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Mountain
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A mountain is a landform that stretches above the surrounding land in a limited area usually in the form of a peak. A mountain is generally steeper than a hill. The adjective montane is used to describe mountainous areas and things associated with them., and the study of mountains is known as Orology.
There is no universally-accepted definition of mountain, though some definitions have been created and applied.
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A mountain is a landform that stretches above the surrounding land in a limited area usually in the form of a peak. A mountain is generally steeper than a hill. The adjective montane is used to describe mountainous areas and things associated with them., and the study of mountains is known as Orology.
There is no universally-accepted definition of mountain, though some definitions have been created and applied. Often, whether something is called a mountain depends on what it is called by the local people. One more objective definition is that mountains are land areas that are >2,500m in height, or 1500-2499m if their slope is >2 degrees, or 1000-1499m if their slope is >5 degrees and local (radius 7km) elevation is >300m, or 300-999m if their local (radius 7km) elevation is >300m.
On the basis of this definition, mountains cover 64% of Asia, 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.
All 50 of the world’s tallest mountains are in Asia.
Exogeology also deals with mountains, which in that branch of science are usually called montes (sing.: mons). The highest mountain in the Solar System is Olympus Mons on Mars (elevation 21,171 m).
Characteristics
High mountains, and mountains located close to the Earth's poles, reach into the colder layers of the atmosphere. They are consequently 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. Mountains can be eroded and weathered, altering their characteristics over time.
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. Alpine forests are forests on mountain sides which attract moisture from the trees.
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.
Types of mountains
Mountains can be characterized in several ways. Some mountains are volcanoes and can be characterized by the type of lava. 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.
Geology
Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everest.
rect 58 14 160 49 Chomo Lonzo
rect 200 28 335 52 Makalu
rect 378 24 566 45 Mount Everest
rect 188 581 920 656 Tibetan Plateau
rect 250 406 340 427 Rong River
rect 333 149 409 186 Changtse
rect 550 284 677 303 Rongbuk Glacier
rect 478 196 570 218 North Face
rect 237 231 346 267 East Rongbuk Glacier
rect 314 290 536 309 North Col north ridge route
rect 531 79 663 105 Lhotse
rect 582 112 711 130 Nuptse
rect 603 232 733 254 South Col route
rect 716 165 839 206 Gyachung Kang
rect 882 147 967 183 Cho Oyu
rect 1 1 999 661
desc bottom-left
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]. 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.
Some isolated mountains were produced by volcanoes, including many apparently small islands that reach a great height above the ocean floor.
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.
See also
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