Volcanoes: What They are and What They Teach

by John W. Judd

If, however, the uncompensated effect of the external forces acting on the earth's crust is calculated to bring about the destruction of those conditions upon which the existence of life depends, the uncompensated effect of the internal forces acting on the earth's crust are fraught with at least equal dangers to those necessary conditions.

CONTRAST BETWEEN THE EARTH AND MOON.

In our nearest neighbour among the planets—the moon—the telescope has revealed to us the existence of a globe, in which the internal forces have not been checked and controlled by the operation of any external agencies—for the moon appears to be destitute of both atmosphere and water.

Under these circumstances we find its surface, as we might expect, to be composed of rocks which appear to be entirely of igneous origin; the mountain-masses, unworn by rain or frost, river or glacier, being of most prodigious dimensions as compared with those of our own globe, while no features at all resembling valleys, or plains, or alluvial flats are anywhere to be discerned upon the lunar surface.

But by the admirable balancing of the external and internal forces on our own globe, the conditions necessary to animal and vegetable existence are almost constantly maintained, and those interruptions of such conditions, produced by hurricanes and floods, by volcanic outbursts and earthquakes, may safely be regarded as the insignificant accidents of what is, on the whole, a very perfectly working piece of machinery.

The ancients loved to liken the earth to a living being—the macrocosm of which man was the puny representative or microcosm; and when we study the well-adapted interplay of the forces at work upon the earth's crust, both from within and without, the analogy seems a scarcely strained one. In the macrocosm and the microcosm alike, slight interferences with the regular functions occasionally take place, and both of them exhibit the traces of a past evolution and the germs of an eventual decay.

CHAPTER XI.
WHAT VOLCANOES TEACH US CONCERNING THE NATURE OF THE EARTH'S INTERIOR.

In entering upon any speculations or enquiries concerning the nature of the interior of our globe, it is necessary before all things that we should clearly realise in our minds how small and almost infinitesimal is that part of the earth's mass which can be subjected to direct examination. The distance from the surface to the centre of our globe is nearly 4,000 miles, but the deepest mines do not penetrate to much more than half a mile from the surface, and the deepest borings fall far short of a mile in depth. Sometimes, it is true, the geologist finds means for drawing inferences as to the nature of the rocks at depths of ten or fifteen miles below the surface; but the last-named depth must be regarded as the utmost limit of that portion of our globe which can be made the object of direct observation and study. This thin exterior film of the earth's mass, which the geologist is able to investigate, we call the 'crust of the globe'; but it must be remembered that in using this term, it is not intended to imply that the outer part of our globe differs in any essential respect from the interior. The term 'crust of the globe' is employed by geologists as a convenient way of referring to that portion of the earth which is accessible to their observation.

But if we are unable to make direct investigations concerning the nature of the internal portions of the globe, there are nevertheless a number of facts from which we may draw important inferences upon the subject. These facts and the inferences based upon them we shall now proceed to consider.

First in importance among these we may mention the results which have been obtained by weighing our globe. Various methods have been devised for accomplishing this important object, and the conclusions arrived at by different methods agree so closely with one another, that there is no room for doubt as to the substantial accuracy of those results. It may be taken as proved beyond the possibility of controversy that our globe is equal in weight to five and a half globes of the same size composed of water, or, in other words, that the average density of the materials composing the globe is five and a half times as great as that of water.