Mountain, a landform rising prominently above the surrounding land or bordering sea. Mountains generally occur in groups called ranges, which in turn form chains, systems, or Cordilleras. There is no exact distinction between a mountain and a hill; what is considered a mountain in one area may be called a hill in another. The Siwalik Hills of India, for example, are higher than most peaks in the Appalachian Mountains of the eastern United States.
A peak is a projecting point on a mountain. (The term is also used to mean the highest point of a mountain, and is sometimes used as a part of a proper name in place of mountain, as in “Pikes Peak.”) The summit is the highest point of a peak or mountain. Foothills are hills lying at the base of a mountain. Elongated depressions between mountains or ranges are valleys.
The height of a mountain is measured by various surveying methods and is expressed as the number of feet (or meters) above sea level. Because surveying methods vary in accuracy, and because many mountains are inaccessible, heights cannot always be exactly determined, and must be periodically revised.
The temperature at any point on a mountain depends on the latitude, season, exposure to the sun, and elevation. As the elevation increases, the temperature falls and the air becomes thinner. Trees cease to grow above a certain level; this level is called the timberline. The snow line is the lower limit of permanent snow accumulation. If snow accumulates faster than it can melt, snow-fields and glaciers develop.
Most of the world's mountain ranges were formed by vast uplifts of the earth's crust, accompanied by folding and faulting (fracturing). Some mountains have been formed through volcanic accumulations of ash and lava. Mountains of this type are often isolated peaks.-Mount Fuji in Japan is an example. According to the theory of plate tectonics, uplifting of the crust and volcanic activity occur largely along the the boundaries between plates, or rigid sections, of the crust moving in relation to each other. Other mountains have been formed by erosion (wearing away) of high plateau regions, leaving the harder parts standing above the surrounding area. Parts of the Ozarks were formed in this way.
A mountain is said to be young, mature, or old, depending upon the degree of erosion. Young mountains have high, sharp peaks not yet worn by erosion. The Himalayas of Asia—highest in the world—are an example. The Appalachians consist of both mature and old mountains, which have been worn down and rounded by erosion.
The effect on the surrounding lands is determined by the height, mass, and location of a mountain or range. Mountains lying in the path of moist winds force the winds to rise, cool, and release their moisture. Thus the area on the windward side of such, a mountain has a wetter and milder climate than that on the leeward side. Many desert regions are the result of the obstruction of moisture-bearing winds. Mountains may also hinder the flow of air masses, causing some areas to be hot while other areas are cold.
A warm, dry wind flowing from mountain areas is common in some regions. As this wind generally occurs in winter or spring, it has the beneficial effects of moderating temperatures and melting snow. In the western part of the United States and Canada it is called a chinook. In the European Alps it is a foehn.
In earlier times mountains were looked upon as forbidding barriers, and were often the legendary homes of gods. By hindering man's movements mountains discouraged communication and cultural exchange.
As better methods of transportation and communication developed, mountains lost their ominous character. Today most mountains are no longer barriers, although their harsh climates and rugged terrain have discouraged extensive human use. Many mountains now provide scenic and recreational enjoyment, as well as valuable resources such as timber and minerals.