quite limited.
But volcanic principles are the same everywhere, differing only in
violence. In the Lipari Islands is situated the volcanic cone of
Stromboli, which has been in a state of constant activity, though very
mild, for at least two thousand years. This affords excellent
opportunities for study, and much of our most valuable information on
the topic is derived from careful observation of it. When the wind is
steady in any quarter, a person may sit to windward for hours within a
few yards of the boiling mass, while the noxious vapors and gases are
borne away in the other direction.
The expulsive agent is in all cases steam, mingled to a greater or less
degree with other vapors or gases. Its operation may be simply
illustrated. Pure water does not readily boil over in any open vessel of
ordinary dimensions. But if the vessel be very deep in proportion to its
width, and the heat is applied at the base only, it boils over more
readily. Now, if instead of water, we substitute porridge, thick
molasses, or any similar thick or viscid material, the bubbles of steam
rise slowly; and if rapidly generated, they force the matter out at the
top ere they escape. Such bubbles as reach the top, burst, throwing tiny
particles of the mass into the air.
How great a portion of the material expelled from volcanoes consists of
steam and other gases is not easy to determine. But that the quantity of
vapor is enormous is indisputable. Vesuvius is noted for the “pine tree”
of vapor that overhangs it. The ascending steam and gases, on reaching
an upper atmosphere as light as themselves, spread out horizontally in
every direction, thus much resembling in outline the stone-pines that
are a prominent feature in the Neapolitan landscape.
Some effort has been made to connect volcanic eruptions with atmospheric
pressure; for, say the theorists, a fall of two inches in the barometer
removes a pressure of over 2,000,000 tons from each square mile. A
sufficient answer to this is, that this, after all, is only one pound to
the square inch; while the force that can cast up volumes of melted
matter from a great depth must needs be many tons to the square inch.
Clearly these gentlemen would perch us on a sort of universal fire-box,
and poise the lid on a hair trigger.
But heavy rainfalls and terrific thunder storms are almost invariable
accompaniments of explosive eruptions. That these are the result and not
the cause of volcanic action is clear. An electrical machine was
invented by Sir William Armstrong, in which electricity was generated by
forcing steam at great speed through a narrow orifice. This same
principle would produce volcanic thunder storms. The immense volumes of
vapor, reaching the open air, must rapidly cool and be precipitated as
rain. The Italians dread these torrents, sweeping down immense
quantities of mud, more than they do the streams of lava.
If an eruption causes an immediate fall of two inches of rain over an
area seven miles square, it will be found that such a rainfall amounts
to more than seven millions of tons of water. Yet the rainfall often is
greater, and the area affected is larger; while it is not to be supposed
that the entire volume of vapor cast forth is at once precipitated on
the earth. This computation can not be assumed as anything more than a
mere illustration of the tremendous forces brought into operation.
The solid substances emitted by volcanoes are popularly styled ashes,
cinders, or scoria and lava. But what is called ashes would be more
appropriately named dust; for it is merely finely divided lava, and in
no way resembles genuine ashes.
Lavas present a general resemblance to the slag and clinkers of smelters
and brick-kilns, but vary considerably in appearance and chemical
composition. We need not touch this question further than to state that
oxygen forms nearly one-half the weight of all lavas, silicon one-fourth
of most, and aluminum one-tenth. From fifteen to twenty per cent. is
made of various others, magnesium, calcium, iron, sodium and potassium
being most common. Hence, the compounds present are always of the class
known to chemists as silicates, substances requiring great heat to melt.
These, from being long melted, abound more or less in crystals; but if
any one re-melts them and cools them suddenly, the result is a simple
glassy mass, with no trace of crystals.
[Illustration: VIEW IN ACTIVE CRATER.]
Scoria or cinders differ from ordinary lava only in the peculiarity of
having partially crystallized in some portions and then stiffened or
solidified while large bubbles were yet imprisoned or in the act of
bursting; thus leaving the mass very ragged and cellular. But if the
lava contains no readily formed crystals, the imprisoned bubbles of
steam slowly rise to the surface, and being greatly elongated by the
flowing of the lava, produce the beautiful material known as “pumice.”
It is to lava exactly what froth or foam is to water. Usually it is much
lighter colored than the lava on which it floats, for the same reason
that well-worked molasses candy is nearly white: they both contain a
vast number of minute air bubbles. Pumice floats on water, and its
decomposition being generally very slow, it drifts about the sea
currents, and is often found thousands of miles from any volcanic
region. In the immediate neighborhood of volcanoes it often accumulates
on the sea to such an extent that vessels can hardly force their way
through it. In the Sunda Islands it has been seen on the sea three feet
in depth. During the year 1878 the accumulation of pumice near the
Solomon Isles was so great that it took ships three days to force their
way through. Sometimes such masses accumulate along the coast line to
such an extent that a person can not readily tell where the shore line
is. One may land and walk about on the great floating raft of pumice,
unable to guess within even a mile of the actual shore. Deep sea
soundings show that the entire ocean bottom is covered more or less with
the pulverized pumice and volcanic dust. From the wide distribution it
is not probable that the layer attains any great thickness.
The Mangaians of the South Pacific told the earlier missionaries of a
feat of one of their heroes which at first was unaccountable. This
demigod, Maui, a sort of Pacific Hercules, raised the sky to its
present position. Not getting it high enough to suit him, he put his
head between the legs of his old father Ru, and heaved him and the
half-raised sky up together. Ru stuck fast among the stars, and Maui
left him there till his body dropped to pieces and his bones fell over
the ground below. To prove the truth of their story they brought the
missionaries bits of pumice, which they said were the bones of Ru.
Singularly enough the white porous stone looked much like bone. The myth
had been invented by the simple folk to explain the origin of pumice.
The ashes, or volcanic dust, is excessively minute, and in consequence
readily penetrate crevices that are hardly visible. Professor Bonney,
examining dust thrown out by Cotopaxi, has calculated that it would
require from four thousand to twenty-five thousand particles to make up
a grain in weight. The substance known as tufa is merely volcanic dust
or ash upon which rain has fallen while the former was still hot. The
resultant paste solidifies into a porous and loosely compacted rock.
Why some volcanoes nearly always throw out dust and fragments, while
others throw out mere molten streams, was for a time not clearly
understood. Some suggested the dust was the result of the continual
collision of fragments as they rose and fell, and hence would increase
in quantity as the eruption continued. This clearly would not meet the
case of volcanoes which, without showing any previous signs of activity,
burst into action with tremendous volumes of dust.
It was at length noticed that dust and fragments always were accompanied
by tremendous explosions; while eruptions of melted lava were far more
quiet, shocks being few and the explosions insignificant as compared
with the former. This gave a clue to the mystery.
Many liquids and solids have the power of absorbing vast quantities of
gas. Under pressure their absorbing powers may be vastly increased.
Sometimes the property appears only at high temperatures. Silver when
melted absorbs twenty-two times its volume of oxygen. If suddenly cooled
the oxygen is given off with a rapidity verging on explosion. This is
called the “spitting” of silver. Tiny cones and melted streams appear on
the cooled surface--volcanoes in miniature. The same property belongs to
the oxide of lead, and some other metals.
Now water can be made to absorb more than a thousand times its bulk of
ammonia; more than five hundred times its bulk of hydrochloric acid.
Alcohol may absorb three hundred times its volume of sulphurous acid.
Charcoal may absorb one hundred times its volume of ammonia, eighty-five
times its volume of hydrochloric acid, sixty-five times its volume of
sulphuretted hydrogen, fifty-five times its volume of sulphurous and
thirty-five times its volume of carbonic acid. Iron, steel and melted
sulphur absorb many gases.
We have already seen that immense volumes of gases are thrown off in
volcanic action. Now if a column of lava rises comparatively slowly in
its “chimney,” the imprisoned gases rapidly escape, producing violent
boiling, but not a positive explosion. But if it rises very rapidly, the
sudden removal of the pressure causes so sudden an expansion of the
compressed gases in its upper portion as to amount to a tremendous
explosion, which reduces the lava to microscopic dust.
This very principle was made practical use of in a mechanical
contrivance invented to make paper pulp out of common cane, such as the
farmer’s boy delights in for a fishing pole. The hard, woody fibre was
placed in a powerful iron cylinder full of water. A strong lid being
adjusted, the whole was heated far above the boiling point of water.
Naturally, every cell would be forced full of moisture by the immense
pressure. After some hours heating, the lid was suddenly removed, and by
the sudden expansion of the water into steam the cane was blown to
atoms.
[Illustration: CRATER OF ORIZABA.]
A beautiful product of the volcano of Kilauea is the substance known as
“Pele’s Hair.” Small particles of glass shot violently into the air
leave behind them long, glittering filaments, like gossamers. Birds
often build their nests of these beautiful threads. Man, taking a hint
from nature, has learned to manufacture the glass hair for himself by
passing jets of steam through the molten slag of iron furnaces. It much
resembles cotton wool, and is used for packing boilers and piston-heads,
and similar purposes.
The appearance of fire at the summit of a volcano is rarely ever real
flame. Any who has seen the peculiar appearance occasioned by brilliant
illumination on a moist or foggy evening may readily perceive the cause.
The phenomenon popularly known as the “sun drawing water” is of the same
character. The immense cloud of vapor ascending from the volcano glows
with the light sent up from the molten mass below. So it may be seen
brilliant by night, and only a dark cloud by day. Stromboli has been
called the light-house of the Mediterranean. In constant action, the
brilliant light at night slowly fades: then suddenly breaks out as
bright as before. This alternating results from the bursting of bubbles
in the crater, which expose a new, hot surface. This rapidly cools; then
another bubble bursts; and so the process continues. This may have
suggested the alternating light now in common use in great light-houses.
In the Galapagos, and other volcanic islands of the Pacific, occurs
another curious feature of volcanic action. Some places abound in
seeming mounds or domes, which may be sometimes readily broken in with a
heavy stone. These are produced by bubbles which partially cooled, when
the lava below found some rent or outlet in another quarter and flowed
away, leaving the solidified bubble.
Sometimes the cavern left by the retreating lava abounds in strange
beauties. A sailor, who with a comrade, explored one of these volcanic
caverns, gives the following account of it:
“In a sharp, deep valley of Albemarle we had broken in the roof of a
bubble; and as we looked in we saw we had opened the way into a tunnel
about fifteen feet in width, and extending either way as far as we could
see from our position. By the lights which entered from above, we made
out the floor as about twenty feet beneath us, and that the walls were
curiously marked with columnar forms. My companion, who had dabbled in
the sciences, proposed that we should take an underground view of
volcanic action and appearances.
“So, on the following day, provided with a couple of lamps, a coil of
knotted line, and a couple of waist-lines and iron poles for staves, we
proceeded on our exploration. We descended with the knotted rope around
our bodies, and stuck our feet into the rough side, lighted in our way
by a single lamp. We carefully watched for any side openings which might
confuse us or lead us astray in returning, but we saw none and felt
safe. It soon became evident that the tunnel had not been formed by a
rent of the mass after cooling, but rather by the molten lava’s having
drained away after a crust had formed upon it. This may account for the
singular and beautiful formations by which we found ourselves
surrounded. After proceeding some distance through a passage with a
pretty uniform width of fifteen to twenty feet, and of about equal
height, we paused to examine the formation of the cavern. The dim light
of our lamps illuminated the pilastered walls, and a roof raftered and
groined with straight and curved beams of crystalline structure many
feet in length. Some of these were of a reddish appearance, and others
had a vitreous lustre, resembling immense crystals, in places broken
into the semblance of foliage, which reflected an olive green light. The
gloomy splendor of this solemn architecture was relieved by the gold or
amber reflections of crystals of sulphur, which, like marigold or
sunflower, gleamed in the passage.
“The broad bases of the pilasters were enriched with counterfeits of
fern, palms, and growths intricate and delicate as the penciling of the
frost spirit’s pictures. But these metallic pictures, under the limning
of the fire-fiend had been inlaid with the brilliant facets of igneous
minerals, green and brown in tint. Tempted onward by the increasing
beauty of the scene, our lamp revealed new objects of interest in the
increasing lustre of the arched ceiling, and the carved and painted
walls. Our lamp was multiplied by the sparkle from the faces of unknown
minerals. In places the passage was divided by central columns of basalt
crystals, which terminated in curves, and were in form and tracery
varied beyond man’s power. The rude Goth for his cathedral, the Moslem
for his mosque, the Celestial for his pagoda, might have drawn
inspiration from this solemn portal to Nature’s vast workshop.
“As we advanced further into the recesses of the mountain, the character
of the cave changed. The angular crystalline forms which indicated the
sudden withdrawal of the molten matter, or the deposit of elements
sublimed by intense heat, yielded to smooth and rounded structures, like
the worn rocks of the river side, giving the impression that the walls
had served as a sluice to fiery torrents pouring from the volcano. A few
steps farther showed us the singular curtain-like foldings of a
substance resembling lampblack. Absolutely without lustre, and absorbent
of every ray of light, it was present, as it were, only to the touch.
With certain misgivings under this curtain of gloom, we entered a cavern
the form or extent of which could only be known by touch of hands, for
no possible brilliancy of light would command an answering reflection
from the absorbent surface. Broken as was the surface to the touch, to
the eye it was without form. The floor was invisible, and we were
guided in our steps by our staves alone. It was like stepping into
primal chaos, before light and form had birth. A profound chasm seemed
to yawn at our feet; yet the rocky floor rang to the blow of the staff,
and with cautious tread we proceeded. The flame of the lamp met no
responsive glow; save from the two intruders who stood awe stricken in
this strange emptiness; it stood in the still blackness unflickering,
like a solid. Feeling the broken walls, the hand was met by an oily
softness; the eye was useless, and even the touch now failed to guide
us. Solid walls were not to the eye; rocky barriers seemed simply
impenetrable darkness to the hand.
“From repeated contact with sooty walls, we also became covered with
this strange, light-absorbing powder, until we were enveloped in an
invisible mantle, and also passed from each other’s sight. Eye alone
answered to eye in their reflections of light. Too deeply impressed for
conversation, we stood still with outstretched hands. My comrade asked
at length, ‘May it not be even so in the valley of the Shadow of Death?’
And we looked for strength into each other’s eyes and linked our arms
that we might have the companionship of touch. We were now thoroughly
frightened, and turned to retrace our steps; but which way? We stood in
a sea of nothingness--locked in the foundations of the mountain. The
walls were lost to the sight, and were nothing to the touch. We stooped
to the deep dust of the floor and held the flame to read our
foot-prints; but the soil absorbed the light, as the sand of the desert
does the raindrop. We reached forward, and the hand failed to meet the
wall; we reached downward; there, too, was empty space. The light showed
no defining edge between the solid rock and the void. We swung the lamp
from the brink on which we lay; it revealed nothing. We dropped a heavy
stone into the chasm and listened for the rebound. No sound was
returned as it sank into the profound. We cast another stone across to
test the width, but this, too, was lost to the senses. Silently they
passed away, as the mist wreath on the hill side. And then we knew we
had been preserved from death. A careless step and we had found a grave
in the depths of the world’s foundations. We realized that we were lying
in trembling safety on the threshold of the extinct volcano, and lifting
our useless eyes from the impenetrable blackness, the awful whisper
‘Lost!’ passed between us. We were afraid to move; but the wasting oil
of our lamp warned us that time must not be lost. Presently our ears
caught the heat of surf on the rock as the tide came in, and following
this direction, we finally reached the entrance, almost fainting from
joy when we stood beyond this chamber of gloom. Once more we stood under
the wondrous tracery and reflections of the outer gates of the
inter-world of mysterious.”
A most thrilling experience, and one giving a fine picture of what may
be found in the mysterious depths of a lava bubble. In some cases the
bubbles are very thin; and an unwary passer might be suddenly plunged
into unfathomable depths should he tread on one. Usually, however, they
are formed over horizontal currents or passages.
We have endeavored to give the well-established facts concerning the
principles of volcanic action. It only remains, ere we leave this phase
of the subject, that we notice the one point on which as yet our
knowledge is not clear. That point is, the source of the heat which
produces the remarkable effects.
Several theories are advanced. One class of scientists believes that the
earth is a mass of molten matter, with only a thin outer shell of cooled
material. That a very
[Illustration: ERUPTION OF MOUNT VESUVIUS.]
high temperature exists at no very great distance from the surface is
beyond a doubt. The observations made in mines and artesian wells show
that the average increase in the temperature is one degree for every
fifty-five feet in depth. One noted variation exists in the deep wells
at Buda-Pesth, in Austria, where the temperature increased up to 3,000
feet; but beyond that depth it became cooler again. The Comstock lode in
Nevada, the richest mineral vein in the world, is nearly at the limit of
practicable working, the normal temperature being as high as one hundred
and fifty degrees.
Even if it be conceded that the material of the earth is a molten mass,
there would be two theories to explain it: one, that the earth was
originally in a state of fusion, and was slowly cooling; the other, that
the great pressure from without keeps an otherwise solid center greatly
compressed and heated, and consequently liquid. Either supposition is
based on well established facts; but it does not appear clear that the
molten globe with a cool shell can settle the entire question.
The objections to this are several. One, the complete absence of
uniformity in the increase of heat as we descend. While the total
average is as given above, the variations are so many and vast, that
there does not seem to be any general law, as there should be if the
molten interior possessed the least uniformity. In some shafts the
increase is one degree for twenty feet; in others, one for every one
hundred; in some, the temperature increases much more rapidly at great
depths, in others, much less rapidly.
A second objection is, the vast difference in the character of lavas,
even in districts very near each other. Thirdly, there seems no definite
connection between volcanoes in the same region. Two adjacent ones may
exhibit very different conditions. Mauna Loa is about 10,000 feet above
Kilauea, a great crater of the same mountain. Yet the upper is often in
a violent state of eruption when the latter is perfectly quiet. It would
be difficult to conceive how these are supplied from the same source. If
the interior were a molten mass in a state of equilibrium, as would be
necessary if the uniformity of its motions in the solar system were to
be preserved, any undue pressure would compel the molten matter to
escape from the lowest opening. This would be in accordance with the
simplest laws of liquids. Then we should find volcanic action most
vigorous at the lowest active volcano; but such is not the case. The
idea of a uniformly liquid interior seems hardly tenable.
There is still other objection to this theory. Experiments have been
made with various materials to ascertain the change affected in them by
heat. It is found that a block of granite five feet long, by a change of
ninety-six degrees in temperature, is expanded .27792 of an inch;
crystalline marble, .03264; sandstone, .0549. If, then, a portion of the
earth’s crust ten miles in thickness be heated six hundred degrees, its
crust would be raised two hundred feet; or a change of one degree, the
rate of expansion being fairly uniform to five hundred or six hundred
degrees, would raise the surface four inches. How important this matter
is may be better understood when we consider that if the interior of the
earth be a uniformly molten mass, with a crust ten miles thick, a
contraction of one-twelve thousandth of an inch should force out of the
crust a cubic mile of lava. We should find then a change in temperature
one forty-eight thousandth of a degree should effect this, if the crust
were ten miles thick.
We are then forced to conclude that the earth is not cooling to any
appreciable extent; or that the liquid interior is still capable of
indefinite compression without necessarily being forced out through
orifices in the crust; or that the interior is not a uniformly molten
mass.
Such are the arguments against a melted interior.
The reader should avoid the assumption of a uniform rate of contraction
or expansion of heat. Within very narrow limits, such a hypothesis may
be allowed; but to assume that it is universal, would be to affirm that
if you could only make the earth cold enough it would shrink to nothing
at all! The earth and the temperature would swallow each other, like the
two snakes, till neither was left. To illustrate more seriously, suppose
a race of men existed whose only experience of temperature ranged
between forty and two hundred degrees. They could consistently
calculate, from the change of water between these limits, that it would
require a temperature of many thousands of degrees to expand it to
seventeen hundred times its bulk. Yet we know they would have to raise
it only to two hundred and twelve degrees to produce the required
effect. And if they could go below forty degrees, they would be
astonished to find that water then expanded instead of contracting.
That the earth, if it cools, does so very slowly is clear, from the
character of the materials thrown out. Lava from Mt. Ætna has been
observed slowly moving nine months after the eruption. Lower portions of
the beds have been found to be abnormally heated ten years after pouring
out. Compare the thickness of a lava bed with the depth from which it is
thrown, and it will be seen that little heat is lost in the subterranean
depths. One instance, showing how slowly the lava is to part with its
heat, may be given:
In the year 1828 a great mass of ice was discovered on Mt. Ætna. In
consequence of the protracted heat of the season, supplies of ice at
Catania and the adjacent regions failed entirely, and the people
suffered considerably for the want of an article considered necessary to
health as well as comfort in that hot climate. The Catanian authorities
caused search to be made for some crevasse or natural grotto on Mt.
Ætna, where drift-snow might exist. Near the base of the highest cone
was found a vast mass of ice, covered by a lava bed. How old it was
there is no means of knowing; nor can we tell how much of the ice might
have originally been melted by the overflowing current. But there it
was, so hard and firm that the workmen quarried it with great
difficulty.
Lastly, it appears that the causes of earthquakes and volcanic action
must be the same. A violent volcanic outbreak causes earthquake shocks
at once, as though relieved by a safety valve. The experiments of Mr.
Robert Mallet, the best known authority on earthquake phenomena, tend to
prove that the shocks necessarily originate at a comparatively short
distance below the surface. So, from two independent lines of
investigation, the same conclusion is reached.
These objections have caused inquiry to be made as to what causes might
locally develop heat. Here there are so many possible methods that
scientists may not be expected to unite. One is the chemical theory:
water coming in contact with quick-lime, or metallic sodium or
potassium, would evolve intense heat. One or two locomotives have been
invented which need no fuel, obtaining their heat supply thus.
But it is objected to this that the products of volcanic action are not
such as would result from such a cause; that all experience indicates
that water has already penetrated every portion of the earth, the
deepest borings always crossing veins, and all great mines requiring to
be artificially drained.
One other theory is, that the slow contraction of the globe from the
radiation of heat into space necessarily affects the outer portion most
directly, and in consequence, the shrinking of the crust at the weakest
points produces unusual pressure there, which can evolve intense heat,
as is shown by the fact that nearly all rocks so twisted or strained
are more or less changed in their internal structure by heat. It is also
evident that a region once thus weakened and seriously broken would
necessarily form a fairly permanent volcanic tract. As the work of
nature all goes to preserve equilibrium or balance of forces, an unusual
upheaval would necessitate unusual subsidence near at hand; and in fact
the highest mountain is always near the deepest ocean.
This explanation, combined with that of chemical action seems to us the
more tenable. None of these theories conflict with the nebular
hypothesis of Kant and Laplace.
In looking over the areas of volcanic action, we will find they have
changed considerably from the areas of the past.
In the continent of Europe there is but one active volcano--Vesuvius;
but there are six others on islands in the Mediterranean. Africa has
four active volcanoes on the west coast, and six on the east; while ten
others are to be found on adjacent islands. Austria has no volcanoes, so
far as is known. In Asia are twenty-four active volcanoes; but twelve of
these are on the peninsula of Kamtschatka. On the American continent we
find a larger proportion. North America has forty-five, most of which
are in Mexico and Central America; and South America has thirty-seven.
Of these continental volcanoes all are near the sea, except four which
are reported to lie in the great unexplored tableland between Siberia
and Thibet; and some are said to exist in the Chinese province of
Mantchooria. No white man has visited them.
But it is in the island world that we must look for the most numerous
volcanoes.
A great ridge runs through the Atlantic; and along this lie a number of
islands with active centres. Jan Mayen, in the Arctic circle, has an
active volcano; Iceland, thirteen; the Azores, six; the Canaries, three;
east African islands, eight; the West Indies, six; three submarine
volcanoes have been observed at different times in the Atlantic. But
through the same region the number of extinct volcanoes is far greater.
Of those which exist several seem approaching extinction.
But in the isles of the Pacific and between the Pacific and Indian
oceans we have a vast series of volcanic vents of wonderful activity. In
the Aleutian Islands are thirty-one; in the Kurile Isles, ten or more;
Japan and the adjacent groups have twenty-five. Southeast of the Asiatic
continent is the most active region on the globe. Fifty volcanoes are
here known. Farther south are four in New Guinea, one or two submarine
vents, a number in New Britain, the Solomon group, the New Hebrides,
three in New Zealand, and Erebus and Terror in the Antarctic circle. Add
to these the islands of the Central Pacific, and we have more than
one-half the volcanoes of the globe. Besides there are a large number of
nearly perfect volcanic cones which must recently have become extinct.
In conclusion, we find all the oceanic islands are either of volcanic or
coral formation; and as we find that the coral polyp can not live at a
greater depth than one hundred and twenty feet; as we know the ocean in
the immediate neighborhood of these islands to be many thousand of feet
in depth; as we know coral islands to be circular, often enclosing a
lagoon of water, it is fair to suppose that the polyps have not built
through long ages of subsidence, as is usually supposed; but that they
have built upon the rims of extinct craters lying near the surface. The
fact that these circular reefs always have one or more breaks in their
circuit is additional reason for the belief. The fact of a coral island
lying within a barrier reef, then,
[Illustration: CORAL REEFS.]
resolves itself into a volcanic crater with an inner cone, as every
active volcano has. It is rather ludicrous to suppose that polyps, among
the lowest of created beings, leading an ephemeral existence, should yet
have such unanimity of purpose, such perfect mutual understanding, as to
undertake to build their reefs in a more or less circular form; it is
preposterous to suppose the unvarying form of the structure is the
result of mere chance. Clearly we must find some other influences; and
the most reasonable is to suppose the foundations of these islands were
laid by the same agency that raised all other Oceanic islands from the
bed of the sea.
The volcano thus plays an important part in the earth’s economy. Not
only does it add to land areas by upheaval from the deep. The amount of
material thrown out by the Javanese volcanoes alone during the past
hundred years is greater far than all the silt borne to the sea by
American rivers during the same period. Krakatoa, in its recent
eruption, threw out more than the Mississippi bears to the sea in sixty
years.
There is some doubt as to how much volcanoes effect by direct upheaval.
The formation of many observed cones shows that the majority are mainly
built up by the materials thrown out, and not by any great elevation of
the adjacent surface. In the case of a volcano already existing, it is
of course not easy to know what proportion of its mass is merely
accumulation of lava, cinders, or tufa.
As to the form of volcanic cones, those of ashes, cinders, and scoria
are of course steepest; those of lava thrown out when liquid having a
very gradual slope. The difference may be readily illustrated by
comparing a heap of sand and pebbles with a heap of stiffening molasses
candy. One is steeply conical; the other, rounded or dome-like. But
either form of volcano may abound in crevices and apertures from which
issue sulphurous vapors and gases. These fumaroles, as they are called,
are usually surrounded with mineral deposits, often resembling the most
delicate filigree work.
Having considered the general phases and principles of volcanic action,
we may now notice some of the more famous eruptions of the past.