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 or chains are linked together, they constitute a mountain belt (e.g., the Andes and Rocky Mountains). The major mountain belts of the world belong to either of two enormous systems, the Circum-Pacific System and the Alpine-Himalayan (or Tethyan) System. Mountains are formed by the folding, faulting, or upwarping of the Earth's surface or by the emplacement of volcanic rock onto the surface. The origin of the major mountain systems is intimately bound up with the interaction of the enormous rigid plates that are thought to make up the lithospherethe outermost shell of the Earth. Most mountains have been elevated above adjacent areas relatively recently with respect to geological time; and erosional processes, if given sufficient time, will reduce them to comparatively low levels. Such processes are primarily responsible for determining the specific shape of a mountain, though its topography is also influenced by its structure and the kinds of rock of which it consists. A collision between lithospheric plates causes extreme compressional folding and the uplifting of large areas. For example, the northward movement of the plate that comprises the Indian subcontinent and its contact with the Eurasian plate has thrust up the Himalayan Mountain Range. Farther west the northward movement of the African plate has resulted in the closure of the Tethys Ocean, of which the Mediterranean is a remnant, and the upthrust of the Alps, Pyrenees, and Atlas Mountains. The mountain ranges around the Pacific Basin are commonly attributed to what is termed the subduction processthe sinking of one plate beneath another at convergent plate boundaries. As the leading edge of the plates underlying the Pacific Ocean are subducted, a portion of the rock material carried into the asthenosphere (layer of molten rock beneath the lithosphere) is melted and subsequently ejected as lava and pyroclasts at the surface. This gives rise to a landscape dominated by steep volcanic cones, such as those in the Cascades of western North America and in Japan. Along other stretches of the Pacific Basin, crustal thickening on the overriding continental plate has led to the formation of mountain belts in which volcanoes constitute only a very small part of the relief, as, for example, the Andes of South America. In some regions, mountain ranges are created by crustal shortening within a continental plate rather than along the boundaries of two colliding plates. Such is the case with the Rockies of North America and the Atlas Mountains of northwestern Africa. Mountains can be classified into a variety of types according to the way they are formed and their resultant structure. The major types include dome, fault-block, fold, and volcanic. Dome mountains are produced by fractureless upwarping of the surface. They have a comparatively flat, dissected surface that gradually slopes toward the adjacent lowlands. This type of mountain is typified by the Black Hills of South Dakota. Fault-block mountains are composed of segments of the Earth's crust that have been uplifted along linear fracture zones in the form of enormous blocks. These upthrown fault blocks are usually separated by valleys or basins. The Sierra Nevada and Teton ranges of western North America are familiar examples of fault-block mountains. Fold mountains are formed by lateral compression and attendant uplift. They tend to occur where extensive basins have been filled with layers of sedimentary rock material. This cover of stratified, unmetamorphosed sedimentary rock reacts differently from the underlying basement rock of granite or gneiss when subjected to compressive forces. Simple-fold mountains are created wherever the sedimentary rock cover is folded by sliding laterally over the basement. The Swiss Jura is representative of this type of mountain, as are certain members of the Appalachian system in eastern North America. In general, volcanic mountains are associated with the island arcs that occur near subduction zones and with the fault zones that are attendant on major orogenic activity. Mountains resulting from volcanic activity fall into one of two major groups. First, there are mountains that result directly from volcanism. These are the ash, cinder, and lava cones that are built up by active volcanoes. Some volcanoes, such as Mount Fuji in Japan, give rise to steep composite cones. If the lava is fairly fluid, then broad-shield volcanoes, as exemplified by Mauna Loa in Hawaii and Cameroon Mountain in West Africa, may result. Second, the residual products of volcanism can produce mountains. Often resistant lavas solidified in volcanic pipes and necks are exhumed by erosion to produce spectacular mountain forms such as Ship Rock in New Mexico or the Devils Tower, Wyoming. On a much larger scale, magma intruded into the crust may be uncovered by erosion to stand up as mountain areas. The granitic Cairngorms of Scotland are such a laccolith. 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. 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 usually lies near sea level. 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, Huascarn, and the high plateau, the Altiplano, in southern Peru and western Bolivia. Peter H. Molnar Additional reading Detailed introductions to the European Alps are E.R. Oxburgh, The Geology of the Eastern Alps (1968), concentrating on Austria; and R. Trmpy, An Outline of the Geology of Switzerland, vol. 1 of Geology of Switzerland: A Guide-Book, ed. by the Schweizerische Geologische Kommission (1980). Introductory articles on other mountain belts of the world are found in Scientific American, including the following: Don L. Anderson, The San Andreas Fault, 225(5):5268 (November 1971); Peter Molnar and Paul Tapponnier, The Collision Between India and Eurasia, 236(4):3041 (April 1977), describing the evolution of mountain belts in eastern Asia; and David L. Jones et al., The Growth of Western North America, 247(5):7084 (November 1982), describing the accretion of exotic terrains to that area. Two useful articles in Geological Society of America Bulletin are Tanya Atwater, Implications of Plate Tectonics for the Cenozoic Tectonic Evolution of Western North America, 81(12):351335 (December 1970), a classic article that changed the way geologists view that region; and John F. Deway et al., Plate Tectonics and the Evolution of the Alpine System, 84(10)313780 (October 1973), a summary of motions of lithospheric plates and mountain building in the Mediterranean area. See also Peter Molnar, The Geologic History and Structure of the Himalaya, American Scientist, 74(2):144154 (March-April 1986), describing how the structure of the Himalayas allows the chain to achieve its great height. Peter H. Molnar