Orogeny

Orogeny is the process of mountain-building. It takes place when two tectonic plates come together. Orogens develop while a continental plate is crumpled and thickened to form mountain ranges, and involve a great range of geological processes collectively called orogenesis. 
A convergent boundary is a frequent cause:
Another, less common cause is when a plate moves over a hot spot in the Earth's mantle. Examples:
There is a third process, called "delamination". Here an unstable portion of cold root of the lithosphere drips down into the mantle, decreasing the density of the lithosphere and causing buoyant uplift. Example:
Areas that are rifting apart, such as mid-ocean ridges and the East African Rift have mountains due to thermal buoyancy caused by the hot mantle underneath them, which pushes them up. This thermal buoyancy is known as "dynamic topography". Examples: