Three principal models of isostasy are used:

- The Airy?Heiskanen model ? where different topographic heights are accommodated by changes in crustal thickness, in which the crust has a constant density
- The Pratt?Hayford model ? where different topographic heights are accommodated by lateral changes in rock density.
- The Vening Meinesz, or flexural isostasy model ? where the lithosphere acts as an elastic plate and its inherent rigidity distributes local topographic loads over a broad region by bending.

Airy and Pratt isostasy are statements of buoyancy, whereas flexural isostasy is a statement of buoyancy when deflecting a sheet of finite elastic strength.

Over geologic time scales, the crust and asthenosphere act like fluids. Just as millions and billions of years is a challenge for your mind, so is the incredibly slow flow of the earth, or the plate motions.

You can review the passive continental margin, and the differences in density and crustal thickness.

Last revision 8/24/2016