Mountain scene in the Himalayas. Credit: Michael Greenwell.
A mountain is a landmass elevated substantially above its surroundings. The difference between a mountain and a hill is essentially one of size: the exact borderline is not clearly defined. Plateaus, or table-mountains, unlike most other mountains, have a large summit area as compared with that at their base. Mountains may stand up above the general level of a plateau, or a plateau may be carved by deep river valleys or canyons to formed a dissected plateau. An intermontan plateau is completely surrounded by mountains.
Most mountains occur in groups, ranges, or chains. The processes involved in mountain building are termed orogenesis. Orogenies can be largely explained in terms of the theory of plate tectonics. Thus the Andes have formed where the Nazca oceanic plate is being subducted beneath (forced under) the South American continental plate, and the Himalayas have arisen at the meeting of two continental plates.
Mountains are traditionally classified as volcanic, block, or folded. Volcanic mountains occur where lava and other debris (e.g., pyroclastic rocks) build up a dome around the vent of a volcano. They are found in certain well-defined belts around the world, marking plate margins. Block mountains occur where land has been uplifted between faults in a way akin to that leading to the formation of rift valleys (see also horst). Folded mountains occur through the deformation of the Earth's crust (see geosyncline), where vast quantities of sediments whose weight causes deformation, accumulate. Erosion eventually reduces all mountains to plains. But it may also play a part in the creation of mountains, as where most of the elevated stretch of land has been eroded, leaving a few resistant outcrops or rock.
A massif is a plateau-like upland area, with abrupt margins and often complex geologic structure.