# 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 It might be outdated or ideologically biased.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

## 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 ?
The mixtures (pristine and doped powders) were initially pelletized in the samples of 8 mm x 6 mm x 1 mm under 8 MPa of uniaxial pressure and then isostatically pressed at 200 MPa.
Long-term sea level rise is confirmed by tidal gauge measurements, showing that the isostatically controlled declining trend of mean sea level in Finland and northern Estonia, which was evident up to the early 1980s, has changed to a nearly stable trend (Johannsson et al.
(1) Phase I occurred more than 15 000 ya and represents deglaciation in a proglacial marine environment and formation of the DeGeer Sea by a marine incursion that occupied isostatically depressed portions of southeastern New Brunswick.
Models have included thermal buoyancy from an underlying positive temperature anomaly in upper mantle (ten Brink et al., 1997), thicker crust giving the origin to an isostatically buoyant load (Studinger et al., 2004) or possible collapse of a high-standing plateau with subsequent uplift and denudation (Bialas et al., 2007).
where [[alpha].sub.g](T; [chi]g) denotes the temperature dependent thermal expansion secant coefficient of the isostatically compacted green as-received pure PTFE, which is characterized by the constant crystalline weight fraction, [chi]g.
The Z2 is an isostatically pressed clay-graphite crucible that provides optimal life when melting copper and copper alloys as well as in induction furnace applications.
The powder was pressed isostatically at a pressure of 150 MPa to dense cylindrical shapes by using a Cold Isostatic Press supplied by EPSI N.V., SO 10036, Belgium.
Isostatically molded to a PFA-covered cable, the probe and cable have the identical chemical resistance at temperatures from -200C to 280C.
In the CPM process, pre-alloyed molten steel is atomized into a spray of micro ingots of uniform size and composition, and isostatically pressed to 100% density.
"At that stage," said Halverson, "we knew we could hot isostatically press (HIP) the boron carbide-aluminum mix to full density, and also that we could sinter it to a porous state, so we figured that a pressureless route to full consolidation could be accomplished.
The pellets were then cold isostatically pressed at 200 MPa for 1 min and sintered using two different methods, namely, conventional and gas pressure sintering by purging nitrogen ([N.sub.2]) gas at 1250[degrees]C.
* The new product is a high pressure, isostatically pressed clay-graphite crucible for melting copper-based alloys and induction furnace applications.

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