The mapping process in India by using plumb lines and theodolites in the 19th century was responsible for discovering new facts. The Himalayan peaks did not deflect the plumb line as much as predicted by the surveyors. The different hypothesis was put forward for the anomalous plumb-line deflections by several researchers during the second half of the nineteenth century. The large mountains have low-density roots was first assumed during topographic surveys in India and the Himalayan mountains.
G.B. Airy and J.H. Pratt were the two prominent researchers who contributed their ideas to the advancement of this particular phenomenon known as Isostasy. Their hypotheses have in common the compensation of the extra mass of mountain above the sea level by a less dense region below the sea level, but they differ in their opinion as far as the way the compensation is reached. For instance, according to Airy, when isostatic compensation is completed, the mass deficiency of the root equals the excess load on the surface. The isostatic compensation is equivalent to applying the popular Archimedes ‘s principle.
Thus the problem was commenced with the identification that in spite of the additional terrain volume, the mountains are associated with negative Bouguer anomalies. Airy’s research tells us that the mountains have a crustal root which does compensate for the relief. Whereas Pratt’s view is that the values of density are variable laterally which is similar to the lateral variations of temperature, composition, etc, In both the views or models, the mountains float on the denser mantle in equilibrium, which is known as an isostatic equilibrium or simply the isostasy. Thus according to the isostatic condition, the weight of columns of the rocks at some depth called the depth of compensation is equal everywhere.
Airy’s detailed studies revealed that the density of the crustal material is uniform, the depth of root penetration is variable and there are deeper roots below the mountains but smaller under the plain. Pratt’s views are exactly opposite and also included that there is no root formation but a level of compensation only. Thus Pratt concluded that topography is a function of lithospheric density whereas Airy viewed that the topography is a function of lithospheric thickness!.
If it were not for the low-density roots, the gravity surveys across the mountainous regions would reveal positive gravity anomalies. The fact that no such anomaly does exist indicates that the mass excess is not present. Hence, some of the dense mantle at depths must be displaced by the lighter crustal material.
The uploading of the crust does cause it to respond by raising upward until the equilibrium is reached again, which is known as isostatic rebound. The earth’s crust does respond isostatically to the erosion and also to the sedimentary deposition. Examples can be seen in Antarctica and Greenland, where the Earth’s crust responds to loading when the glaciers form and depresses the crust into the mantle to maintain the isostatic equilibrium.