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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. This concept 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. Isostasy is not a process that upsets equilibrium, but rather one which restores it (a negative feedback). 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.

Contents

Isostatic models

Airy (left) and Pratt (right) models of isostasy

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.
- where the lithosphere acts as an elastic plate and its inherent rigidity distributes local topographic loads over a broad region by bending. This hypothesis was suggested to explain how large topographic loads such as seamounts (eg. Hawaiian Islands) could be compensated by regional rather than local displacement of the lithosphere.

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

See also

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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

"ISOSTASY, in Geology. - When the Great Trigonometrical Survey of India was initiated, it was found that the deflection of the plumb-line by the Himalayas was much less than the calculated amount due to the theoretical attraction of the visible mass of the mountains. Sir G. B. Airy suggested that this might be caused by the presence of a mass of matter, of less than the average density, under the mountains; this explanation was further investigated by Archdeacon J. H. Pratt, who applied the term compensation to the negative effect of the underlying defect of density, in compensating the direct effect of the attraction of the visible mass of the mountains. In 1892 Major C. E. Dutton, discussing the greater problems of physical geology, deduced a general principle that the weight of matter under any unit area of the earth's surface tended to become uniform, and suggested that this was brought about by an underground transfer of material to balance the visible surface transport from regions of erosion to those of deposition. To this principle he gave the name isostasy ('loos, equal, and aTacLS, position), not as a synonym for Pratt's compensation, but as a name for the principle and process by which it was brought about. In 1909 there appeared a very complete and elaborate investigation of the subject, by J. F. Hayford, in which the word isostasy is used throughout as synonymous with what Pratt called compensation, and this use of the term has since become general among geodesists. Some inconvenience results from this change in the meaning attached to the word, for it is still largely understood by geologists in the sense intended by its inventor, as the process by which the fact implied by Pratt's word, compensation, is brought about (see also Geology).

See G. B. Airy, Phil. Trans., cxlv., 1855, p. for; T. H. Pratt, Phil Trans., cxliv., 1859, p. 745; C. E. Dutton, Bull. Phil. Soc. Washington, xi., 1892, p. 51; J. F. Hayford, The Figure of the Earth and Isostasy, from Measurements in the United States (Washington 1909). (R. D. 0.)


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