The word Isostasy has been derived from the Greek word lsostasios meaning equal standing or in equipoise. The theory of isostasy explains, the tendency of the earth's crust to attain equilibrium and the distribution of the material in the earth's crust which conforms to the observed gravity values.
This theory was developed from gravity surveys in the mountains of India, in 1850. The term was first proposed by Clarence Dutton, an American geologist in 1889.
This doctrine states that wherever equilibrium exists in the earth's surface, equal mass must underlie equal surface areas; in other words a great continental mass must be formed of lighter material than that supposed to constitute the ocean-floor.
Thus, there exists a gravitational balance between crustal segments of different thickness. According to Dutton, the elevated masses are characterized by rocks of low density and the depressed basins by rocks of higher density.
In order to compensate for its greater height these lighter continental material 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 stable equilibrium.
Accordingly, a level of uniform pressure is thought to exist where the pressure due to elevated masses and depressed areas would be equal. This is known as the 'Isopiestic-Level'.
Isostatic balance is upset if enough matter is transferred from one region to the other on the earth's surface. Intrusion of igneous material, accumulation of snow and ice, deposition of sediments etc. puts additional load whereas denudation, melting of ice etc. causes unloading.
This process of loading and unloading disturbs the balance and the process of compensation takes place to restore it. With the removal of material from the top of the mountains through erosion, it becomes lighter. Materials should, therefore, move into the roots of the mountains at depth through the interior of the earth.
This movement is termed as compensation which takes place in the form of elevation and depression. This is because, at depths, rocks apparently flow slowly outward from an overloaded area, that subsides to form a depression, to an under loaded area which gets higher and higher to form an elevation. The isopiestic level is the level of compensation.
The zone between the level of compensation and the surface of the earth is the zone of compensation or lithosphere. The zone below the isopiestic level is called the asthenosphere.
Three theories have been propounded to explain the concept of isostasy:
1. Airy's Theory
He presumes that the crustal blocks are of equal density and unequal thickness. As such the blocks constituting the mountains are thicker than those on which the plains lie and therefore they stand higher up as is the case with the masses of ice floating in water.
Floating ice is eight-ninths submerged and the higher the ice rises above the water level the deeper is the submerged portion. Thus the roots of the mountains sink in the basaltic substratum to depths proportional to the heights above.
This has greater support from recent geophysical data. For example Mt. Everest in the Himalayas rises to a height of about 9 Kms. whereas right beneath it the crust is about 80 Kms thick.
Thus Airy suggested that blocks of the lithosphere had a constant density of 2.7 gm. per cubic centimetre and floated in the asthenosphere of density 3.3 gm per cubic centimetre.
2. Pratt's Theory
According to this theory, there is a difference in the density of rocks in the crust and at the heights of the crustal blocks are determined by their densities. As such blocks made up of lighter material are at higher elevation than those consisting of denser material.
Lighter material, has therefore, been assumed to lie under mountains and heavier material under ocean and there also exists a boundary, between the upper blocks and the lower dense rocks, at a uniform depth known as the level of compensation.
Thus, the rocks constituting the elevated masses and depressed areas exert equal pressure at the level of compensation.
3. Heiskanen's Theory
The assumptions of both Airy and Pratt have been combined in this theory. Here it is assumed that density varies both between crustal blocks and within each block.
It has been observed that the average density of rocks of sea-level is more than those at higher elevations and this variation of density is thought to continue further downwards causing the deeper rocks more dense than the shallower ones.
Thus different blocks are thought to have different densities and accordingly extend downward to different depths. It explains for the roots of mountains and for the variations in density in different parts of the crust.
The theory of isostasy convincingly explains the vertical uplift of the mountains but it has not yet been possible to establish that isostasy is the factor initiating tectonic movements.
The role of isostasy in the developments of the earth's crust is rather modest and not decisive.
The idea of isostasy is supported by the fact that the melting of ice from the glaciers in Scandinavia led to a reduction of load and the consequent rise of the area.
The theory of isostasy is also confirmed by the seismic data.