Isostasy is a term used in Geology to refer to the state of ice above stasy and is angravitational 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). This article includes a list of works cited but its sources remain unclear because it lacks in-text citations. ... Gravity is a force of attraction that acts between bodies that have mass. ... Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... The tectonic plates of the Lithosphere on Earth. ... Earth cutaway from core to exosphere. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... Perspective view of the Himalayas and Mount Everest as seen from space looking south-south-east from over the Tibetan Plateau. ... For other uses of India, see India (disambiguation). ...

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. Archimedes (Greek: c. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... The geologic time scale is used by geologists and other scientists to describe the timing and relationships between events that have occurred during the history of the Earth. ...

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.

George Biddell Airy Sir George Biddell Airy (July 27, 1801 – January 2, 1892) was British Astronomer Royal from 1835 to 1881. ... Earth cutaway from core to exosphere. ... John Henry Pratt (June 4, 1809 - December 28, 1871), an English clergyman and mathematician, devised a theory or curstal balance which would become the basis for the isostasy principle. ... The rocky side of a mountain creek near Orosí, Costa Rica. ... In physics, density is mass m per unit volume V. For the common case of a homogeneous substance, it is expressed as: where, in SI units: ρ (rho) is the density of the substance, measured in kg·m-3 m is the mass of the substance, measured in kg V is... Vening Meinesz with his gravimeter Felix Andries Vening Meinesz (the Hague July 30, 1887 - Amersfoort August 10, 1966) was a Dutch geophysicist and geodetist. ...

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. Iceberg west of Ilulissat inlet, Greenland Iceberg, Tèmpanos, Patagonia, Argentina. ...

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. Changes in the elevation of Lake Superior due to glaciation and post-glacial rebound Post-glacial rebound (sometimes called continental rebound, isostatic rebound or isostatic adjustment) is the rise of land masses that were depressed by the huge weight of ice sheets during the last ice age, through a process... Snowflakes by Wilson Bentley, 1902 Ice is the name given to any one of the 14 known solid phases of water. ... // Template:Othi fucking love ya babeeruses2 The Baltic Sea is located in Northern Europe, from 53°N to 66°N latitude and from 20°E to 26°E longitude. ... Hudson Bay, Canada. ...

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. Schematic of sea level (black) and rate of change (blue) over the last 25 kyr There are multiple complex factors may influence sea level change. ... It has been suggested that this article or section be merged with sea level. ...

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.

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 [1]