Isostasy

isostasy (īsŏs`təsē): see continent.

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isostasy

Theory describing the mass balance in the Earth's crust, which treats all large portions of the crust as though they were floating on a denser underlying layer, about 70 mi (110 km) below the surface. In this theory, a mass above sea level is supported below sea level, so high mountains must be regions where the crust is very thick, with deep roots extending into the mantle. This is analogous to an iceberg floating on water, in which the greater part of the iceberg is under water.

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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.

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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