isostázis

Isostasy is the gravitational equilibrium between the Earth’s lithosphere and the, underlying, asthenosphere. In this state, the topography of continents and ocean basins is supported by buoyancy forces in the partially molten mantle. The crust adjusts its thickness or density so that columns of rock extending to depth are balanced by the weight of surrounding mantle. When additional load is placed on the crust or is removed, vertical movements of crust and mantle occur as the system moves toward a new equilibrium.

Two classic models describe isostatic support. The Airy model postulates compensation by variations in crustal thickness: thicker crust under tall mountains provides the necessary buoyancy. The Pratt model assumes lateral changes in crustal density with a relatively uniform thickness, so less dense regions rise. Real crust often combines elements of both mechanisms.

Isostatic adjustments occur over timescales ranging from thousands to millions of years and are driven by processes such as erosion, sedimentation, glaciation, and deglaciation. Post-glacial rebound is a well-known example, where land previously loaded by ice sheets rises as the load is removed and the mantle slowly relaxes.

Evidence for isostasy includes gravity field data, seismic imaging, and geodetic measurements (GPS and leveling) that show ongoing vertical movements in formerly glaciated regions and in response to sedimentation and tectonic loading. While useful as a first approximation, isostasy interacts with dynamic mantle flow, lithospheric flexure, and tectonics, which can cause local deviations from simple models.