Volcanoes: What They are and What They Teach

by John W. Judd

The close connection between the phenomena of volcanoes and earthquakes cannot be doubted. It is true that some of those vibrations or tremors of the earth's crust, to which we apply the name of earthquakes, occur in areas which are not now the seat of volcanic action; and it is equally true that the stratified rock-masses of our globe, far away from any volcanic centres, exhibit proofs of violent movement and fracture, in the production of which, concussions giving rise to earthquake vibrations, could scarcely fail to have occurred. But it is none the less certain that earthquakes as a rule take place in those areas which are the seats of volcanic action, and that great earthquake-shocks precede and accompany volcanic outbursts. Sometimes, too, it has been noticed that the manifestation of activity at a volcanic centre is marked by the sudden decline of the earthquake-tremors of the district around, as though a safety-valve had been opened at that part of the earth's surface.

Mr. Mallet has shown that by the careful study of the effects produced at the surface by earthquake-vibrations, we may determine with considerable accuracy the point at which the shock or concussion occurred which gave rise to the vibration. Now it is a most remarkable fact that such calculations have led to the conclusion that, so far as is at present known, earthquake shocks never originate at greater depths than thirty miles from the surface, and that in some cases the focus from which the waves of elastic compression producing an earthquake proceed is only at the depth of seven or eight miles. As we have already seen, there can be no doubt that in the great majority of instances the forces originating earthquake-vibrations and volcanic outbursts are the same, and independent lines of reasoning have conducted us to the conclusion that these forces operate at very moderate distances from the earth's surface.

Under these circumstances, geologists have been led to enquire whether there are any means by which we can conceive of such an amount of heat, as would be competent to produce volcanic outbursts, being locally developed at certain points within the earth's crust. Recent discoveries in physical science which have shown the close relation to one another of different kinds of force, and their mutual convertibility, have at least suggested the possibility of the existence of causes by which such high temperatures within certain portions of the earth's crust may be originated.

DAVY'S CHEMICAL THEORY.

When, at the commencement of the present century, Sir Humphry Davy discovered the remarkable metals of the alkalies and alkaline earths, and at the same time demonstrated the striking phenomena which are exhibited if these metals be permitted to unite with oxygen, he at once perceived that if such metals existed in an uncombined condition within the earth's crust, the access of water and air to the mass might give rise to the development of such an amount of heat, as would be competent to produce volcanic phenomena at the surface. It is true that at a later date Davy recognised the chemical theory of volcanoes as being beset with considerable difficulties, and was disposed to abandon it altogether. It was argued, with considerable show of reason, that if the heat at volcanic centres were produced by the access of water to metallic substances, great quantities of hydrogen would necessarily be evolved, and this gas ought to be found in prodigious quantities among the emanations of volcanoes. The fact that such enormous quantities of hydrogen gas are not emitted from volcanic vents has been held by many authors to be fatal to the chemical theory of volcanoes.

But the later researches of Graham and others have made known facts which go far towards supplying an answer to the objections raised against the chemical theory of volcanoes. Various solids and liquids have been shown to possess the power of absorbing many times their volume of certain gases. Among the gases thus absorbed in large quantities by solids and liquids, hydrogen is very conspicuous. In some cases gases are absorbed by metals or other solids in a state of fusion, and yielded up again by them as they cool.

It is a very remarkable circumstance that some meteorites are found to have absorbed large quantities of hydrogen gas, and this is given off when they are heated in vacuo. Thus it has been demonstrated that certain meteorites have contained as much as forty seven times their own volume of hydrogen gas.

We have already pointed out that there are reasons for believing the internal portions of our globe to be composed of materials similar to those found in meteorites. If such be the case, the access of water to these metallic substances may result in the formation of oxides, attended with a great local development of heat, the hydrogen which is liberated being at once absorbed by the surrounding metallic substances. That this oxidation of the metallic substances in the interior of our globe by the access of water and air from the surface is continually going on, can scarcely be doubted. We may even look forward to a far-distant period when the whole of the liquid and gaseous envelopes of the globe shall have been absorbed into its substance, and our earth thereby reduced to the condition in which we now find the moon to be.

There is a second method by which high temperatures might be locally developed within the earth's crust, which has been suggested by Vose, Mallet, and other authors.

We have good grounds for believing that the temperature of our globe is continually diminishing by its radiation of heat into space. This cooling of our globe is attended by contraction, which results in movements of portions of its crust. It may at first sight appear that such movements would be so small and insignificant as to be quite unworthy of notice. But if we take into account the vast size of our earth it will be seen that the movements of such enormous masses may be attended with the most wonderful results.