isostasy

Isostasy is the geological principle describing the gravitational equilibrium between the Earth’s lithosphere and the more ductile mantle beneath it. The lithosphere behaves as a buoyant shell that “floats” on the viscous mantle, so surface topography is supported by buoyancy forces. When a load is added or removed—such as ice during glaciations, sediment deposition, or erosion—the lithosphere adjusts vertically toward a new balance, a process known as isostatic adjustment.

Two classical models explain how the compensation occurs. In Airy isostasy, crustal thickness varies while density remains roughly constant: high topography is supported by a deeper, thicker crust that forms a buoyant root. In Pratt isostasy, crustal density varies laterally with relatively uniform thickness: variations in elevation are produced by density contrasts within the crust and mantle beneath. Real Earth likely exhibits a combination of these mechanisms, influenced by mantle rheology and dynamic processes rather than a single idealized scheme.

Isostasy has observable consequences. Post-glacial rebound occurs when ice sheets melt, and previously loaded regions rise slowly today, as seen in parts of Fennoscandia and North America. Gravity and seismic data reveal crustal roots and density variations consistent with isostatic compensation. While a useful first-order framework, isostasy is not a strict static condition; long-term plate tectonics, mantle convection, and dynamic topography can modify or override simple vertical equilibrium in some regions.