Volcanoes: What They are and What They Teach

by John W. Judd

It has been shown that if a part of the earth's crust fifty miles in thickness were to have its temperature raised 200° Fahrenheit, its surface would be raised to the extent of 1,000 or 1,500 feet Le Conte has pointed out that if we conceive the conduction of heat to take place at slightly different rates along different radii of our globe, we should at once be able to account for the existing inequalities of the earth's surface, and for all those continental movements which can be shown to have taken place in past geological periods.

DYNAMICAL THEORIES.

But if we admit, as we have good grounds for doing, that the loss of heat from the external portions of our globe goes on more rapidly than in the case of the central masses, we have thereby introduced another powerful agent for the production of high temperatures within the earth's crust. The external shell of the globe will tend to contract upon the central mass, and in so doing a series of tangential strains will result which will be capable of folding and crumpling the rocks along any lines of weakness. That such crushing and crumpling has during all geological periods taken place along lines of weakness in the earth's crust, is proved, as we have seen, by the phenomena presented by mountain-ranges. Now these crushings, crumplings, and other violent movements of great rock-masses must result in the development of a vast amount of heat, just as the forcing down of a break upon a moving wheel produces heat. This conclusion is strikingly confirmed by the well-known geological fact that nearly all rocks which have undergone great movement and contortion are found to present evidence of having been subjected to such chemical and crystalline actions, as would result from the development of a high temperature within their mass.

RECAPITULATION OF SEVERAL THEORIES.

Let us sum up briefly the various methods which have been suggested to account for the high temperatures within certain parts of the earth's crust by which volcanic phenomena are produced.

Our globe may be conceived of as an incandescent liquid mass surrounded by a cooler, solid shell. If we regard this liquid interior mass as supplying directly the various volcanic vents of the earth, it must be conceded that the outer shell is of comparatively slight thickness. But astronomers are almost universally agreed that such a thin outer shell and inner liquid mass are quite incompatible with that rigidity which our planet exhibits under the attractions of its neighbours. Geologists are almost equally unanimous in regarding this hypothesis of a liquid nucleus and thin, solid shell as contradicted by the stability of the conditions which have been maintained during such long past periods, and which exist at the present day. The extent and character of volcanic action do not indicate a condition of general instability in our earth, but one of stability subject to small and local interferences The grandest volcanic disturbances appear small and insignificant, if we take into account the vast dimensions of the globe upon which they are displayed.

If, on the other hand, we consider the outer solid shell to be of great thickness, we are met by the difficulty of accounting for the upheaval of liquid matter through such vast thicknesses of a solid shell. The differences in character of lavas extruded from closely adjoining volcanic districts seem equally difficult of explanation on any theory of a central, fluid nucleus and a solid, outer shell. Nor is the distribution of heat within the earth's crust so uniform as might be anticipated, if the source of that heat be a great central mass of highly heated materials.

Under these circumstances, geologists and physicists have enquired whether any other conditions can be imagined as existing in the earth's interior, which would better account for the observed phenomena than does the hypothesis of a liquid nucleus and a solid outer shell. Two such alternative hypotheses have been suggested.

Mr. Hopkins, adopting the theory that the earth has solidified both at the centre and its outer surface, endeavoured to explain the occurrence of volcanoes and earthquakes by supposing that cavities of liquid material have been left between the solid nucleus and the solid shell, and these cavities full of liquid material constitute the sources from which the existing volcanoes of the globe draw their supplies. But this hypothesis is found to be beset with many difficulties when we attempt to apply it to the explanation of the phenomena of volcanic action. It entirely fails, among other things, to account for the remarkable fact that during past geological periods the scene of volcanic action has been continually shifting over the surface of the earth, so that there is probably no considerable area of our globe which has not at one time or other been invaded by the volcanic forces.

By some other theorists, who have felt the full force of this last objection, an attempt has been made to explain the phenomena of volcanoes by supposing that the globe is solid from its surface to its centre, but that the internal portions of the globe are at such a high temperature that they are only retained in a solid condition by the enormous pressure to which they are subjected. The central masses of the globe are thus regarded as being in an actually solid, but in a potentially liquid condition, and any local relief of pressure is at once followed by the conversion of solid to liquefied materials, in the district where the relief takes place, resulting in the manifestation of volcanic phenomena at the spot. It may be granted that this hypothesis better accords with the known facts of Vulcanology than any of those which we have previously described, but it is impossible to shut our eyes to the fact that not a few serious difficulties still remain. Thus it is based upon the assumption that the law of the elevation of the point of fusion by pressure is true at temperatures and pressures almost infinitely above those at which we are able to conduct observations; but neither experiment nor analogy warrant this conclusion, for the former shows that the elevation of the point of fusion by pressure goes on in a continually diminishing ratio, and the latter famishes us with the example of volatile liquids which, above their critical points, obstinately remain in a gaseous condition under the highest pressures. Nor is it easy upon this hypothesis to account for the very irregular distribution of temperatures within the earth's crust, as demonstrated by observations in mines, wells, and borings. The hypothesis further requires the assumption that, at such very moderate depths as are required for the reservoirs of volcanoes, the effects of pressure and temperature on the condition of rock-materials are so nicely balanced that the smallest changes at the surface lead to a disturbance of the equilibrium.