Schematic representation of the principle of isostatic equilibrium in geodynamics. The three situations in the figure are 1: a mountain range or plateau; 2: a totally filled basin (surface is at sea level); 3 an underfilled basin (with sea water forming the top part of the column). The formulas to the right give the lithostatic pressure at reference height. Symbols used: ρL = density of the lithosphere; ρA = density of the asthenosphere; ρS = density of the sediments in the basin; ρW = density of sea water. The heights of different sections of the columns are represented by a letter h; while g is the gravitational acceleration. 
In the case of isostatic equilibrium, the (lithostatic) pressure should be equal for all three cases at the reference height, in formula: P1 = P2 = P3.
Own work
Woudloper