Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the Earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. It is invoked to explain how different topographic heights can exist at the Earth's surface. When a certain area of lithosphere reaches the state of isostasy, it is said to be in isostatic equilibrium. Certain areas (such as the Himalaya) are not in isostatic equilibrium, which has forced researchers to identify other reasons to explain their topographic heights (in the case of the Himalaya, by proposing that their elevation is being "propped-up" by the force of the impacting Indian plate).

In the simplest example, isostasy is the principle observed by Archimedes in his bath, where he saw that when an object was immersed, a volume of water equal to that of the object was displaced. On a geological scale, isostasy can be observed where the Earth's strong lithosphere exerts stress on the weaker asthenosphere which, over geological time flows laterally such that the load of the lithosphere is accommodated by height adjustments.

Isostatic models

Two principal models of isostasy are used:

• The Airy hypothesis

- where different topographic heights are accommodated by changes in crustal thickness.

• The Pratt hypothesis

- where different topographic heights are accommodated by lateral changes in rock density.

Isostatic rebound

Isostatic rebound is observed in areas where a loading force has been removed. A key example is in areas which were once covered by ice-sheets, such as around the Baltic Sea and Hudson Bay. As the ice retreats, the load on the lithosphere and asthenosphere is reduced and they rebound back towards their equilibrium levels. In this way, it is possible to find sea-cliffs, and associated wave-cut platforms hundreds of metres above present-day sea-level.

Eustasy and Relative Sea Level Change

Eustasy is another cause of relative sea level change quite different from isostatic causes. The term "eustasy" or "eustatic" refers to changes in the amount of water in the oceans, usually due to global climatic changes. When the Earth's climate cools, water is evaporated from the oceans and is precipitated on land masses as year-round ice and snow, causing a relative sea level fall (relative to a stable land mass). The refilling of ocean basins by glacier meltwater at the end of ice ages is an example of eustatic sea level rise.

A second significant cause of eustatic sea level rise is thermal expansion of sea water, when the Earth's mean temperature increases. Current estimates of global eustatic rise vary from as little as zero to +2 mm/a, its determination being difficult due to the need to separate out local vertical crustal movements.

When the term "relative" is used in context with "sea level change", the implication is that both eustasy and isostasy are at work, or that the author does not know which cause to invoke. It means that the position of the coastline is changing without specifying whether the land is rising/sinking or the oceans are filling/emptying.

Further reading

• Lisitzin E. (1974) "Sea level changes". Elsevier Oceanography Series, 8