Volcanoes: What They are and What They Teach

by John W. Judd

DIFFICULTIES NOT YET EXPLAINED.

It is the weight of these several objections that has led geologists in recent years to regard with greater favour those hypotheses which seek to account for the production of high temperatures within parts of the earth's crust, without having recourse to a supposed incandescent nucleus. If it can be shown that there are any chemical or mechanical forces at work within the crust of the globe which are capable of producing local elevations of temperature, then we may conceive of a condition of things existing in the earth's interior which is free from the objections raised by the astronomer on the score of the earth's proved rigidity, and by the geologist on the ground of its general stability, and which at the same time seems to harmonise better with the observed facts of the distribution of temperature within the earth's crust. How far the existence of such chemical and mechanical agencies capable of producing high temperatures within the crust of the globe have been substantiated, we have already endeavoured to point out.

It must be admitted, then, that the questions of the nature of the earth's interior and the cause of the high temperatures which produce volcanic phenomena, are still open ones. We have not yet got beyond the stage of endeavouring to account for the facts observed by means of tentative hypotheses. Some of these, as we have seen, agree with the facts, so far as they are at present known, much better than others; but the decision between them or the rejection of the whole of them in favour of some new hypothesis, must depend on the results of future observation and enquiry.

It may be well, before leaving this subject, to remark that they are all equally reconcilable with the nebular theory of Kant and Laplace. Granting that the matter composing our globe has passed successively through the gaseous and liquid conditions, it is open to us to imagine the earth as now composed of a liquid nucleus with either a thick or a thin solid shell; of a solid nucleus and a solid shell with more or less liquid matter between them; or, lastly, to conceive of it as having become perfectly solid from the centre to the surface.

CAUSE OF THE PRESENCE OF WATER IN LAVAS.

But it is not upon the existence of a high temperature within certain parts of the earth's crust that the production of volcanic activity alone depends. The presence of water and other liquid and gaseous substances in a state of the most intimate admixture with the fused rock-masses, is, as we have seen, the main cause of the violent displays of energy exhibited at volcanic centres. And We shall now proceed to notice the hypotheses which have been suggested to account for the presence of these liquid and gaseous bodies in the midst of the masses of incandescent materials poured out from volcanic vents.

There is an explanation of this presence of water and various gases in the masses of molten rock-materials within the earth's crust which at once suggests itself, and which was formerly very generally accepted. Volcanoes, as we have seen, are usually situated near coast-lines, and if we imagine fissures to be produced by which sea-water finds access to masses of incandescent rock-materials, then we can regard volcanic outbursts as resulting from this meeting of water with rock-masses in a highly healed condition. This supposition has been thought to receive much support from the fact that many of the gases evolved from volcanic vents are such as would be produced by the decomposition of substances present in sea-water.

But it frequently happens that an explanation which at first sight appears to be very simple and obvious, turns out on more critical examination to be quite the reverse, and this is the case with the supposed origination of volcanic outbursts by the access of sea-water to incandescent rock-material by means of earth-fissures. It is difficult to understand how, by such means, that wonderfully intimate union between the liquefied rock and the water, evolved in such quantities during volcanic outbursts, could be brought about; and moreover, we can scarcely regard the production of fissures in the earth's crust as being at the same time both the cause and the effect of this influx of water to the deep-seated rock-masses at a high temperature.

ABSORPTION OF GASES BY LIQUIDS AND SOLIDS.

During recent years the attention of both geologists and physicists has been directed to a remarkable property exhibited by many liquids and solids, as supplying a possible explanation of the phenomena of volcanic action. The property to which we refer is that whereby some liquid and solid substances are able to absorb many times their volume of certain gases—which gases under different conditions may be given off again from the liquids or solids. This power of absorption is a very remarkable one; it is not attended with chemical combination, but the amount of condensation which gases must undergo within the solid or liquid substances is sometimes enormous. Water may be made to absorb more than 1,000 times its volume of ammonia, and more than 500 times its volume of hydrochloric acid. Alcohol may absorb more than 300 times its volume of sulphurous acid. Charcoal may absorb 100 times its volume of ammonia, 85 times its volume of hydrochloric acid, 65 times its volume of sulphuretted hydrogen, 55 times its volume of sulphurous acid, and 35 times its volume of carbonic acid. Platinum-black absorbs many times its volume of oxygen and other gases.