Mountain, land mass elevated substantially above its surroundings. Most mountains occur in ranges, chains, or zones. The earth's crust is made up of various moving fragments; hence, land masses are in constant motion. Thus the Andes have formed where the Nazca oceanic plate is being forced under the South American continental plate, and the Himalayas have arisen at the meeting of 2 continental plates.
Mountains are classified as volcanic, block, or folded. Volcanic mountains occur when lava and other debris build up a dome around the vent of a volcano. Block mountains occur where land has been uplifted between earthquake faults. Folded mountains occur through deformations of the earth's crust; when vast quantities of sediments accumulate, their weight causes deformation. Erosion eventually reduces all mountains to plains.