Theory of isostasy by Heiskenen 1933 - Geography for You

Theory of isostasy by Heiskenen 1933

Isostasy or isostatic equilibrium is the state of gravitational equilibrium between Earth's crust (or lithosphere) and mantle such that the crust "floats" at an elevation that depends on its thickness and density. This concept is invoked to explain how different topographic heights can exist at Earth's surface. Although initially defined in terms of continental crust and mantle,[1] it has subsequently been interpreted in terms of lithosphere and asthenosphere, particularly with respect to oceanic island volcanoes, such as the Hawaiian Islands.

Although Earth is a dynamic system that responds to loads in many different ways, isostasy describes the important limiting case in which crust and mantle are in static equilibrium. Certain areas (such as the Himalayas and other convergent margins) are not in isostatic equilibrium and are not well described by isostatic models.

The general term 'isostasy' was coined in 1882 by the American geologist Clarence Dutton.

Theory of isostasy by Heiskenen 1933

The Heiskanen hypothesis, developed by Finnish geodesist Weikko Aleksanteri Heiskanen, is an intermediate, or compromise, hypothesis between Airy’s and Pratt’s. 

This hypothesis says that approximately two-thirds of the topography is compensated by the root formation (the Airy model) and one-third by Earth’s crust above the boundary between the crust and the substratum (the Pratt model).

Accord­ing to him density of rocks varies within the column (section of the earth) and between the columns. For example, rocks of a column at sea level have a higher density (say 2.76 gram cm-3) than at higher elevation of the same column (say 2.70 gram cm-3) which means as we go down the rocks of a section of the earth’s crust become denser i.e. density increases downward. Similarly, the density of rocks of different sections (col­umns) of the earth’s crust also varies. Thus, it appears that the density of rocks varies both vertically and horizon­tally.

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