A better understanding of the role of mountains in creating equilibrium and stability of the Earth requires a knowledge of the essence and structure of different components of the Earth's crust . These components )like mountains,plains and seas) are not in the form of irregular ridges and depressions which are located in the upper section of the crust all of the components are however,in a relative equilibrium;this state results from a decrease or increase in the density and also from a change in their thickness.
According to isostasy hypothesis there is another layer parallel and under the surface of the Earth on which the pressure exerted from mountains,plains,and seas is equal.This layer is knows as equilibrium or isostasic layer. To get a clearer image of isostasy, a simple example would be to imagine mountains as great pieces of ice floating in the sea . As we know the excess weight of the piece of ice above the surface of water counterbalances the lost weight of the portion under water . To explain isostasy different hypothesis have been proposed, among which the most important ones are :
1 - Pratt hypothesis , according to this hypothesis the higher parts of the crust have a lower density and the pressure exerted from all parts of the crust is equal on the equilibrium surface.It worths mentioning that the specific gravity of the upper portion of the crust of the Earth which is mainly made of granite rock and is called sial is less than that of the lower portion which is made of basalt rock and is called sima;it has also been determined that the thickness of sial in continents is to its maximum amount , and in most oceans it is absent or scanty.
2 - Airy hypothesis , according to this hypothesis the specific gravity of the Earth's crust is equal under its components, however the dimensions of its components are in such a way the higher they are from the surface of the Earth the more their depth in the ground is.It's worth mentioning that the thickness of the crust is between 30 to 40 km. in the plainareas and between 50 to 60 km. in mountain ranges.In fact mountains have roots;the more their height is from the ground,the more the depth of their root is.
3 - Heiskaen hypothesis,this hypothesis is actually in the middle point of the two aforesaid hypothesis , it states that the specific gravity of the components of the crust varies proportionately with their depth.Considering the facts mentioned,it is quite obvious that mountains are the most important factors for creating a balance in the Earth's movement and stability through their lesser specific gravity ) in comparison with the lower and shorter parts ( and also their deeper roots in the Earth's crust.