VI.—Radio-activity and the Earth's Thermal History

Extract

The distribution of land and sea implies that the earth's outer shell is in a condition of approximate hydrostatic equilibrium. Otherwise the equatorial regions should be girdled by a continental protrusion, or else each of the polar regions should be occupied by a continental bulge. The observed fact that the inequalities of the earth's surface exhibit neither of these conditions proves that the general ellipticity of the lithosphere does not differ greatly from that which would be assumed by a liquid spheroid having a similar distribution of density in depth. The incapacity—thus demonstrated—of the lithosphere to endure permanent stresses leads naturally to an inquiry into the conditions that maintain continents and mountain ranges above sea-level. Investigations based on the deviations of the plumb-line from the vertical and on the varying intensity of gravity indicate beyond doubt that the elevated tracts of the globe owe their support to a deficiency of density in their deep-seated foundations, while the great sunken areas owe their depression to a corresponding excess of density in the underlying rocks. Thus has arisen the conception of isostasy, a word which was coined by Dutton in 1889 to express the state of hydrostatic balance that maintains in position elevated and depressed columns of the lithosphere.