The earth’s crust is made up of large plates called tectonic plates that fit into each other. These plates keep moving a few centimeters every year. Mountains form along the boundaries where the tectonic plates move towards each other (convergent boundaries). The tectonic plates collide triggering deformation and thickening of the crust. This in turn leads to crustal uplift and mountain formation. This process is a horizontal compression that leads to deformation folding and faulting of layers into folds or wrinkles along the convergent plate boundaries. This crustal uplift can be either a hill or a mountain depending upon the height and slope of the formation. But also to balance the weight of the earth surface, much of the compressed rock is forced downward, producing deep mountain roots making mountains for both upward and downward.
Convergence due to converging plates can be either continental-oceanic convergence, oceanic-oceanic convergence or continental-continental convergence.
- When a plate of continental crust converges with a plate of oceanic crust, the heavier oceanic crust will move under the continental crust and this process is called subduction. This is the process through which mountains and volcanoes are formed when the subducted oceanic crust is melted and recycled to the surface (e.g. West coast of North and South America).
- When a place of oceanic crust converges with another plate of oceanic crust, the older crust will subduct under the newer crust that is less dense leading to volcanic ring islands (e.g. Japanese islands).
- When two plates of continental crusts come into contact with each other, neither of them will subduct beneath the other due to their densities. So this collision leads to formation of big mountains with fragments of oceanic sediments in them even in the highest peaks (e.g. Alps in Europe, Himalayas in Asia).
source – owlcation.com