It may seem at first as if it were hopeless that man should ever know
anything about the earth’s interior. Just think what a huge ball this
globe of ours is, and you will see that after all, in living and moving
over its surface, we are merely like flies walking over a great hill. All
that can be seen from the top of the highest mountain to the bottom of
the deepest mine is not more in comparison than the mere varnish on the
outside of a school globe. And yet a good deal can be learnt as to what
takes place within the earth. Here and there, in different countries,
there are places where communication exists between the interior and
the surface; and it is from such places that much of our information on
this subject is derived. Volcanoes are among the most important of the
channels of communication with the interior.
Let us suppose that you were to visit one of these volcanoes just before
what is called “an eruption.” As you approach it, you see a conical
mountain, seemingly with its top cut off. From this truncated summit a
white cloud rises. But it is not quite such a cloud as you would see on
a hill top in this country. For as you watch it you notice that it rises
out of the top of the mountain, even though there are no clouds to be
seen anywhere else. Ascending from the vegetation of the lower grounds,
you find the slopes to consist partly of loose stones and ashes, partly
of rough black sheets of rock, like the slags of an iron furnace. As you
get nearer the top the ground feels hot, and puffs of steam, together
with stifling vapors, come out of it here and there. At last you reach
the summit, and there what seemed a level top is seen to be in reality a
great basin, with steep walls descending into the depths of the mountain.
Screening your face as well as possible from the hot gases which almost
choke you, you creep to the top of this basin, and look down into it.
Far below, at the base of the rough red and yellow cliffs which form its
sides, lies a pool of some liquid, glowing with a white heat, though
covered for the most part with a black crust like that seen on the
outside of the mountain during the ascent. From this fiery pool jets of
the red hot liquid are jerked out every now and then, stones and dust are
cast up into the air, and fall back again, and clouds of steam ascend
from the same source and form the uprising cloud which is seen from a
great distance hanging over the mountain.
This caldron-shaped hollow on the summit of the mountain is the crater.
The intensely heated liquid in the sputtering boiling pool at its bottom
is melted rock or lava. And the fragmentary materials—ashes, dust,
cinders, and stones—thrown out, are torn from the hardened sides and
bottom of the crater by the violence of the explosions with which the
gases and steam escape.
The hot air and steam, and the melted mass at the bottom of the crater,
show that there must be some source of intense heat underneath. And as
the heat has been coming out for hundreds, or even thousands of years, it
must exist there in great abundance.
But it is when the volcano appears in active eruption that the power
of this underground heat shows itself most markedly. For a day or two
beforehand, the ground around the mountain trembles. At length, in a
series of violent explosions, the heart of the volcano is torn open,
and perhaps its upper part is blown into the air. Huge clouds of steam
roll away up into the air, mingled with fine dust and red hot stones.
The heavier stones fall back again into the crater or on the outer
slopes of the mountain, but the finer ashes come out in such quantity,
as sometimes to darken the sky for many miles round, and to settle down
over the surrounding country as a thick covering. Streams of white hot
molten lava run down the outside of the mountain, and descend even to the
gardens and houses at the base, burning up or overflowing whatever lies
in their path. This state of matters continues for days or weeks, until
the volcano exhausts itself, and then a time of comparative quiet comes,
when only steam, hot vapors, and gases are given off.
About 1800 years ago, there was a mountain near Naples shaped like a
volcano, and with a large crater covered with brushwood. No one had ever
seen any steam, or ashes, or lava come from it, and the people did not
imagine it to be a volcano, like some other mountains in that part of
Europe. They had built villages and towns around its base, and their
district, from its beauty and soft climate, used to attract wealthy
Romans to build villas there. But at last, after hardly any warning,
the whole of the higher part of the mountain was blown into the air
with terrific explosions. Such showers of fine ashes fell for miles
around, that the sky was as dark as midnight. Day and night the ashes
and stones descended on the surrounding country; many of the inhabitants
were killed, either by stones falling on them, or from suffocation by
the dust. When at last the eruption ceased, the district, which had
before drawn visitors from all parts of the old world, was found to be
a mere desert of grey dust and stone. Towns and villages, vineyards
and gardens, were all buried. Of the towns, the two most noted were
called Herculaneum and Pompeii. So completely did they disappear, that,
although important places at the time, their very sites were forgotten,
and only by accident, after the lapse of some fifteen hundred years,
were they discovered. Excavations have since that time been carried on,
the hardened volcanic accumulations have been removed from the old city,
and you can now walk through the streets of Pompeii again, with their
roofless dwelling houses and shops, theaters and temples, and mark on
the causeway the deep ruts worn by the carriage wheels of the Pompeians
eighteen centuries ago. Beyond the walls of the now silent city rises
Mount Vesuvius, with its smoking crater, covering one half of the old
mountain which was blown up when Pompeii disappeared.
Volcanoes, then, mark the position of some of the holes or orifices,
whereby heated materials from the inside of the earth are thrown up to
the surface. They occur in all quarters of the globe. In Europe, beside
Mount Vesuvius, which has been more or less active since it was formed,
Etna, Stromboli, and other smaller volcanoes, occur in the basin of the
Mediterranean, while far to the northwest some volcanoes rise amid the
snows and glaciers of Iceland. In America a chain of huge volcanoes
stretches down the range of mountains which rises from the western margin
of the continent. In Asia they are thickly grouped together in Java and
some of the surrounding islands, and stretch thence through Japan and the
Aleutian Isles, to the extremity of North America. If you trace this
distribution upon the map, you will see that the Pacific Ocean is girded
all round with volcanoes.
Since these openings into the interior of the earth are so numerous over
the surface, we may conclude that this interior is intensely hot. But we
have other proofs of this internal heat. In many countries hot springs
rise to the surface. Even in England, which is a long way from any active
volcano, the water of the wells of Bath is quite warm (120° Fahr.). It
is known, too, that in all countries the heat increases as we descend
into the earth. The deeper a mine the warmer are the rocks and air at its
bottom. If the heat continues to increase in the same proportion, the
rocks must be red hot at no great distance beneath us.
It is not merely by volcanoes and hot springs, however, that the
internal heat of the earth affects the surface. The solid ground is made
to tremble, or is rent asunder, or is upheaved or let down. You have
probably heard or read of earthquakes; those shakings of the ground,
which, when they are at their worst, crack the ground open, throw down
trees and buildings, and bury hundreds or thousands of people in the
ruins. Earthquakes are most common in or near those countries where
active volcanoes exist. They frequently take place just before a volcanic
eruption.
Some parts of the land are slowly rising out of the sea; rocks, which
used always to be covered by the tides, come to be wholly beyond their
limits; while others, which used never to be seen at all, begin one by
one to show their heads above water. On the other hand some tracts are
slowly sinking; piers, sea walls, and other old landmarks on the beach,
are one after another enveloped by the sea as it encroaches further and
higher on the land. These movements, whether in an upward or downward
direction, are likewise due in some way to the internal heat.
Now when you reflect upon these various changes you will see that through
the agency of this same internal heat land is preserved upon the face
of the earth. If rain and frost, rivers, glaciers, and the sea were to
go on wearing down the surface of the land continually, without any
counterbalancing kind of action, the land would necessarily in the end
disappear, and indeed would have disappeared long ago. But owing to the
pushing out of some parts of the earth’s surface by the movements of the
heated materials inside, portions of the land are raised to a higher
level, while parts of the bed of the sea are actually upheaved so as to
form land.
This kind of elevation has happened many times in all quarters of the
globe. As already mentioned most of our hills and valleys are formed of
rocks, which were originally laid down on the bottom of the sea, and have
been subsequently raised into land.
This earth of ours is the scene of continual movement and change. The
atmosphere which encircles it is continually in motion, diffusing heat,
light, and vapor. From the sea and from the waters of the land, vapor is
constantly passing into the air, whence, condensed into clouds, rain and
snow, it descends again to the earth. All over the surface of the land
the water which falls from the sky courses seaward in brooks and rivers,
bearing into the great deep the materials which are worn away from the
land. Water is thus ceaselessly circulating between the air, the land,
and the sea. The sea, too, is never at rest. Its waves gnaw the edges of
the land, and its currents sweep round the globe. Into its depths the
spoils of the land are borne, there to gather into rocks, out of which
new islands and continents will eventually be formed. Lastly, inside the
earth is lodged a vast store of heat by which the surface is shaken, rent
open, upraised or depressed. Thus, while old land is submerged beneath
the sea, new tracts are upheaved, to be clothed with vegetation and
peopled with animals, and to form a fitting abode for man himself.
This world is not a living being, like a plant or an animal, and yet you
must now see that there is a sense in which we may speak of it as such.
The circulation of air and water, the interchange of sea and land; in
short the system of endless and continual movement by which the face of
the globe is day by day altered and renewed, may well be called the Life
of the Earth.