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.
Mountains and mountain ranges have varied origins. Some are the erosional remnants of plateaus; others are cones built up by volcanoes, such as Mt. Rainier in Washington, or domes pushed up by intrusive igneous rock (see rock), such as the Black Hills of South Dakota and the Henry Mts., Utah. Fault-block mountains (see fault) 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 fold 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 subsidence (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 ocean). 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 continent). 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 tectonics theory, the lithosphere 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 spreading; continental drift).
See W. M. Bueler, Mountains of the World (1970); K. Hsu, Mountain Building Processes (1986); A. J. Gerrard, Mountain Environments (1990).
Acute reaction to a change from low altitudes to altitudes above 8,000 ft (2,400 m). Most people gradually adapt, but some have a severe reaction that can be fatal unless they return to low altitude. Normal adaptations to the reduced oxygen at high altitude (e.g., breathlessness, racing heartbeat) are exaggerated; other manifestations include headache, gastrointestinal upsets, and weakness. Pulmonary edema is quickly reversed with oxygen and evacuation to a lower area.
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Bighorn sheep (Ovis canadensis).
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Mountain range, southern Montana and northern Wyoming, U.S. It is a range of the northern Rocky Mountains extending 120 mi (193 km), rising abruptly 4,000–5,000 ft (1,200–1,500 m) above the Great Plains and Bighorn Basin. The highest summit is Wyoming's Cloud Peak, at 13,165 ft (4,013 m). Bighorn National Forest covers part of the range. On Medicine Mountain is the Medicine Wheel, a prehistoric stone-spoked circle 70 ft (20 m) in diameter.
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Species (Puma concolor) of large, graceful cat that lives in a wide variety of habitats in the Americas, from southern Alaska to Patagonia. In many regions, the species is restricted to wilderness areas, and some subspecies are considered endangered. Cougars' coloration ranges from pale buff to reddish brown, with dark ears and tail tip and white rump and belly. The adult weighs from 77 to more than 220 lb (35 to 100 kg). A male may be about 9 ft (3 m) long, one-third of which is tail, and stand 24–30 in. (60–75 cm) tall at the shoulder. Since the cougar occasionally kills livestock, it has been intensively hunted by farmers, especially in North America, and has been basically exterminated from the eastern U.S. It is valuable for preventing overpopulation of prey animals (mostly deer, in North America). In North America, cougar attacks on humans occur a few times per year, some being fatal.
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Mountain goat (Oreamnos americanus)
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Sport of attaining, or attempting to attain, high points in mountainous regions, mainly for the joy of the climb. The pleasures of mountaineering lie not only in the conquest of the peak but also in the physical and spiritual satisfactions brought about through intense personal effort, ever-increasing proficiency, and contact with natural grandeur. The greater rewards do not come without considerable risk and danger. The first great peak ascended in modern times was Mont Blanc, in 1786. Other Alpine peaks followed, capped by the ascent of the Matterhorn in 1865. By the 1910s, most peaks of the Andes, the Rockies, and other Western Hemisphere ranges had been climbed, including Mount McKinley (1913). Beginning in the 1930s a series of successful ascents of mountains in the Himalayas occurred; the summits of many of the Himalayan mountains were not reached until the 1950s, however. Of these climbs, the best known is the 1953 ascent of Mount Everest by Edmund Hillary and Tenzing Norgay. In the 1960s mountaineering became an increasingly technical sport, emphasizing the use of specialized anchoring, tethering, and grappling gear in the ascent of vertical rock or ice faces.
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Any of several shrubs or trees of the genus Sorbus, in the rose family, native to the Northern Hemisphere. They are widely cultivated as ornamentals for their white flower clusters and bright-orange fruits. Most noteworthy are the handsome American mountain ash, or dogberry (S. americana), and European mountain ash (S. aucuparia), also called rowan, or quickbeam. The European species grows to 60 ft (18 m), twice as high as the American species.
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Landform that rises well above its surroundings, generally exhibiting steep slopes, a relatively confined summit area, and considerable local relief (inequalities of elevation). Mountains are considered larger than hills, but the term has no standardized geologic meaning. Mountains are formed by the folding, faulting, or upwarping of the Earth's surface due to the movement of plates (see plate tectonics) or by the emplacement of volcanic rock onto the surface. For example, the Himalayan Mountains where India meets the Eurasian Plate were formed by a collision between plates that caused extreme compressional folding and the uplifting of large areas. The mountain ranges around the Pacific basin are attributed to the sinking of one plate beneath another. Seealso plateau.
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Typhus-like disease first seen in the Rocky Mountain region, caused by the bacterium Rickettsia rickettsii (see rickettsia) and transmitted by various ticks. In severe cases the rash bleeds more and is especially prominent on the wrists and ankles. Central nervous system involvement causes restlessness, insomnia, and delirium. Prostration may progress to coma, with death possible in a week or more. Mortality increases with age. Recovery is slow but usually complete as visual disturbances, deafness, and mental confusion pass. Prompt antibiotic treatment hastens it and reduces mortality. Prevention depends on avoiding tick bites, by wearing long, light-coloured clothing and insect repellent and inspecting for ticks. A vaccine reduces the risk of infection somewhat and of death greatly.
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Breed of Swiss working dog brought to Switzerland over 2,000 years ago by invading Romans. The hardy breed was widely used to pull carts and to drive cattle to and from their pastures. It has a broad chest, hanging V-shaped ears, and a long, silky, black coat. Markings include brown spots on the chest and forelegs and over the eyes, and sometimes white on the chest, nose, feet, and tail tip. The breed stands 21–28 in. (53–70 cm) high and weighs about 90 lbs (40 kg).
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A mountain is a landform that extends above the surrounding terrain in a limited area, with a peak. A mountain is generally steeper than a hill, but there is no universally accepted standard definition for the height of a mountain or a hill although a mountain usually has an identifiable summit. Mountains cover 64% of Asia, 36% of North America, 25% of Europe, 22% of South America, 17% of Australia, and 3% of Africa. As a whole, 24% of the Earth's land mass is mountainous. 10% of people live in mountainous regions. Most of the world's rivers are fed from mountain sources, and more than half of humanity depends on mountains for water.
The adjective montane is used to describe mountainous areas and things associated with them. Orology is its specialized field of studies, though the term is mostly replaced by "mountain studies". (Not to be confused with horology.)
Some authorities define a mountain as a peak with a topographic prominence over a defined value: for example, according to the Britannica Student Encyclopedia, the term "generally refers to rises over 2,000 feet (610 m)". The Encyclopædia Britannica, on the other hand, does not prescribe any height, merely stating that "the term has no standardized geological meaning".
The height of a mountain is measured as the elevation of its summit above mean sea level. The Himalayas average 5 km above sea level, while the Andes average 4 km. The highest mountain on land is Everest, in the Himalayas.
Other definitions of height are possible. The peak that is farthest from the center of the Earth is Chimborazo in Ecuador. At above sea level it is not even the tallest peak in the Andes, but because Chimborazo is very close to the equator and the Earth bulges at the equator, it is further away from the Earth's center than Everest. The peak that rises farthest from its base is Mauna Kea on Hawaii, whose peak is above its base on the floor of the Pacific Ocean. Mount Lamlam on Guam also lays claim to the tallest mountain as measured from it base. Although its peak is only above sea level, it measures to its base at the bottom of the Marianas Trench.
Even though Everest is the highest mountain on Earth today, there have been much taller mountains in the past. During the Precambrian era, the Canadian Shield once had mountains in height that are now eroded down into rolling hills. These formed by the collision of tectonic plates much like the Himalaya and the Rocky Mountains.
At (Fraknoi et al., 2004), the tallest known mountain in the solar system is Olympus Mons, located on Mars and is an ancient volcano. Volcanoes have been known to erupt on other planets and moons in our solar system and some of them erupt ice instead of lava (see Cryovolcano). Several years ago, the Hale telescope recorded the first known images of a volcano erupting on a moon in our solar system.
High mountains, and mountains located closer to the Earth's poles, have elevations that exist in colder layers of the atmosphere. They are consequently often subject to glaciation and erosion through frost action. Such processes produce the popularly recognizable mountain peak shape. Some of these mountains have glacial lakes, created by melting glaciers; for example, there are an estimated 3,000 glacial lakes in Bhutan.
Sufficiently tall mountains have very different climatic conditions at the top than at the base, and will thus have different life zones at different altitudes. The flora and fauna found in these zones tend to become isolated since the conditions above and below a particular zone will be inhospitable to those organisms. These isolated ecological systems are known as sky islands and/or microclimates. Tree forests are forests on mountain sides which attract moisture from the trees, creating a unique ecosystem. Very tall mountains may be covered in ice or snow.
Mountains are colder than lower ground, because the Sun heats Earth from the ground up. The Sun's radiation travels through the atmosphere to the ground, where Earth absorbs the heat. Air closest to the Earth's surface is, in general, warmest (see lapse rate for details). Air as high as a mountain is poorly warmed and, therefore, cold. Air temperature normally drops 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit) for each 300 meters (1000 feet) of altitude.
Mountains are generally less preferable for human habitation than lowlands; the weather is often harsher, and there is little level ground suitable for agriculture. At very high altitudes, there is less oxygen in the air and less protection against solar radiation (UV). Acute mountain sickness (caused by hypoxia - a lack of oxygen in the blood) affects over half of lowlanders who spend more than a few hours above 3,500 meters (11,483 feet).
A number of mountains and mountain ranges of the world have been left in their natural state, and are today primarily used for recreation, while others are used for logging, mining, grazing, or see little use of any sort at all. Some mountains offer spectacular views from their summits, while others are densely wooded. Summit accessibility ranges from mountain to mountain; height, steepness, latitude, terrain, weather, and the presence or lack thereof of roads, lifts, or tramways are all factors that affect accessibility. Hiking, backpacking, mountaineering, rock climbing, ice climbing, downhill skiing, and snowboarding are recreational activities typically enjoyed on mountains. Mountains that support heavy recreational use (especially downhill skiing) are often the locations of mountain resorts.
Mountains can be characterized in several ways. Some mountains are volcanoes and can be characterized by the type of lava and eruptive history. Other mountains are shaped by glacial processes and can be characterized by their glaciated features. Still others are typified by the faulting and folding of the Earth's crust, or by the collision of continental plates via plate tectonics (the Himalayas, for instance). Shape and placement within the overall landscape also define mountains and mountainous structures (such as butte and monadnock). Finally, many mountains can be characterized by the type of rock that make up their composition. More information on mountain types can be found in List of mountain types.
A mountain is usually produced by the movement of lithospheric plates, either orogenic movement or epeirogenic movement. The compressional forces, isostatic uplift and intrusion of igneous matter forces surface rock upward, creating a landform higher than the surrounding features. The height of the feature makes it either a hill or, if higher and steeper, a mountain. The absolute heights of features termed mountains and hills vary greatly according to an area's terrain. The major mountains tend to occur in long linear arcs, indicating tectonic plate boundaries and activity. Two types of mountain are formed depending on how the rock reacts to the tectonic forces – block mountains or fold mountains.
The compressional forces in continental collisions may cause the compressed region to thicken, so the upper surface is forced upward. In order to balance the weight of the earth surface, much of the compressed rock is forced downward, producing deep "mountain roots" [see the Book of "Earth", Press and Siever page.413]. These roots are deeply embedded in the ground, thus, a mountain have a shape like peg [See Anatomy of the Earth, Cailleus page.220]. Mountains therefore form downward as well as upward (see isostasy). However, in some continental collisions part of one continent may simply override part of the others, crumpling in the process.
Block mountains are created when large areas are widely broken up by faults creating large vertical displacements. This occurrence is fairly common. The uplifted blocks are block mountains or horsts. The intervening dropped blocks are termed graben: these can be small or form extensive rift valley systems. This form of landscape can be seen in East Africa, the Vosges, the Basin and Range province of Western North America and the Rhine valley. These areas often occur when the regional stress is extensional and the crust is thinned.
The mid-ocean ridges are often referred to as undersea mountain ranges due to their bathymetric prominence.
Where rock does not fault it folds, either symmetrically or asymmetrically. The upfolds are anticlines and the downfolds are synclines; in asymmetric folding there may also be recumbent and overturned folds. The Jura mountains are an example of folding. Over time, erosion can bring about an inversion of relief: the soft upthrust rock is worn away so the anticlines are actually lower than the tougher, more compressed rock of the synclines.