Useful notes on Arthur Holmes’ Views on the the earth’s surface

According to Arthur Holmes, the higher relief features of the earth’s surface are composed of lighter materials and their larger portions penetrate into the heavier and denser materials lying below to keep them in a state of hydrostatic balance.

He was of the opinion that mountains have long roots which are composed of sialic rocks and which go down to depth of 80 to 96 km. Thus, the compensating deficiencies of mass in the columns underlying the mountain ranges are made good.

It may be pointed out that the views of Holmes were the same as those of Airy, who for the first time made the suggestion that the crust is supported by the underlying denser material (sima) and that the weight of mountains is balanced by light materials extending as ‘roots’ into the denser material just as icebergs are balanced in water. Remember that this view of Airy was expressed in 1855.

If the regions concerned remain undisturbed for long period of time, by the activities of the earth, the crystal columns of equal area extend downwards to the same depth at which the weight of each column exerts nearly same pressure on underlying material despite the difference of its surface elevation. The depth of 50 km selected for these columns would be the depth of isostatic compensation.

To sum up, “the whole subject of isostasy is one of great difficulty and its investigation is still proceeding. There is little doubt that isostasy has passed from the stage of theory to that of fact, but there are still many serious problems to be solved before it can be said that it is fully understood”.

According to Steers, the views of Airy on isostasy have got general acceptance from most of the geologists.

More recent studies in the form of Hayford anomalies bear out the fact that by and large isostasy exists in the Himalayas. The order of under compensation in the Outer Himalayas is about 10%, and that of over-compensation over the alluvial Gangetic plain is 40%. The processes of erosion and deposition tend to maintain isostatic equilibrium in the region as a whole.

As indicated by Heiskanen and others isostasy is now an accepted principle and it cannot be contradicted by regional exceptions. Probably the most remarkable exception to the principle of isostasy is presented by the Indian region.