isostasi

Isostasy, also known as isostasia, is the state of gravitational vertical equilibrium between the Earth’s lithosphere and the underlying mantle. In this framework, the crust behaves as a floating rigid shell whose elevation is set by buoyancy forces in the mantle, so continents and ocean basins rise or fall to balance their mass. The concept explains why continental crust is thicker and less dense than oceanic crust, yet sits at different elevations.

Two classic models describe how isostatic balance can be achieved. Airy isostasy posits that topography is supported by variations in crustal thickness: mountains have deep “roots” that extend into the mantle to equalize pressure at depth. Pratt isostasy argues that horizontal variations in crustal density, with lighter regions supporting higher topography, can provide the same surface elevations without thick roots. In practice, both crustal thickness and density variations, as well as mantle flow, contribute to isostatic balance; the real Earth exhibits a combination of these effects and dynamic adjustments.

Isostasy also encompasses time-dependent processes. Glacial isostasy refers to the slow vertical readjustment of crust after the retreat of huge ice sheets, causing uplift in formerly loaded regions and subsidence elsewhere as the mantle re-equilibrates. Ongoing vertical motions due to tectonics, sediment loading, and mantle convection reflect deviations from strict static isostasy.

Applications and limitations: isostasy informs interpretations of topography, gravity anomalies, crustal thickness, and regional geoid variations. However, it is an idealized concept; dynamic mantle flow and non-buoyant forces can cause deviations from simple isostatic balance.