Isostasy

Isostasy is the principle of gravitational equilibrium between Earth's rigid outer lithosphere and the more fluid asthenosphere beneath it, under which elevated topography such as mountain ranges is buoyantly supported by a corresponding low-density root extending downward, in a manner analogous to an iceberg floating in water with most of its bulk submerged. The principle emerged from mid-nineteenth century surveys in the Himalayas, where measured deflections of a plumb line near the mountains were far smaller than calculations based on the mountains' visible mass predicted, implying a compensating mass deficit at depth. George Airy and John Pratt each proposed a differing model in 1855 to explain the compensation, Airy holding that crustal blocks of uniform density vary in thickness while Pratt held that they vary in density above a uniform depth, and the general term isostasy for the equilibrium concept was introduced later, in 1889, by American geologist Clarence Dutton. Isostasy is used today to explain phenomena such as the ongoing rebound of land once depressed by the weight of Ice Age glaciers.