Mountain, landform that rises prominently above its surroundings, generally exhibiting steep slopes, a relatively confined summit area, and considerable local relief. Mountains generally are understood to be larger than hills, but the term has no standardized geological meaning. Very rarely do mountains occur individually. In most cases, they are found in elongated ranges or chains. When an array of such ranges is linked together, it constitutes a mountain belt. For a list of selected mountains of the world, see below.
A mountain belt is many tens to hundreds of kilometres wide and hundreds to thousands of kilometres long. It stands above the surrounding surface, which may be a coastal plain, as along the western Andes in northern Chile, or a high plateau, as within and along the Plateau of Tibet in southwest China. Mountain ranges or chains extend tens to hundreds of kilometres in length. Individual mountains are connected by ridges and separated by valleys. Within many mountain belts are plateaus, which stand high but contain little relief. Thus, for example, the Andes constitute a mountain belt that borders the entire west coast of South America; within it are both individual ranges, such as the Cordillera Blanca in which lies Peru’s highest peak, Huascarán, and the high plateau, the Altiplano, in southern Peru and western Bolivia.

Mountainous terrains have certain unifying characteristics. Such terrains have higher elevations than do surrounding areas. Moreover, high relief exists within mountain belts and ranges. Individual mountains, mountain ranges, and mountain belts that have been created by different tectonic processes, however, are often characterized by different features.
Chains of active volcanoes, such as those occurring at island arcs, are commonly marked by individual high mountains separated by large expanses of low and gentle topography. In some chains, namely those associated with “hot spots” (see below), only the volcanoes at one end of the chain are active. Thus, those volcanoes stand high, but with increasing distance away from them erosion has reduced the sizes of volcanic structures to an increasing degree.

Mountains and mountain belts exist because tectonic processes have created and maintained high elevations in the face of erosion, which works to destroy them. The topography of a mountain belt depends not only on the processes that create the elevated terrain but also on the forces that support this terrain and on the types of processes (erosional or tectonic) that destroy it. In fact, it is necessary to understand the forces that support elevated terrains before considering the other factors involved.
Two properties of rocks contribute to the support of mountains, mountain belts, and plateaus, namely strength and density. If rocks had no strength, mountains would simply flow away. At a subtler level, the strength of the material beneath mountains can affect the scale of the topography.
In terms of strength, the lithosphere, the thickness of which varies over the face of the Earth from a few to more than 200 kilometres, is much stronger than the underlying layer, the asthenosphere . The strength of the lithosphere is derived from its temperature; thick lithosphere exists because the outer part of the Earth is relatively cold. Cold, thick, and therefore strong lithosphere can support higher mountain ranges than can thin lithosphere, just as thick ice on a lake or river is better able to support larger people than thin ice.

Tectonic processes that destroy elevated terrains
Besides erosion, which is the principal agent that destroys mountain belts, two tectonic processes help to reduce high elevations. Horizontal crustal extension and associated crustal thinning can reduce and eliminate crustal roots. When this happens, mountain belts widen and their mean elevation diminishes. Similarly, the cooling and associated thermal contraction of the outer part of the Earth leads to a reduction of the average height of a mountain belt.

Major Types Of Mountain Belts
Mountain belts differ from one another in various respects, but they also have a number of similarities that enable Earth scientists to group them into certain distinct categories. Each of these categories is characterized by the principal process that created a representative belt. Moreover, within individual belts different tectonic processes can prevail and can be associated with quite different landforms and topography. Thus, for any category there are exceptions and special cases, as well as subdivisions.

Major Mountain Belts Of The World
Most mountains and mountain ranges are parts of mountain belts that have formed where two lithospheric plates have converged and where, in most cases, they continue to converge. In effect, many mountain belts mark the boundaries of lithospheric plates, and these boundaries in turn intersect other such boundaries. Consequently, there exist very long mountain systems where a series of convergent plate boundaries continue from one to the next. A nearly continuous chain of volcanoes and mountain ranges surrounds most of the Pacific basin—the so-called Circum-Pacific System. A second nearly continuous chain of mountains can be traced from Morocco in North Africa through Europe, then across Turkey and Iran through the Himalayas to Southeast Asia; this chain, the Alpine-Himalayan (or Tethyan) System, has formed where the African, Arabian, and Indian plates have collided with the Eurasian Plate. Nearly all mountain ranges on the Earth can be included in one of these two major systems and most that cannot are residual mountains, which originated from ancient continental collisions that occurred hundreds of millions of years ago.