Volcanoes: What They are and What They Teach

by John W. Judd

But although we may conclude that at the moderate depths we have indicated such high temperatures exist, it would not be safe to infer, as some have done, that at a distance of only 40 or 50 miles from the surface the materials composing our globe are in a state of actual fusion. Both theory and experiment indicate that under increased pressure the fusing point of solid bodies is raised; and just as in a Papin's digester we may have water retained by high pressure in a liquid condition at a temperature far above 212° F., so in the interior of the earth, masses of rock may exist in a solid state, at a temperature far above that at which they would fuse at the earth's surface. We may speak of such rock-masses, retained in a solid condition by intense pressure, at a temperature far above their fusing point at the earth's surface, as being in a 'potentially liquid condition.' Upon any relief of pressure such masses would at once assume the liquid state, just as the superheated water in a Papin's digester immediately flashes into steam upon the fracture of the strong vessel by which it is confined. We have already seen how the action at volcanic vents often appears to indicate just such a manifestation of elastic forces, as would be exhibited by the relief of superheated masses from a state of confinement by pressure.

In reasoning upon questions of this kind, however, we must always be upon our guard against giving undue extension to principles and laws which seem to be clearly established by experiment at the earth's surface. It is well to remember how exceedingly limited is our command of extreme pressures and high temperatures, when compared with those which may exist within a body of the dimensions of our globe.

EFFECT OF PRESSURE ON FUSION-POINT.

If we were to imagine a set of intelligent creatures, who were able to command only a range of temperatures from 50° to 200° F., engaged upon an investigation of the properties of water, we shall easily understand how unsafe it may be to extend generalisations far beyond the limits covered by actual experiment. Such beings, from their observation of the regular changes of volume of water at all the temperatures they could command, might infer that at still higher and lower temperatures the same rates of expansion and contraction would be maintained. Yet, as we well know, such an inference would be quite wide of the truth; for a little above 200° F. water suddenly expands to 1,700 times its volume, and not far below 50° F. the contraction is suddenly changed for expansion.

It has been argued by the late Mr. David Forbes and others that, inasmuch as experiment has shown that—though the fusing points of solids are raised by pressure, yet that this rise of the fusing points goes on in a diminishing ratio as compared with the pressures applied—a limit will probably be reached at which the most intense pressure will not be sufficient to retain substances at a high temperature in their solid state. The fact that gases cannot be retained in a liquid condition by the most intense pressure at a temperature above their critical point, may seem by analogy to favour the same conclusion. Hence, David Forbes, Dana, and other authors, have argued in favour of the existence of a great liquid nucleus in our globe covered by a comparatively thin, solid crust. And if we accept the supposed proofs of a constant increase of temperature from the surface to the centre of the globe, such a conclusion appears to be at least as well founded as that which regards the central masses of the earth as maintained in a solid condition by intense pressure.

A little consideration will, however, convince us that the facts which have been relied upon as proving the intensely heated condition of the central masses of our globe, are by no means so conclusive as has been supposed.

The earth's form, which mathematicians have shown to be exactly that which would be acquired by a globe composed of yielding materials rotating on its axis at the rate which our planet does, has often been adduced as proving that the latter was not always in a rigid and unyielding condition. In the same way, all the remarkable facts and relations of the bodies of the solar system, which have been shown by astronomers to lend such support to the nebular hypothesis, have been thought, at the same time, to favour the view that our earth is still in a condition of uncompleted solidification.

But it is quite admissible to accept the nebular hypothesis and the view that our globe attained its present form while still in a state of fluidity, and at the same time to maintain that our earth has long since reached its condition of complete solidification. And there are not a few facts which appear to lend support to such a conclusion.

SUPPOSED PROOFS OF LIQUID NUCLEUS.

If the rapid rate of increase in temperature which has been demonstrated to occur at so many parts of the earth's surface be maintained to the centre, then, as argued by David Forbes and Dana, it is difficult to conceive of our earth as being in any other condition than that of a liquid mass covered by a comparatively thin crust. The objection to this view, both upon geological and astronomical grounds, we have pointed out in the previous chapter.