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.