A highly simplified isostatic balance between continental and oceanic crustal columns is shown in the figure below. With the approximation that gravity is constant through the depths shown, we can write the balance as
where the greek letter rho represents density of the crust, (rho)c, mantle, (rho)m, and water, (rho)w, respectively. Hc is the thickness of the continental crust, Ho the thickness of the oceanic crust, Hw the depth of water, and R the thickness of the crustal "root" of the continent, or the thickness of the sub-oceanic mantle that enters into the balance. The assumed condition is that the weights of columns above a level taken at the base of the continental crust are equal.
For the values shown, R = 28.2 km and the elevation h = 1.8 km; For Hc = 35 km, the elevation is reduced to 1.1 km.