Approximate gravitational compensation of loads by subsurface mass redistribution. Airy and Pratt models use local column balances; flexural support spreads the load laterally. Real surface topography can also be maintained by dynamic stresses.
A cold linear plate profile has excess column mass relative to hot mantle. Equal hydrostatic pressure below the plates is restored by replacing part of the rock column with water. The displayed elevation follows to first order in thermal expansion; its denominator is the rock-minus-water density difference, not the rock density alone.
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Isostasy is a geological concept that describes the state of gravitational equilibrium between the Earth's lithosphere (the rigid outer layer of the Earth) and the underlying asthenosphere (a semi-fluid layer of the upper mantle). According to the principle of isostasy, the Earth's crust "floats" on the denser, more fluid mantle beneath it, much like an iceberg floats in water.