The epeirogenic uplifts are thought to be caused due to a phenomenon known as Isostasy, which refers to the state of hydrostatic balance of the crustal segments of different thickness and density.
As we know, due to weathering, erosion, transportation and deposition, the loads of rock on the surface of the earth are shifted from one place to the other. This disturbs the isostatic equilibrium.
Usually the load is shifted on the earth’s surface from high relief features to lower areas. In order to restore the isostatic balance upset by the shifting of load, the mantle rocks (which always show plastic behaviour) move towards the area from which mass of rocks has been removed, at the bottom of the crust or beneath it, tending to elevate it.
Isostatic movements are always vertical. The isostatic movements are also thought to be significant in areas which were earlier the sites of accumulation of large quantities of ice and snow, but later on there was melting away of ice as the glaciers retreated and the weight was suddenly reduced upsetting the isostatic balance.
ADVERTISEMENTS:
The state of isostasy is then re-established by compensating uplift of those areas.
The epeirogenic processes are rather slow but continuous and affects large portions of the crust. The sea-leval is mostly used as a datum-line to observe and measure the elevation or subsidence of the lands caused due to epeirogenic movements.
Changes in position of the sea-level relative to the land are called ‘eustatic changes’, which are either due to the changes in the capacity of the sea-basin and or in the volume of the sea water.