Isostasy is a geological theory explaining the general equilibrium in the earth’s crust, which is supposed to be maintained by the yielding or flow of rock material beneath the surface under gravitative stress. By the theory of isostasy each unit column of the earth, from surface to center, has approximately the same weight, and the continents stand higher than the ocean beds chiefly because the material of the crust has there less density. A theory of the condition of approximate equilibrium in the outer part of the earth, such that the gravitational effect of masses extending above the surface of the geoid in continental areas is approximately counterbalanced by a deficiency of density in the material beneath those masses, while deficiency of density in ocean waters is counterbalanced by an excess in density of the material under the oceans.
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