Harper's Round Table, January 5, 1897

by Various

Each time the lava rose up into the basin it swelled quickly in its middle part, and in the twinkling of an eye it was broken by an explosion of such violence that a quantity of the fluid rock was tossed in fragments high into the air. As this sped upward and downward, it had a chance partly to cool, so that as it fell on the edge of the cone opposite to where I was the roar of its striking was very suggestive of what would happen if the wind should die away.

Although the circumstances were such as made it hard to observe closely, I had no difficulty in seeing that the vapor which blew out at each explosion was steam. As it came forth, it was of the steel-blue color which we see just where the steam comes from the safety-valve of a very hot boiler. As it rose in the crater it soon became white, and as it whirled around me it had the well-known odor of steam, mingled with that of sulphur. In a word, it was evident that it was the vapor of water which was the cause of the explosions.

After I had watched this fascinating scene for about half an hour, with much inconvenience from the heat of the earth and from the shaking of the ground on which I lay, the explosions, which were at first at the rate of three or four each minute, became more and more frequent and violent, and the strong wind began to die away, so that a speedy retreat was necessary to escape the bits of lava, which were now falling heavily. Looking back from the base of the cone, I noted that the explosions came faster and faster, so that it sounded as a continuous roar. It was just as when a locomotive starts on its journey. At the outset we can count the puffs; as the cylinders move faster and faster the escape sounds perfectly continuous.

From the base of the cinder cone there flowed out a small lava stream. This lava was evidently full of steam, which poured forth from all parts of the surface. This is seen in all eruptions. Clouds of steam hung over the streams of lava. They are often visible ten miles or more away from the current of molten rock. In a great eruption the steam given forth from the crater often forms, as it condenses, into rains, that fall in fearful torrents about the cone. It is evident, in a word, that the explosions of volcanoes are formed by the escape of the vapor of water. They are, indeed, like the explosions of boilers.

The question now arises as to the way in which this steam gets into the lava. This we can decide by a simple bit of study of the facts. Taking a map which shows the positions of several hundred active volcanoes, we find at once that they are all situated on the sea floor, from which they rise to form islands on its surface; or, when they are on the continents, they are never more than two hundred and fifty miles from the ocean. This shows that the activity of a volcano is, in some way, related to the sea-water. The only way in which we have been able to reasonably conceive of the sea bringing about volcanic explosions will now be described.

On the sea floor there is a constant laying down of sediments—limestones, sandstones, etc. We know by the parts of the old sea floor that have been uplifted into dry lands that such beds have been formed, to the thickness in all of one or two hundred thousand feet. These beds are made of small bits of rocky matter and fragments of dead animals and plants. These bits do not fit closely together, and the interspaces are filled with sea water, so that as much as one-twelfth of the rock is usually made up of the fluid in which it was formed. As the ages go on, these beds, with the water which they hold, are buried deeper and deeper by the newer rocks which are laid down upon them, until it may be that they are thus brought to lie twenty miles or more below the surface of the solid earth.

Next let us see as to the heat to which these rocks, with their imprisoned water, are exposed. We know, from a great number of studies which have been made in mines that for each mile we go downward in the earth there is an increase in heat, differing a good deal in different places, but on the average amounting to about one hundred degrees. Therefore, at the depth of twenty miles the imprisoned water would have a temperature of about two thousand degrees. In other words, it would be about as hot as the melted iron that comes from the blast-furnace. Thus heated, the water of the tiny cells of the rock would tend to explode with something like the intensity of gun-powder when it was fired; but as it is sealed in by the great thickness of the rock above, it cannot burst into vapor—just as in the steam-boiler the water stays as a fluid even when it is heated twice as hot as it needs be to become steam when it is not confined.

Let us now suppose that a rift, or, as geologists call it, a fault, is formed in the rocks leading from the surface downward to the level where this very explosive water lies. We can readily fancy that at once the fluid would flash into steam; and as this occurred in the myriads of little cavities in the rocks, which were so heated that they tended to become melted, great quantities of the beds would be forced along with the escaping steam in the form of lava. We see also that this would account for the fact that when the lava comes to the surface of the earth it is commonly filled with steam. When it rises quickly to the air, it is blown to fine dust by the expanding vapor; or if it does not fly to pieces, the little bits of water expand into bubbles, forming pumice or lava, so full of little cavities that it will float on the water like cork.

This view as to the origin of volcanoes, although it would not be accepted by all the students of these strange features of the earth, seems most probable, for the reason that it accounts for the fact that all the seats of present volcanic activity are on the floor of the seas or near their borders, and that the extinct volcanoes which we have had a chance to study lost their activity at a time when, by the changes in the shape of the land, the sea was moved away from the region where they were found. We easily perceive that it is only where, as in the sea, beds are being laid down, one on top of another, that the heat is rising in the rocks, and the water in their crevices becoming hotter; beneath the land the rocks are always becoming less heated, so that the water which they contain is constantly cooling down.

I have spoken of the water contained in the very heated rocks as if it remained in the state of fluid. It is likely that, when in its very hot state, it may be changed into its gases, oxygen and hydrogen, of which it is composed, and that these gases would again become the vapor of water as they rose toward the surface and were somewhat cooled. This and other matters of chemical detail which go on in the wonderful laboratory of the under-earth do not hinder our believing that volcanoes are due to the escape of the water which is constantly being buried in the rocks as they are built. So large is the amount of this water which lies thus buried that probably it amounts to somewhere near as much as is held in all the seas. Were it not for the return of the buried fluid through the volcanoes, the oceans would doubtless be much smaller than they are. They might, indeed, have long since disappeared in the crevices of the earth.