A theory describing the state of mass balance in the Earth's crust which can be considered as less dense blocks floating on the denser semi-molten mantle. Thus high mountains must be regions where the crust is thickest, with deep roots extending into the mantle. Also, continents uplift (elastic rebound) when material is removed by erosion or an overburden of ice is melted away, as at the end of an ice age.
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. Certain areas (such as the Himalayas) 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, an amount of water equal in volume 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.Isostatic effects of deposition and erosion
When large amounts of sediment are deposited on a particular region, the immense weight of the new sediment may cause the crust below to sink.
An analogy may be made with an iceberg - it always floats with a certain proportion of its mass below the surface of the water.
Isostatic effects of plate tectonics
When continents collide, the continental crust may thicken at their edges in the collision.
However, some continental collisions are far more complex than this, and the region may not be in isostatic equilibrium, so this subject has to be treated with caution.
Isostatic effects of ice-sheets
The formation of ice-sheets can cause the Earth's surface to sink.
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.
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.
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