Isostasy (Greek, isos: equal, stasis: standstill) in earth science is used as a concept of the gravitational equilibrium that explains the balance of topographical undulations on the Earth’s surface. The need to achieve natural balance or isostasy takes place on Earth wherever a large amount of weight is present. This weight might be due to a large mountain, ice from an ice age or even from human structures such as the weight from large man made lakes. Isostasy also takes place when a large amount of weight is removed from an area causing that portion of the Earth’s crust to rise such as when ice caps melt. In effect any excess mass, say high mountains must be compensated by deficit of mass, and any low land below sea level, say ocean basin must be underlain by a mass heavier than normal.
One way to understand isostasy is to think about a large ice cube floating in lemonade. A certain part of the ice cube always remains below the surface. As it melts, the ice cube floats up higher. Heavy objects such as mountains behave in a similar manner. As the mountain grows, it sinks into the Earth. As it is eroded, mountains often rise back up out of the surface.
Various hypotheses about isostasy take into account density, thickness (Airy hypothesis) and pressure variations to explain topographic variations among lithospheric plates. In the mid-nineteenth century two premises emerged: Airy’s hypothesis (Airy, G B, 1855, Phil Trans, Roy Soc London) and Pratt’s hypothesis (Pratt, J H, 1859, Phil Trans, Roy Soc London). Airy’s hypothesis proposes that there exists a ‘root’ or thicker crust below mountain and ‘anti root’ or thinner crust below ocean basin. His hypothesis is explained simply by the Archimedes principle that hydrostatic state of the Earth’s crust supported by a denser (viscous) liquid substratum (called mantle) is like that of a raft made of logs of different heights floating on water, larger the height deeper it is in the water. Four years later, Pratt suggested that density of rocks also have a role to play for complete isostatic equilibrium, he believed lower density rocks lay below mountains and higher density below ocean basins.
With the recent geophysical techniques, seismological and gravity observations, it is found that root below the Himalaya (thicker crust ~ 45-50 km) and anti root below the Indian Ocean (thinner crust ~15-20 km) exist. The density too is less for the sedimentary crust (~ 2.6 gm/cc) below Himalaya compared to the density of the oceanic crust (~2.7 gm/cc) below the Indian Ocean. Further, in geophysical observations it is found that all parts of the Earth’s crust is not in complete isostatic equilibrium; for example the overlying crust of southern Himalaya is yet to achieve isostatic equilibrium. Isostatic equilibrium of the Earth’s crust thus is an ongoing geological process with continued plate movements.