Volcanoes: What They are and What They Teach

by John W. Judd

Every completed scientific investigation must consist of four series of operations. In the first of these an attempt is made to collect the whole of the facts bearing on the question, by means of observation and experiment; the latter being only observation under conditions determined by ourselves. In the second stage of the enquiry, the attention is directed to classifying and grouping the isolated facts, so as to determine their bearings upon one another, and the general conclusions to which they appear to point. In the third stage, it is sought to frame an hypothesis which shall embrace all the observed facts, and shall be in harmony with the general conclusions derived from them. In the fourth stage, this hypothesis is put to the most rigid test; comparing the results which must follow, if it be true, with the phenomena actually observed, and rejecting or amending our hypothesis accordingly. Every great scientific theory has thus been established by these four processes—observation, generalisation, hypothesis, and verification.

The enquiry concerning the nature and causes of volcanic action is far from being a completed one. It is true that many hypotheses upon the subject have been framed, but in too many instances these have not been based on accurate observations and careful generalisations, and can be regarded as little better than mere guesses. Indeed, the state of the enquiry at the present time would seem to be as follows. Although much remains to be done in the direction both of observation and experiment, the main facts of the case have been established upon irrefragable evidence. The classification and comparison of these facts have led to the recognition of certain laws, which seem to embrace all the known facts. To account for these facts and their demonstrated relations to one another, certain tentative hypotheses have been suggested; but in no case can it be truly said that these latter have so far stood the test of exact enquiry as to deserve to rank as demonstrated truths. A complete and consistent theory of volcanic action still remains to be discovered.

VALUE AND LIMITS OF HYPOTHESES.

In accordance with the plan which we have sketched out for ourselves at the commencement of this work, we shall aim at following what has been the order of investigation and discovery in our study of volcanic action; and in this concluding chapter we shall indicate the different hypotheses by which it has been proposed to account for the varied phenomena, which we have discussed in the preceding pages, and their remarkable relations to one another. We shall endeavour, in passing, to indicate how far these several hypotheses appear to be probable, as satisfying a larger or smaller number of those conditions of the problem which have been established by observation, experiment, and careful reasoning; but we shall at the same time carefully avoid such advocacy of any particular views as would tend to a prejudgment of the question. Hypothesis is, as we have seen, one of the legitimate and necessary operations in scientific investigation. It only becomes a dangerous and treacherous weapon when it is made to precede rather than to follow observation and experiment, or when being regarded with paternal indulgence, an attempt is made to shield it from the relentless logic of facts. Good and bad hypotheses must be allowed to 'grow together till the harvest;' such as are unable to accommodate themselves to the surrounding conditions imposed by newly-discovered facts and freshly-established laws will assuredly perish; and in this 'struggle for existence' the true hypothesis will in the end survive, while the false ones perish.

It may well happen, however, that among the hypotheses which have up to the present time been framed, none will be found to entirely satisfy all the conditions of the problem. New discoveries in physics and chemistry have suggested fresh explanations of volcanic phenomena in the past, and may continue to do so in the future; and the true theory of volcanic action, when it is at last discovered, may combine many of the principles which now seem to be peculiar to different hypotheses.

Let us, in the first place, enquire what are the facts which must be accounted for in any theory of volcanic action. We have already been led to the conclusion that the phenomena exhibited by volcanoes were entirely produced by the escape of imprisoned water and other gases from masses of incandescent and fluid rock. Our subsequent examination of the problem confirmed the conclusion that in all cases of volcanic outburst we have molten rock-materials from which water and other gases issue with greater or less violence. The two great facts to be accounted for, then, in any attempted explanation of volcanic phenomena, are the existence of this high temperature at certain points within the earth's crust, and the presence of great quantities of water and gas, imprisoned in the rocks. We shall perhaps simplify the enquiry if we examine these two questions separately, and, in the first place, review those hypotheses which have been suggested to account for high temperatures in the subterranean regions, and, in the second place, examine those which seek to explain the presence of large quantities of imprisoned water and gases.

INCREASE OF TEMPERATURE WITH DEPTH.

That a high temperature exists in the earth's crust at some depth from the surface is a £act which does not admit of any doubt. Every shaft sunk for mining operations, and every deep boring made for the purpose of obtaining water, proves that a more or less regular increase of temperature takes place as we penetrate downwards. The average rate of this increase of temperature has been estimated to be about 1° Fahrenheit for every 50 or 60 feet of depth.

Now if it be assumed that this regular increase of temperature continues to great depths, a simple calculation proves that at a depth of 9,000 feet a temperature of 212° Fahrenheit will be found—one sufficient to boil water at the earth's surface—while at a depth of 28 miles the temperature will be high enough to melt cast-iron, and at 34 miles to fuse platinum.

So marked is this steady increase of temperature as we go downwards, that it has been seriously proposed to make very deep borings in order to obtain supplies of warm water for heating our towns. Arago and Walferdin suggested this method for warming the Jardin des Plantes at Paris; and now that such important improvements have been devised in carrying borings to enormous depths, the time may not be far distant when we shall draw extensively upon these supplies of subterranean heat. At the present time the city of Buda-Pesth is extensively supplied with hot-water from an underground source. Should our coal-supply ever fail it may be well to remember that we have these inexhaustible supplies of heat everywhere beneath our feet.