EUSTASY

Changes in the shape and level of the surface of the world’s oceans in equilibrium with the gravitational field of the Earth. Rise and fall in this surface may be due to water volume changes (mainly as a result of glacio-eustasy), to changes in the shape of the equipotential surface (geoidal eustasy) and to changes in the shape of ocean basins (tectono-eustasy). The best-known eustatic changes are glacio-eustatic changes caused by the addition and removal of water to the total ocean volume as a result of climate-induced variations in the dimensions of the world’s glaciers and ice sheets. Geoidal-eustatic changes are less well known and arise because the surface of the world’s ocean is uneven due to regional variations in the Earth’s gravitational field. The surface of oceans represents an equipotential surface of the Earth’s gravitational field, known as the geoid.

Tectono-eustatic changes are long-term alterations in the shape of ocean basins as a result of plate motion (see plate tectonics). For example, it has been argued that as a result of sea-floor spreading, ocean basins have been widening at an average rate of 16 cm/year during the Late quaternary. If correct, this would mean that the volume of the world’s ocean basins has increased by 6 per cent since the last interglacial. The situation is complex, however, since it can be argued that during the Quaternary, the total amount of global sea-floor spreading has been offset by reductions in ocean volume caused by subduction processes.

[See alsoisostasy, sea level, sea-level change]