Isostasy

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 accomodated by height adjustments.

Isostatic models

Two principle models of isostasy are used:

• The Airy hypothesis

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

• The Pratt hypothesis

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

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 100s of metres above present-day sea-level.