Mountain, name usually applied to region of land that is raised rather steeply above the surrounding terrain. Mountains are distinguishable from plateaus by mountains’ usually limited summit area (mountains are generally much narrower at the top than at the base) and they are distinguishable from hills by mountains’ generally higher elevation. The elevation, or altitude, of a mountain is given as the height of the summit above sea level. Therefore, a mountain with an elevation of 4000 m (13,100 ft) may rise to a level of only 3000 m (9840 ft) above the surrounding land.
Mountains are normally found in groups or ranges consisting of peaks, ridges, and intermontane (between mountains) valleys. Except for certain mountains that occur singly, the smallest unit pertaining to mountains is the range, comprising either a single complex ridge or a series of ridges generally alike in origin, age, and form. Several closely related ranges in a parallel alignment or chainlike cluster are known as a mountain system; an elongated series of systems forms a mountain chain; and an extensive complex of ranges, systems, and chains is known as a belt, or cordillera.
Geologists believe that most mountains are formed by movements in the earth’s crust. The plate tectonics model has helped explain many of these processes. This model describes the crust of the earth as made up of a number of vast plates that move about at the rate of a few centimeters a year leading to the collision and separation of continents and the subsequent development of mountain belts.
Movements that result in collisions between plates tend to raise the crust by faulting, folding, or arching up layers of rock. Movements that result in separation, or rifting, of plates cause some blocks of crust to sink, leaving other blocks to tower above them. Volcanic eruptions also raise mountains, and much of the world’s volcanic activity is concentrated along the active margins of crustal plates. Finally, some ranges of low mountains are created by nontectonic processes, chief among these being the sculpturing effects of erosion, which wears away softer rock and leaves the harder rock.
Collisions between plates of the earth’s crust trigger various geologic processes that result in crustal uplift. A common process, produced by horizontal compression, is the deformation of crustal strata into folds or wrinkles (see Geomorphology) or the thrusting of vast, thick sheets of rock over one another (see Fault). The Himalayas, for example, were raised by the compression that accompanied collision of the Indian plate with the Eurasian one. Europe’s Alps and Jura Mountains also formed because of horizontal compression, generated in their case by collision of the African plate with the Eurasian one. Similarly, the Appalachian Mountains rose in response to forces caused by repeated collisions between the North American plate and the African and European ones, and most of the Rocky Mountains rose because of the compressive effects generated by collisions between the North American and Pacific plates.
Formation of a basin-and-range structure, best exemplified by the basin-and-range areas in Nevada and Utah (see Great Basin), is the result of movement of rock along faults, or major deep cracks, in the crust of the earth. Occasionally, such movement, called faulting, causes rock bordering on faults to be lifted vertically in great blocks. The raised edges of the blocks then appear as mountains, the depressed edges as valleys. Such mountains are also widely known as fault-block mountains.
A third type of mountain formed by uplifting is called the dome. The dome structure is typified by the Henry and Abajo Mountains in Utah, and by the Adirondack Mountains in New York. These mountains are created by deep-seated intrusion of igneous, or molten, rock that arches the rocks at the surface.
Rock on the surface of the earth is constantly exposed to erosion. Because rocks of different composition resist erosion differently, areas of relatively hard rock may stand high above areas of softer, more easily eroded rock. Mountains resulting from this erosive sculpturing of the land may be linear in appearance if the resistant rock is the upturned edge of a sedimentary rock unit, flat-topped buttes or mesas if the harder rock is a flat-lying unit, or complex and irregular ranges if the resistant rocks are an uncovered intrusive igneous mass. Portions of the Ozark Plateau (also known as the Ozark Mountains) in Arkansas and Missouri are good examples of mountains created by the forces of erosion.
Mountains formed by volcanic action are well known because of their usually isolated occurrence and periodically dangerous aspect. Most spectacular and probably most familiar are the conical peaks composed of lava and volcanic debris, such as Mount Rainier and Mount Saint Helens in the western United States, Mount Erebus in Antarctica, Mount Vesuvius in Italy, and Fuji in Japan. Many of these volcanic mountains have summit craters that still emit steam and debris; others no longer showing signs of volcanic activity may be only dormant, not extinct. Shield volcanoes, typified by Mauna Loa and Mauna Kea in Hawaii, are less spectacular even when quite high, since the physical properties of their formative material have shaped them into broad shieldlike masses that deemphasize their height.
Mountains affect life in many ways. Apart from their mineral, forest, agricultural, and recreational resource value, they exert a significant influence on climate and determine the course of economic or historical trends. Especially high mountain ranges, such as the Sierra Nevada in the U.S., the Andes in South America, and the Himalayas in Asia, markedly affect climate and weather patterns over vast areas of the earth because they stand as barriers to regularly circulating air masses. Moisture carried inland by winds from the Pacific Ocean, for example, is lost in the form of rain and snow on the windward sides of the Sierra Nevada and Andes; the leeward, or inland, side is drier, and the land beyond is frequently arid.
The importance of mountains with regard to the history and economy of various nations can be shown by their influence upon the development of the western United States. The first travelers and settlers, and then the earliest railroads, avoided mountain crossings because of the dangers and costs involved. Later, however, the vast deposits of minerals that became so vitally important to the development of several western states were found exposed principally in mountainous areas, and the lure of “striking it rich” drew people and railroads west despite the hardships encountered in traversing the passes. As a result, transportation routes and patterns, with large populations centered about them, were established; most of these remain today.
The political significance of mountains has been noticeable throughout human history. Mountain barriers with their relatively narrow and easily defendable passes have made various ranges throughout the world natural political boundaries, second in strategic importance only to oceans and seas.
Most of the world’s highest mountains lie in the great Himalayan system and the cordillera stretching through North and South America. The accompanying table includes the highest mountains found on each continent.
See Erosion; Geology; Volcano. For additional information, see separate articles on most mountains and mountain systems mentioned.
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