Isostasy


Isostasy

Isostasy (Greek isos = "equal", stásis = "standstill") 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". It is important to note that isostasy is not a process that upsets equilibrium, but rather one which restores it. It is generally accepted that the earth is a dynamic system that responds to loads in many different ways, however isostasy provides an important 'view' of the processes that are actually happening. Nevertheless, 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 Himalayas, 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 of Buoyancy 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

Three principal models of isostasy are used:
* The Airy-Heiskanen Model: - where different topographic heights are accommodated by changes in crustal thickness.
* The Pratt-Hayford Model: - where different topographic heights are accommodated by lateral changes in rock density.
* The Vening Meinesz Model: - where the crust acts as an elastic plate and its inherent rigidity spreads topographic loads over a broader region. This hypothesis was put forward to explain how feautures of grand magnitude like the Himalayas could be explained using a regional isostatic compensation rather than a localised one which is the case for the first two models.

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. Similarly, when large amounts of material are eroded away from a region, the land may rise to compensate. Therefore, as a mountain range is eroded down, the (reduced) range rebounds upwards (to a certain extent) to be eroded further. Some of the rock strata now visible at the ground surface may have spent much of their history at great depths below the surface buried under other strata, to be eventually exposed as those other strata are eroded away and the lower layers rebound upwards again.

An analogy may be made with an iceberg - it always floats with a certain proportion of its mass below the surface of the water. If more ice is added to the top of the iceberg, the iceberg will sink lower in the water. If a layer of ice is somehow sliced off the top of the iceberg, the remaining iceberg will rise. Similarly, the Earth's lithosphere "floats" in the asthenosphere.

Isostatic effects of plate tectonics

When continents collide, the continental crust may thicken at their edges in the collision. If this happens, much of the thickened crust may move "downwards" rather than up as with the iceberg analogy. The idea of continental collisions building mountains "up" is therefore rather a simplification. Instead, the crust "thickens" and "the upper part of the thickened crust" may become a mountain range.

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. Conversely, isostatic post-glacial rebound is observed in areas once covered by ice-sheets which have now melted, 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 former sea-cliffs and associated wave-cut platforms hundreds of metres above present-day sea-level. The rebound movements are so slow that the uplift caused by the ending of the last Ice Age is still continuing.

In addition to the vertical movement of the land and sea, isostatic adjustment of the Earth also involves horizontal movements, changes in the gravitational field, Earth's rotation rate, polar wander, and can induce earthquakes. For details see Postglacial rebound.

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, a greater proportion of the earths water is stored on land masses in the form of Glaciers, snow, etc. This results in a relative fall in global sea levels (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 from tide gauge records and satellite altimetry is about +3 mm/a (see 2007 IPCC report). Global sea level is also affected by vertical crustal movements, changes in the rotational rate of the Earth, (see Postglacial rebound), large scale changes in continental margin and changes in the spreading rate of the ocean floor.

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.

Further reading

* Lisitzin, E. (1974) "Sea level changes". Elsevier Oceanography Series, 8
*Watts, A.B. (2001) "Isostasy and Flexure of the Lithosphere" Cambridge University Press

External links

*Interactive Isostasy Experiment [http://discoverourearth.org/student/topography/isostasy.html]

ee also

*Clarence Dutton, who coined the term "isostasy" in 1889.
* John Fillmore Hayford


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Look at other dictionaries:

  • Isostasy — I*sos ta*sy, n. [See {Iso }; {Stasis}.] The state or quality of being isostatic. Specif. (Geol.), general equilibrium in the earth s crust, supposed to be maintained by the yielding or flow of rock material beneath the surface under gravitative… …   The Collaborative International Dictionary of English

  • isostasy — 1889, from ISO (Cf. iso ) + Gk. stasis setting, weighing, standing (see STET (Cf. stet)). Related: Isostatic …   Etymology dictionary

  • isostasy — [ī säs′tə sē] n. [< ISO + Gr stasis, a standing still: see STASIS] 1. a condition in which there is equal pressure on every side 2. Geol. approximate equilibrium in large, equal areas of the earth s crust, preserved by the action of gravity… …   English World dictionary

  • isostasy — /uy sos teuh see/, n. 1. Geol. the equilibrium of the earth s crust, a condition in which the forces tending to elevate balance those tending to depress. 2. the state in which pressures from every side are equal. Also, isostacy. [1885 90; ISO +… …   Universalium

  • isostasy — noun Etymology: International Scientific Vocabulary is + Greek stasia condition of standing, from histanai to cause to stand more at stand Date: 1889 1. general equilibrium in the earth s crust maintained by a yielding flow of rock material… …   New Collegiate Dictionary

  • isostasy — noun The state of balance of the Earths lithosphere floating on the magma …   Wiktionary

  • isostasy — n. balance between the gravity which tends to depress and the buoyancy which tends to raise a given segment of the earth s crust (Geology) …   English contemporary dictionary

  • isostasy — [ʌɪ sɒstəsi] noun Geology the equilibrium that exists between parts of the earth s crust, which rise if material (such as an ice cap) is removed and sink if material is deposited. Derivatives isostatic adjective Origin C19: from iso + Gk stasis… …   English new terms dictionary

  • isostasy — isos·ta·sy …   English syllables

  • isostasy — i•sos•ta•sy or i•sos•ta•cy [[t]aɪˈsɒs tə si[/t]] n. 1) gel the equilibrium of the earth s crust, a condition in which the forces tending to elevate balance those tending to depress 2) phs the state in which pressures from every side are equal •… …   From formal English to slang


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