ISOSTATIC EQUILIBRIUM AND MECHANICAL EQUILIBRIUM

The sketch below represents a schematic cross-section through a lithospheric plate. Although the thicknesses of both the crust and the lithospheric mantle vary, isostatic equilibrium is maintained.
• Isostatic equilibrium implies that at - and below - a particular depth, called the compensation depth, the pressure becomes hydrostatic (ie pressure shows no lateral variation). In other terms, at or below the compensation level, the weight of vertical columns with the same cross-sectional areas standing on the same gravitational equipotential surface are the same.
• Isostasy controls the elevation of the Earth's surface. Lithospheres with thin/thick continental crusts have lower/higher surface elevation compared to the average lithosphere. However, isostatic equilibrium does not mean mechanical equilibrium. Lateral variations of density along a given equipotential surface produce gravitational forces (body forces) acting inside the lithospheric plate.

Gravitational potential energy and the gravitational force

The sketch bellow illustrates the variation with depth of the lithostatic pressure along two lithospheric columns, B and C, in isostatic equilibrium. It is assumed here that the density of the crust (pink) and that of the mantle (green) are laterally constant (i.e. they are the same in both columns) and that there is no density difference between the lithospheric mantle and the asthenosphere. Because it has a thicker crust, elevation of column C is higher than that of column B. Consequently the lithostatic pressure at any depth within the crust of column C is a higher than the lithostatic pressure at the same depth in column B. However, at or below the compensation depth (here the base of crust in column C) the lithostatic pressure is the same in both column.