Introduction
The word ‘Isostasy' is derived from a Greek word meaning 'in equipoise’ or ‘in balance’. The theory of ‘Isostasy’ postulates a system for the distribution of materials in the earth’s crust which conforms to and explain the observed gravity values. This theory was developed from gravity surveys in the mountains of India (1850). The term was first used by the American Geologist—Dutton 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 materials than that supposed to constitute the ocean floors and further, 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 oceans and continents may remain in stable equilibrium.
According to Dutton, the elevated masses are characterised by rocks of low density and the depressed basins are characterised by rocks of higher density. Accordingly, a level is thought to exist where the pressure due to elevated masses and the depressed areas would be eyual This is called ‘Isopiestic Level’ or the “Level of Uniform Pressure’.
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Any loading due to sedimentation, deposition or intrusion of igneous material etc. or unloading due to denudation or melting of ice will, therefore, disturb the balance and compensation in the form of depression or elevation will foliow to restore the state of bydrostatic balance. The state of isostasy can be maintained only if there Lis a continuous compensation at depth.
Through erosion, material is being removed from the tops of the mountains, which therefore are becoming lighter. Materials should therefore move into the roots of the mountains at depth through the interior of the earth. This movement is termed compensation.
The zone between the isopiestic level and the surface of the earth is called the zone of compensation or lithosphere. The zone below the isopiestic level is called the asthenosphere.
In analyzing the concept of equal pressure or weight, it follows that the blocks in balance have equal masses. If granites and basalts ate considered, in the light of equal mass concept, the granites being 10°, lighter than the basalts, must have higher volume which is xepresented by the highlands composed of sialic matter ; on the contrary the volume of the denser matter, i.e., basalt or sima has ought to be less and that is why they are represented by depressions or ocean basins.
In this regard, three theories have been (put forwarded) as follows :
Airy theory
According to this theory, there is a change in the density of rocks at depth tn the earth’s interior and that the upder lighter material floats on the dense layer. The depth of this change varies from place to place. Airy preferred to assume that the different crustal blocks are of equal density and unequal thickness. According to him, the blocks constituting the mountains ,are thicker than those on which the plains lie and as a result they stand higher up just in the same way in which a huge block of ice projects much -above the smaller ones when allowed to float freely in water. The thicker block at the same time sinks deeper down in the sub-stratum and thus constitutes the root of the correspanding mountains.
According to Airy therefore mountains are supported by their roots which penetrate deep down into the denser sub-stratum and keep the mountain floating due to its buoyancy.
This has greater support from recent geophysical data. For ‘example, Mt. Everest in the Himalayas rises to a height of about. 9 kms, right beneath it the crust is. about 80 kms thick ; whereas the ‘crust is only 32 kms thick in the’ Deccan Plateau and only 12 to 20 km or less near sea-coasts of India.
Pratt theory
According to this theory, the additional mass of a great mountain is compensated by a corresponding deficiency in the density of the rocks constituting the crustal olock on which the mountain stands.
Here, it has also been assumed that the boundary between the upper light materia] and,the lower dense rocks is at a uniform depth, called the depth of compensation. ‘Besides, it is also presumed that there are variations in the.density of the lighter layers which are related to the elevation of the surface.. The weight of columns of rock extending from the surface to the depth of compensation in different parts of the earth is thus the same.
Observations made with the help of seismic studies indicate that more dense material ‘underlies ocean basins than continents, which support Pratt’s assumptions.
Heiskanen’s theory
He combines the assumptions of both Airy and Pratt. It is observed that rocks at sea-level are more dense on the average than those at higher elevations. He assumes this change continues downwards, tending to make deeper rocks more dense than the shallower ones. In addition, different sections are thought to have different densities and different lengths.
This. theory accounts for the roots of mountains and for the variations in density in diffe.ent parts of the crust.