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 . Areas that are rifting apart , such as mid-ocean ridges and the East African Rift , have mountains due to thermal buoyancy related to the hot mantle underneath them ; this thermal buoyancy is known as dynamic topography .