Volcanoes: What They are and What They Teach

by John W. Judd

Now the density of the materials which compose the crust of the globe is very much less than this, varying from about two-and-one-third to three times that of water. Hence we are compelled to conclude that the interior portions of the globe are of far greater density than the exterior portions; that, as a matter of fact, the mass of the globe is composed of materials having twice the density of the rocks exposed at the surface.

DENSITY OF EARTH'S INTERIOR.

It has been sometimes argued that as all materials under intense pressure appear to yield to an appreciable extent, and to allow their particles to be packed into a smaller compass, we may find in this fact an explanation of the great density of the internal parts of the globe. It has in fact been suggested that under the enormous pressure which must be exerted by masses of rock several thousand feet in thickness, the materials of which our earth is composed may be compelled to pack themselves into less than one-half the compass which they occupy at the surface. But the ascription of such almost unlimited compressibility to solid substances can be supported neither by experiment nor analogy. Various considerations point to the probability that solid bodies yield to pressure up to a certain limit and no farther, and that when this limit is reached an increase in pressure is no longer attended with a reduction in bulk.

If then we are compelled to reject the idea of the unlimited compressibility of solid substances, we must conclude that the interior portions of our globe are composed of materials of a different kind from those which occur in its crust. And this conclusion, as we shall presently see, is borne out by a number of independent facts.

The study of the materials ejected from volcanic vents proves that even at very moderate depths there exist substances differing greatly in density, as well as in chemical composition. The lightest lavas have a specific gravity of 2·3, the heaviest of over 3. And that materials of even greater density are sometimes brought by volcanic action from the earth's interior, we have now the clearest proofs.

RELATION BETWEEN EARTH AND OTHER PLANETS.

But in considering a question of this kind, it will be well to remember that analogy may furnish us with hints upon the subject which may prove to be by no means unimportant. There is no question upon which modern science has wrought out a more complete revolution in our ideas, than that of the relation of our earth to the other bodies of the universe. We know, as the result of recent research, that our globe is one of a great family of bodies, moving through space in similar paths and in obedience to the same laws. A hundred years ago the primary and secondary planets of the solar system could be almost numbered upon the fingers; now we recognise the fact that they exist in countless millions, presenting every variety of bulk from masses 1,400 times as large as our earth down to the merest planetary dust. Between the orbits of Mars and Jupiter, more than 200 small planets have been recognised as occurring, and every year additions are made to the number of these asteroids. Comets have now been identified with streams of such planetary bodies, of minute size, moving in regular orbits through our system. The magnificent showers of 'shooting-stars' have been proved to be caused by the passage of the earth through such bands of travelling bodies, and 'the zodiacal light' finds its most probable explanation in the supposition that the sun is surrounded by a great mass of such minute planets. Every increase in the power of the telescope reveals to us the existence of new secondary planets or moons, revolving about the primaries; and the wonderful system of the Saturnian rings is now explained by the proved existence of great streams of such secondary planets circling around it. The solar system was formerly conceived of as a vast solitude through which a few gigantic bodies moved at awful distances from one another. Now we know that the supposed empty void is traversed by countless myriads of bodies of the most varied dimensions, all moving in certain definite paths, in obedience to the same laws, ever acting and reacting upon each other, and occasionally coming into collision.

There are not wanting further facts to prove that the other planets are like our own in many of their phenomena and surroundings. In some of them atmospheric phenomena have been detected, such as the formation of clouds and the deposition of snow, so that the external forces at work on our globe act upon them also. And that internal forces, like those we have been considering in the case of our earth, are at work in our neighbours, is proved by the great solar storms and the condition of the moon's surface.

But the results of spectrum-analysis in recent years have furnished new facts in proof of the close relationship of our earth to the numerous similar bodies by which it is surrounded. So far as observation has yet gone we have reason for believing that not only the members of the solar system, but the more distant bodies of the universe, are all composed of the same elementary substances as those which enter into the composition of our globe.

The most satisfactory information concerning the composition and nature of other planetary bodies is derived from the study of those small planets which occasionally come into collision with our globe, and which have their own proper motion in space thereby arrested. These meteorites, as such falling planetary bodies are called, have justly attracted great attention, and their fragments are treasured as the most valuable objects in our museums.