In plate tectonics, the lithosphere is rigid overlying the weak asthenosphere in geologic time scales. The high mountain ranges either in the ocean or in the continent can be supported by this rigid lithosphere for millions of years. The presence of the weak layer of asthenosphere is implicated from gravity observations near mountain range. The bougur gravity anomaly indicates negative crustal root underneath the mountains, compensating gravitational force of the high topography. This is called isostasy, meaning gravitationally equilibrium.
There are three isostatic models to achieve compensation:
1 Airy model: The high topography is accommodated by the thickness of the crust.
2 Pratt model: The high topography is accommodated by the density of the crust.
The above two belong to the "local compensation", which means there is no regional effect. One can imagine a picture of "floating iceburg", the vertical load is totally supported by the underlaid "fluid" asthenosphere. The horizontal force between adjacent lithosphere is decoupled.
3 regional compensation (elastic plate model): This model is more realistic and reasonable. Airy model actually belongs to one of its extreme case. Basically, it views the lithosphere as an elastic plate with a distinct parameter called flexure rigidity. The flexure rigidity is equivalent to elastic thickness of the plate.
Isostasy is not universal in the world. In some areas, such as Greenland, the mantle is rebounding to unloading because of the melting of icecap. The violation of isostasy or new isostasy models will certainly arouse new questions to the earth scientists.
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