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mountain,high land mass projecting conspicuously above its surroundings and usually of limited width at its summit. Although isolated mountains are not unusual, mountains commonly form ranges, comprising either a single complex ridge or a series of related ridges. A group of ranges closely related in form, origin, and alignment is a mountain system; an elongated group of systems is a chain; and a complex of ranges, systems, and chains continental in extent is called a cordillera, zone, or belt.
Global Distribution and Impact on Humanity
Most of the great mountain systems now in existence were developed fairly late in geologic history. The greatest mountain masses are in North and South America, including the Andes, Rockies, Sierra Nevada, and Coast Ranges of the United States, Canada, and Alaska; and the Eurasian mountain belt, in which lie the Pyrenees, Atlas, Alps, Balkans, Caucasus, Hindu Kush, Himalayas, and other ranges. Among notable single peaks are Everest, K2 (Godwin-Austen), and Kanchenjunga in Asia; Aconcagua, Chimborazo, and Cotopaxi in South America; McKinley, Logan, and Popocatepetl in North America; Mont Blanc and Elbrus in Europe; Kilimanjaro, Kenya, and Ruwenzori in Africa.
Mountains have important effects upon the climate, population, economy, and state of civilization of the regions in which they occur. By intercepting prevailing winds they cause precipitation; regions on the windward side of a great range thus have plentiful rainfall, while those on its lee side are arid. Mountains are in general thinly populated, not only because the cold climate and rarefied atmosphere of high regions are unfavorable to human life, but also because the higher reaches of mountains are unfit for agriculture. Mountains frequently contain valuable mineral ores, deposited out of solution by water or by gases. Mountains act as natural barriers between countries and peoples; they determine the routes followed by traders, migrants, and invading armies. The difficulties of travel and communication in mountain regions tend to favor political disunity.
The Origins of Mountains
Mountains and mountain ranges have varied origins. Some are the erosional remnants of plateausplateau,
elevated, level or nearly level portion of the earth's surface, larger in summit area than a mountain and bounded on at least one side by steep slopes, occurring on land or in oceans.
..... Click the link for more information. ; others are cones built up by volcanoesvolcano,
vents or fissures in the earth's crust through which gases, molten rock, or lava, and solid fragments are discharged. Their study is called volcanology. The term volcano
..... Click the link for more information. , such as Mt. Rainier in Washington, or domes pushed up by intrusive igneous rock (see rockrock,
aggregation of solid matter composed of one or more of the minerals forming the earth's crust. The scientific study of rocks is called petrology. Rocks are commonly divided, according to their origin, into three major classes—igneous, sedimentary, and metamorphic.
..... Click the link for more information. ), such as the Black Hills of South Dakota and the Henry Mts., Utah. Fault-block mountains (see faultfault,
in geology, fracture in the earth's crust in which the rock on one side of the fracture has measurable movement in relation to the rock on the other side. Faults on other planets and satellites of the solar system also have been recognized.
..... Click the link for more information. ) are formed by the raising of huge blocks of the earth's surface relative to the neighboring blocks. The Basin and Range region of Nevada, Arizona, New Mexico, and Utah is one of the most extensive regions of fault-block mountains.
All the great mountain chains of the earth are either foldfold,
in geology, bent or deformed arrangement of stratified rocks. These rocks may be of sedimentary or volcanic origin. Although stratified rocks are normally deposited on the earth's surface in horizontal layers (see stratification), they are often found inclined or curved
..... Click the link for more information. mountains or complex structures in whose formation folding, faulting, or igneous activity have taken part. The growth of folded or complex mountain ranges is preceded by the accumulation of vast thicknesses of marine sediments. It was first suggested in the late 1800s that these sediments accumulated in elongated troughs, or geosynclines, that were occupied by arms of the sea. While some of the sediment was derived from the interior of the continent, great quantities of sediment were apparently derived from regions now offshore from the continent. For examples, sedimentary rocks of the Appalachian Mts. formed in a vast geosyncline that extended from the Gulf states northeastward through the eastern states and New England, and into E Canada. It is now recognized that great thicknesses of sediment can occur wherever there is subsidencesubsidence,
lowering of a portion of the earth's crust. The subsidence of land areas over time has resulted in submergence by shallow seas (see oceans). Land subsidence can occur naturally or through human activity.
..... Click the link for more information. (lowering of the earth's crust).
The best modern analogues of geosynclines appear to be the thick deposits of sediment making up the continental shelves and continental rises (see oceanocean,
interconnected mass of saltwater covering 70.78% of the surface of the earth, often called the world ocean. It is subdivided into four (or five) major units that are separated from each other in most cases by the continental masses. See also oceanography.
..... Click the link for more information. ). Most geologists now believe that the geosynclinal sediments found in mountain ranges were initially deposited under similar conditions. The period of sedimentation is followed by folding and thrust faulting, with most high mountain ranges uplifted vertically subsequent to folding. The movements of the earth's surface that result in the building of mountains are compression, which produces folding, thrust faulting, and possibly some normal faulting; tension, which produces most normal faulting; and vertical uplift. Mountains are subject to continuous erosion during and after uplift. Sharp peaks are formed and are subsequently attacked and leveled. Mountains may be entirely base-leveled, or they may be rejuvenated by new uplifts.
The ultimate cause of mountain-building forces has been a source of controversy, and many hypotheses have been suggested. An old hypothesis held that earth movements were adjustments of the crust of the earth to a shrinking interior that contracted and set up stresses due either to heat loss or gravitational compaction. Another hypothesis suggested that earth movements were primarily isostatic, i.e., adjustments that kept the weights of sections of the crust nearly equal (see continentcontinent,
largest unit of landmasses on the earth. The continents include Eurasia (conventionally regarded as two continents, Europe and Asia), Africa, North America, South America, Australia, and Antarctica.
..... Click the link for more information. ). A third hypothesis, popular from the early 1960s to today, ascribed mountain-building stresses to convection currents in a hot semiplastic region in the earth's mantle.
According to the plate tectonicsplate tectonics,
theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history.
..... Click the link for more information. theory, the lithospherelithosphere
, brittle uppermost shell of the earth, broken into a number of tectonic plates. The lithosphere consists of the heavy oceanic and lighter continental crusts, and the uppermost portion of the mantle.
..... Click the link for more information. is broken into several plates, each consisting of oceanic crust, continental crust, or a combination of both. These plates are in constant motion, sideswiping one another or colliding, and continually changing in size and shape. Where two plates collide, compressional stresses are generated along the margin of the plate containing a continent. Such stresses result in the deformation and uplift of the continental shelf and continental rise sediments into complex folded and faulted mountain chains (see seafloor spreadingseafloor spreading,
theory of lithospheric evolution that holds that the ocean floors are spreading outward from vast underwater ridges. First proposed in the early 1960s by the American geologist Harry H.
..... Click the link for more information. ; continental driftcontinental drift,
geological theory that the relative positions of the continents on the earth's surface have changed considerably through geologic time. Though first proposed by American geologist Frank Bursley Taylor in a lecture in 1908, the first detailed theory of
..... Click the link for more information. ).
See W. M. Bueler, Mountains of the World (1970); K. Hsu, Mountain Building Processes (1986); A. J. Gerrard, Mountain Environments (1990).
What does it mean when you dream about a mountain?
Climbing the mountain and reaching the top denotes achieving one’s goals. Descending the mountain is returning after a success or letting go of insurmountable issues. Looking at the mountain may denote evaluating a major decision.