Theory of Isostasy

In a general way the word “Isostasy” means equilibrium.

 The theory of Isostasy explains the tendency of earth crust to attend equilibrium as the materials on the earth are distributed according to their gravity values.

 The theory of Isostasy was proposed by an American geologist, Clarence Dutton. According to him, “the elevated masses are characterized by rocks of lighter density and the depressed basin by rocks of higher density”.

 This statement promotes that the pressure exerted by the elevated mass and by depressed area should be equal which is known as Isopiestic- level.

 The isopiestic level is called level of compensation.

 In order to compensate for the greater height, lighter continental mass must extend downward to some distance under the continent and below the ocean floor  level in order that unit areas beneath the oceans and continents may remain in the state of equilibrium.

 Here comes the term “compensation”. Just for example: the materials of the higher elevation are carried towards lower elevation or depression due to erosion. Due to which the heavy materials of the top becomes lighter when it is eroded. So to compensate this, materials should move into the root of the mountains through the interior of the earth. So this movement is called compensation which takes place in the form of elevation and depression. So, loading and unloading is natural on earth surface and it is always balanced by level of compensation.

 

Altogether three hypothesis have been proposed to explain the concept of Isostasy. 

 

1)     AIRY’S THEORY:

 

 According to this theory “crustal blocks are of equal density but unequal thickness”. As the blocks constituting the mountains are thicker than that of plains so they stand up higher. Example: If the mountain rises up to the height of 5km then it is submerged to the same height in the earth as its height. That means higher it rises upward, the deeper is the submerged portion. This hypothesis also suggests that the block of the lithosphere has the constant density of about 2.7 gm/cc which floats above the asthenosphere of density about 3.3 gm/cc.

 

2)     PRATT’S THEORY:

 

According to this theory “crustal blocks are of different densities such that their heights are determined by their densities”. As such block made up of lighter materials are at higher elevation than those denser blocks. Lighter blocks having less density are assumed to lie under mountains and denser blocks with high density are assumed to lie under oceans. And there exists a uniform boundary between upper blocks and lower denser blocks at a uniform depth which is known as level of compensation. According to this hypothesis, whether the block is elevated mass or depressed area, both exist uniformly equal pressure at the level of compensation.

 

3)      HEISKANEN’S THEORY:

 

 Both Airy’s and Pratt’s theory are combined to explain the Heiskanen’s theory. According to this theory “density varies both between crustal blocks and within the block”. As the average densities of the rocks of depression (sea level) is more than that of higher elevation (mountains) . And further downward the density of the block is more than shallower part i.e. the density varies between the same block.

 

Recent seismic survey also confirms the theory of isostasy but it has not been able to establish that the isotasy is the factor of initiating the tectonic movements.