Volcanoes: What They are and What They Teach

by John W. Judd

Fig. 8—Minute Liquid-cavity in a Crystal, with a moving Bubble. (The path of the bubble is indicated by the dark line.)

NATURE OF LIQUIDS IN CAVITIES.

What is the nature of the liquids which are thus imprisoned in these cavities contained in the crystals of lavas and granites? Careful experiments have given a conclusive answer to this question. In many cases the liquid is water, usually containing considerable quantities of saline matter dissolved in it. Sometimes the saline matters are present in such abundance that they cannot all pass into solution, but crystallise out, as in [fig. 7]—Nos. 3, 4, 5—where cubic crystals of the chlorides of sodium and potassium are seen floating in the liquid; in other cases the liquid is a hydrocarbon like the mineral oil which is present in great abundance in deep-seated rocks in many parts of the globe. But in some other cases the liquid contained in the cavities of crystals is found to be one which could scarcely be anticipated to occur under such circumstances—the gas known as carbonic add, which under extreme pressure can be reduced to a liquid condition. In cavities containing liquefied carbonic acid, if the rock be warmed up to 86° or 90° Fahrenheit the bubble suddenly vanishes, sometimes with an appearance like ebullition or boiling, as represented in [fig. 9]. Now the temperature which we have indicated is the 'critical point' of carbonic acid, and above that temperature it cannot exist in a liquid condition, however great may be the pressure to which it is subjected. The liquid has been converted into a gas which completely fills the cavity. The carbonic acid in the cavities of crystals has frequently been isolated and its nature placed beyond doubt by spectroscopic and ordinary chemical tests.

The presence of these liquids in the cavities of crystals clearly proves that the latter must have been formed under enormous pressure—a pressure sufficiently great to reduce, not only steam, but also volatile hydrocarbons and even gaseous carbonic acid, to the bulk of a liquid.

Fig. 9.—Cavity in Crystal containing Carbonic-Acid Gas at a temperature of 86° F., and passing from the liquid to the gaseous condition.

Such conditions of enormous pressure we may infer to exist in the deep-seated reservoirs beneath volcanoes, where, besides the weight of the superincumbent rock-masses, we have the compressing force of great quantities of elastic vapour held in confinement. The crystals of which granitic rocks are entirely built up exhibit clear evidence of having been all formed under these conditions of enormous pressure. The glassy base or groundmass of lavas, on the other hand, presents all the characters of materials that have cooled from a state of fusion. Most lavas consist in part of crystals, exhibiting fluid-cavities like those present in granite, and in part of a base, which has evidently been formed by the cooling of a fused mass. We are therefore justified in concluding that the crystals have been formed in subterranean recesses, and that the groundmass or base has consolidated at the surface. The bearing of these conclusions upon some of the great problems presented by volcanoes we shall have occasion to point out in the sequel.

CAUSE OF MOVEMENT OF BUBBLES.

One of the most interesting inquiries suggested by the study of the liquid-cavities in volcanic rocks is that of the cause of the apparently spontaneous movement of the bubbles which we have described as taking place in some of the smaller of them. The ingenious experiments of Mr. Noel Hartley have suggested to Professor Stokes an explanation which is probably the true one. It appears that these minute globes of vapour are in such a state of unstable equilibrium as to be affected by the smallest changes of temperature, and that the variations in the heat of the atmosphere, due to currents of air and the movement of warm or cold bodies through it, are sufficient to cause the oscillation of these sensitively poised bubbles.