isostasy
Isostasy is a theory describing the maintenance of an equilibrium in the total mass of the Earth's crust despite its crustal movement. There exists a balance between the land masses and the mantle of the Earth on which the continental plates float, so that the plates rise and sink on the semi-molten surface of the mantle in such a fashion that the relative mass weighing upon the Earth's crust is constant. The spread of the continental plates by the upwelling of material from deep within the Earth's crust is balanced by the submergence of the opposite edges of the plates.