# isostasy

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## isostasy

(īsŏs`təsē): see continentcontinent,
largest unit of landmasses on the earth. The continents include Eurasia (conventionally regarded as two continents, Europe and Asia), Africa, North America, South America, Australia, and Antarctica.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Isostasy

(or isostatic equilibrium), state of equilibrium of the earth’s crust in which the crust is resting on a solid, heavier substratum as though it were floating on the substratum according to Archimedes’ law. The word “isostasy” is frequently used in a broader and more indefinite sense.

It is because of isostasy that the thicker and the more dense (heavier) the crust, the more deeply its bottom is sunk in the substratum; therefore, mountains usually have “roots,” that is, downward protrusions of the crust beneath them. Isotasy is generally realized on a regional basis, which is to say that not every small sector of the earth’s crust is in equilibrium, but only fairly large (100–200 km wide) blocks. Full realization of isostasy leads to a situation where on any horizontal surface beneath the crust, beginning with the compensation surface and deeper, pressure is constant. Isostasy is found by observing deflections of the plumb line, by measuring the thickness of the earth’s crust by seismic methods, and primarily by determining isostatic gravity anomalies, which indicate a difference between observed values of the acceleration of gravity and the value that should be present at the given point if there were complete isostasy (and no local irregularities in the earth’s crust). The correction that has to be made in the observed or theoretically computed value of the force of gravity in such calculations is called isostatic reduction. The normal crust (with normal thickness) is considered to be an isostatically balanced crust whose surface is located at sea level; under elevated land one must assume a deficit of mass that compensates for the excess load of this land, and under a water basin one must assume an excess of mass that compensates for the lower density of water in comparison with the normal crust. These deficits and excesses of mass are called isostatic compensation.

Observations show that the earth’s crust is in a state very close to complete isostasy almost everywhere. In areas of intensive tectonic movements, however, there are deviations from isostasy, sometimes very significant ones. For example, there are zones of very strong negative isostatic anomalies along the ocean trenches.

Isostasy establishes itself very quickly. For example, during the last ice age the Baltic Shield and the Canadian Shield subsided under the weight of the ice (in the present geological age Antarctica and Greenland are in a similar state), but when the ice thawed these regions began to rise at a speed on the order of several millimeters a year (the present maximum uplift in the Baltic Shield area is 11 mm a year). Therefore, movements that restore isostasy take comparatively little time and are observed today in only a few places. Slower tectonic movements that disrupt isostasy are more common. The tendency of the earth’s crust toward equilibrium plays an important role in geotectonics, but this is a passive role, unlike the active role of tectonic forces that disrupt isostasy. Isostatic forces restrict the scope of tectonic movements, however, and restore equilibrium when tectonic forces weaken.

### REFERENCES

Liustikh, E.N. “Izostaziia i izostaticheskie gipotezy.” Tr>.Geofizicheskogo in-ta AN SSSR, 1957, no. 38.
Artem’ev, M.E. Izostaticheskie anomalii sily tiazhesti i nekotorye voprosy ikh geologicheskogo istolkovaniia. Moscow, 1966.
Artiushkov, E.V. “Ob ustanovlenii izostaticheskogo ravnovesiia zemnoi kory.” Izv. AN SSSR: Fizika Zemli, 1967, no. 1.
Artem’ev, M.E. “Izostaziia.” Zetmila i Vselennaia, 1970, no. 3.

E. N. LIUSTIKH

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

## isostasy

[ī′säs·tə·sē]
(geophysics)
A theory of the condition of approximate equilibrium in the outer part of the earth, such that the gravitational effect of masses extending above the surface of the geoid in continental areas is approximately counterbalanced by a deficiency of density in the material beneath those masses, while deficiency of density in ocean waters is counterbalanced by an excess in density of the material under the oceans.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Koss, "Densification of titanium powder during hot isostatic pressing," Metallurgical Transactions A, vol.
This is done by combing the gravity and seismic crustal structure models in computing the isostatic gravity gradient and solving the VMM inverse problem of isostasy.
The grains' fracture toughness is optimal for isostatic compositions, measured when the Ca/Si molar ration hovers 1.5, while MIT researchers characterize flexible and stressed-rigid compositions, respectively, at 1.7 and 1.3 ratios--the former most representative of typical portland cement.
The sea-level stand may have been due to (i) regional climatic deterioration sufficient to cause significant increase in glacial loading, briefly slowing isostatic uplift to the same rate as ocean-level rise; (ii) briefly increased glacial melting on a regional or wider scale, causing ocean-level to rise at the same rate as isostatic uplift; or (iii) briefly increased sea-ice activity which eroded the bedrock surface, destroying the Hiatella colony, and increasing the supply of inshore gravel to blanket the shore platform so formed.
The elastic properties of Vistamaxx elastomers are a result of a predominantly amorphous ethylene propylene (EP) matrix laced with a network of fine, well dispersed isostatic PP crystallites.
(Kent, Wash.) is constructing an extremely large hot isostatic press (HIP) scheduled to be completed late next year.
One cartel was on the isostatic graphite market - used in electrodes, casting dies and semiconductor applications - and the other in the extruded graphite market - used in electrolytic anodes and cathodes and in boats.
A thermobalanced and artificially aged pearlitic cast iron structure makes the STP-35 EzVision isostatic and unable to deform or distort over time or during temperature changes.
The system features one-micron Heidenhain glass scales and the thermo-balanced, artificially-aged, pearlitic cast-iron structure that ensures the equipment is fully isostatic and will not deform or distort over time or temperature change.
Melting is quick: the land rising under the reduced weight (isostatic adjustment) takes very much longer and will not counteract the additional water in the oceans.
Metso Powdermet AB is a leading supplier of powder metallurgy hot isostatic pressing (HIP) products and components.

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