Ortostazy

Ortostazy, in geology, describes the vertical adjustment of the Earth's lithosphere relative to the asthenosphere in order to achieve gravitational equilibrium. The lithosphere behaves as a buoyant shell that floats on the more ductile mantle, and its elevation adjusts in response to changes in surface load.

Two classical models explain the compensation: Airy isostasy, which assumes crustal columns have the same strength and that topography is produced by variations in crustal thickness while density is constant; and Pratt isostasy, which assumes uniform crustal thickness but lateral density variations cause topography to rise or sink. Modern understanding also includes dynamic and flexural isostasy, incorporating mantle flow and elastic strength of the lithosphere.

Loads such as ice sheets, mountains, sedimentation or erosion alter buoyancy and trigger vertical movements that restore equilibrium over time. When a load is removed, such as after glacial retreat, the crust uplifts in a process known as post-glacial rebound or isostatic rebound.

Evidence for ortostazy comes from gravity data, seismic profiling, and time-dependent geodetic measurements (GPS, leveling). Regions like Fennoscandia and parts of North America have experienced measurable uplift since the last glacial maximum; ocean basins show smaller isostatic responses due to existing hydrostatic balance of oceanic crust.

Ortostazy is a foundational concept in tectonics and geodesy, helping explain long-term topographic differences, continental convergence, and regional sea-level changes.