Page:Popular Science Monthly Volume 91.djvu/124

 Mountains That Float

Why? Because the underlying materials are lighter than the mountains

��IN a remarkable series of researches con- ducted by the U. S. Coast and Geodetic Survey, under the direction, first of Professor John F. Hayford, and later of Mr. William Bowie, Chief of the Division of Geodesy, it has been conclusively proved that mountains and continents, and prob- ably islands, float.

The earth is solid. How, then, can we say that the mountains float?

A hundred or even a thousand years are as a day in the geological calendar; and for such periods the movement of portions of the earth, under any forces which may act on them, would probably be so small as to go unnoticed. But when such forces act for a hundred thousand or a million years or longer, the earth's materials behave as if they were plastic; in other words, they give way to the forces affecting them, and assume a state of equilibrium. Those who work in the deep mines of the earth are familiar with the movement or creeping of the rock which will close old shafts or tunnels.

But what is the evidence that mountains float ? The answer is in the observed tilting of the plumb line and in the measurements of the earth's pull, called gravity.

The Proof Furnished by Gravity

Let us confine our attention to gravity. If the earth's material were a perfect liquid, its surface would be perfectly smooth and the shape of this surface would be that of a ball. Since the earth spins very rapidly, centrifugal force is set up which causes the maximum at the equator and a minimum or zero at the poles. This means a shortening of the axis of rotation by a distance of about twenty-six miles. That is, the distance from the north pole to the south pole would be that much shorter than the distance through the earth's center between two points opposite each other on the equator.

Exactly the same shape would result if the earth's material, though solid, were arranged in layers according to the density, the densest material at the earth's center and the lightest at the surface. The earth would assume that shape because of the yielding under the attractive force of each particle of the earth's material on every

��other particle, and because of the centrif- ugal force due to the earth's rotation which tends to throw its materials out into space.

The resultant of these forces, or gravity, would on this ideal earth vary gradually in intensity from the equator to the poles.

How We Get the Idea of Floating

The earth's materials are not arranged in layers exactly with respect to their densities. As a matter of fact gravity determinations show that the materials under the vast plains along the coasts are arranged very nearly in the normal way, but that the materials under the mountains are found to be lighter than normal. The deficiency of material under a mountain down to a depth of about sixty miles below the sea level is almost exactly equal to the mountain material which is above sea level.

Similarly, under plateaus like those of our Western States, there is a deficiency of materials very nearly or exactly equal to the mass of material between the surface and the imaginary sea level surface beneath. The normal density is that under the coastal plains.

As a result of this counterbalancing of the material above sea level at any part of a continent by material lighter than nor- mal under it, the pressure or weight of material on an imaginary surface about sixty miles below sea level is the same at all points of that surface.

This brings out the idea of floating. If we should see an iceberg floating in the ocean, we would conclude that the ice showing above the water is held up or floated as a consequence of a greater mass of ice under it. As all know, ice is lighter than water. A block of wood thrown in the water has some of its material held above the surface by the portion under the water. The weight of the material of the whole block exactly equals that of the water displaced by the block. Similarly the weight of a mountain mass and the column of material directly under it to a depth of say sixty miles, below the imaginary sea level surface, equals that of the weight of a similar column of material of equal cross section, under the coastal plain, which has little or no material above sea level.

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