This article belongs to Geology. In 1889, Dutton, an American–geologist, proposed the term Isostacy (fin a Greek word meaning 'equipoises' or 'equal standing') for the gravitational equilibrium which determines the heights of continents and the depths of the ocean floors in conformity with the densities of the rocks underlying them. According to this concept, blocks of the earth's crust, because of variations in densities, Would rise to different levels and appear on the surface as mountains, plateaus, plains or ocean basins.

Hutton believed that the relatively up-raised areas such as mountains, plateaus and plains must be -composed of lighter material to "remain afloat" over heavier material underneath. During the 19th century, a survey of India was being conducted under the guidance of Sir George Everest—the Surveyor General of India at that time. The latitude of a place was being determined both by the triangulation method and astronomically. In case of Kalianpur and Kaliana, the two values differed by 5.23 seconds of arc, corresponding to a distance of 168 metres on the ground. This was attributed to the attraction exerted on the plumb line by the enormous rock is composing the Himalayas. Archdeacon Pratt made calculations of the mass of maintains and concluded that the deflection of the plumb line towards the mountains should be at two places Kalianpur and Kaliana, but interestingly, the calculated difference was 15.885 seconds, against the observed distance of 5.23 seconds. This anomaly was explained on the basis of a decrease in density of rocks at lower depths.

Based on the above observation, as well as on the exploration of the crust by earthquake waves, it is inferred that mountain ranges have "roots' going down to depths of 50 km or 60 km, which are largely composed of light sialic rocks. Below the plains near sea level, the thickness of the sial is only 30 km or less. There is no sial beneath the deeper parts of the oceans.

Thus, various features of the earth's crust such as continents, mountains, plateaus and ocean basins are in isostatic equilibrium is disturbed by geological action. For instance, uplands get eroded through various agencies and the eroded material transported and deposited in depressions such as ocean basins. This causes a change in relative masses of the up-raised areas and the lowlands. But isostacy comes into play to restore the isostatic equilibrium, and slow vertical movements take place to raise the eroded uplands and to sink the overburdened lowlands. Also, slow lateral movements of dense material beneath the sial adjust the deeper layers. Isostacy is sometimes the cause of geodynamic movements and may explain the phenomenon of uplift of marine sediments to high levels, for instance.

EVIDENCE IN SUPPORT OF ISOSTACY
1. Sinking of Landmasses and Uplift of Costal Areas: At times the heat emitted by the radioactive elements in the earth's interior gets trapped in sima. This causes sima to expand and lose some density. The sialie continents sink deeper into sima. This allows sea transgress on land. The occasional rise of sea level has been accounted for in this way. Similarly, if the accumulated heat in sima is released through volcanic eruptions, it gains some density. As a result, continents are raised. Many coastal areas, submerged in the sea in the past, but now found above sea-level are a proof of the principle of isostacy.
2. Continental Drift: If the lighter continental masses are floating over denser material, continental drift seems to be a distinct possibility.
3. Uplift of European Landmasses: The deposition of ice during the Ice Age caused large European landmasses to sink but when the glaciation came to an end, the continents started rising again and are still rising. There are 'some scientists like Hopfner, Hubbert and Melton who have challenged the concept of isostacy.