Volcanoes: What They are and What They Teach

by John W. Judd

POSITION OF THE ISOGEOTHERMS.

The deep-sea soundings of the 'Challenger' have shown that the floor of the ocean is constantly maintained at a temperature but little above that of the freezing point of water. This low temperature is probably produced by the absorption of heat from the earth's crust by the waters of the ocean, which distribute it by means of convection currents on the grandest scale. Hence, the isogeotherms, or lines indicating the depths at which the same mean temperature is found within the earth's crust, are probably depressed beneath the great ocean-floors, and rise towards the land-masses. It is to this circumstance, combined with that of the enormous pressure of water on the ocean-beds, that we must probably ascribe the general absence of volcanoes in the deep seas and their distribution near coast-lines.

We have thus briefly reviewed the chief hypotheses which have been suggested in order to account for the two great factors in all volcanic phenomena—namely, the presence of highly heated rock-masses within the earth's crust, and the existence of various vapours and gases in a state of most intimate mechanical, but not chemical, union with these incandescent materials. It must be admitted that we do not at present appear to have the means for framing a complete and consistent theory of volcanic action, but we may hopefully look forward to the time when further observation and experiment shall have removed many of the existing difficulties which beset the question, and when by the light of such future researches untenable hypotheses shall be eliminated and the just ones improved and established.

But if we are constrained to admit that a study of the observed phenomena and established laws of volcanic action have not as yet enabled us to frame any complete and satisfactory theory on the subject, we cannot lose sight of the fact that all modern speculation upon this question appears to be tending in one definite direction. It is every day becoming more and more clear that our earth is bound by ties of the closest resemblance to the other members of that family of worlds to which it belongs, and that the materials entering into their constitution, and the forces operating in all are the same.

We have had occasion in a previous chapter to point out that there are the strongest grounds for believing the interior of our globe to consist of similar materials to those found in the small planetary bodies known as meteorites. That the comets are merely aggregations of such meteorites, and that the planets differ from them only in their greater dimensions, may be regarded as among the demonstrated conclusions of the astronomer. The materials found most abundantly in meteorites and in the interior of our globe are precisely the same as those which are proved to exist in an incandescent state in our sun. Hence we are led to conclude that the whole of the bodies of the solar system are composed of the same chemical elements.

ERUPTIVE ACTION IN THE SUN.

That the forces operating in each of these distant bodies present striking points of analogy is equally clear. The sun is of far greater dimensions than our earth, and is still in great part, if not entirely, in a gaseous condition. The great movements in the outer envelopes of the sun exhibited in the 'sun-spots' and 'solar prominences,' recall to the mind the phenomena of volcanic activity upon our globe. But the vast energy still existing in the intensely heated mass of the sun, and the wonderful mobility of its gaseous materials, give rise to appearances beside which all terrestrial outbursts seem to sink into utter insignificance. Vast cavities of such dimensions that many globes of the size of our earth might be swallowed up in them are formed in the solar envelopes in the course of a few days or hours. Within these cavities or sun-spots incandescent vapours are observed, rushing upwards and downwards with almost inconceivable velocity.

The drawings made by Secchi, and reproduced in figs. [89] and [90], will give some idea of the appearances presented by these great holes in the solar envelopes.

Fig. 89.—A group of Sun-spots. (After Secchi.)