hydroisostasy

Hydroisostasy is the portion of isostatic adjustment of the Earth's crust that results from changes in the weight of the surface water load. Like traditional isostasy, it describes how the crust and the underlying mantle respond to buoyancy forces, but the load in question is the mass of water in oceans, seas, lakes, and rivers. When surface water mass increases, such as during sea-level rise or expansion of large freshwater bodies, the additional load depresses the crust and the mantle flows to restore equilibrium, causing subsidence in the loaded area and uplift nearby. Decreases in water loading have the opposite effect.

In marine and coastal settings, hydroisostatic adjustment interacts with lithospheric flexure and variations in mantle rheology. The result can be changes in seafloor topography, coastline position, and regional gravity anomalies over geological timescales. Hydroisostasy is thus an important component of the broader framework used to understand sea-level changes, shoreline evolution, and the vertical motion of continents and margins.

Relation to other concepts: hydroisostasy is often treated alongside traditional models of isostasy (such as Airy and Pratt) and within mantle-viscoelastic or flexural models. It is closely connected to glacial isostatic adjustment and eustatic sea-level studies, because large-scale changes in water load—whether from melting ice sheets or hydrological cycles—drive part of the vertical crustal response observed in geology and geodesy.

See also: isostasy, glacial isostatic adjustment, sea level change, lithospheric flexure, mantle rheology.