isostasy—A 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, whereas the effect of deficiency of density in ocean waters is counterbalanced by an excess of density in the material under the oceans. The basic principle of isostasy is that the masses of prismatic columns of the outer part of the Earth extending to some constant depth below the surface of the geoid are proportional to the areas of their sea-level sections, regardless of their surface elevations. The depth below sea level to which these hypothetical columns extend is termed the depth of isostatic compensation and is somewhere between 60 and 70 statute miles. The area of the sea-level section of a unit hypothetical column for which isostatic compensation is ordinarily complete has not been determined; it may be uniform for all parts of the Earth or may vary with the character of the relief in the same continental regions. See also isostatic compensation, depth of isostatic adjustment.

isostatic adjustment—The process of restoring and maintaining isostasy in the so-called crust of the Earth. The distribution of material in the outer part of the earth is undergoing continual change by the operation of erosion, sedimentation, and other natural forces. The unbalanced condition which would naturally result from such disturbing processes is offset by the movement of material at considerable depths below the surface of the Earth. See also isostatic compensation, depth of isostasy.

isostatic anomaly—The difference between an observed value of gravity and a theoretical value which has been corrected for elevation of the station above the geoid and for the effect of topography over the whole earth and for its isostatic compensation. See also anomaly.

isostatic compensation—The departure from normal density of material in the lower part of a column of the Earth’s crust which balances (compensates) land masses (topography) above sea level and deficiency of mass in ocean waters, and which produces approximate equilibrium of the Earth’s crust; isostasy. See also topographic deflection.

isostatic compensation, depth of—The depth below sea level at which the condition of equilibrium termed isostasy is complete. See also isostasy; isostatic adjustment.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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