Isostasy is a theory describing the maintenance of an equilibrium in the total mass of Earth's crust despite its crustal movement. There exists a balance between the land masses and the mantle of 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 Earth's crust is constant. The spread of the continental plates by the upwelling of material from deep within Earth's crust is balanced by the submergence of the opposite edges of the plates.