Mountain
A mountain is a large 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. The study of mountains is called Orography.
Exogeology deals with planetary mountains, which in that branch of science are usually called montes (singular—mons). The highest mountain on earth is the Mount Everest 8,848 m (29,029 ft).. The highest known mountain in the Solar System is Olympus Mons on the planet Mars at 21,171 m (69,459 ft).
Definition
There is no universally accepted definition of a mountain. Elevation, volume, relief, steepness, spacing and continuity has been used as criteria for defining a mountain.[1] In the Oxford English Dictionary a mountain is defined as "a natural elevation of the earth surface rising more or less abruptly from the surrounding level and attaining an altitude which, relatively to the adjacent elevation, is impressive or notable."[1]
Whether a landform is called a mountain may depend on usage among the local people. The highest point in San Francisco, California, is called Mount Davidson, notwithstanding its height of 990 feet, which makes it ten feet short of the minimum for a mountain in American appellation. Similarly, Mount Scott outside Lawton, Oklahoma is only 824 feet from its base to its highest point.
Definitions of "mountain" include:[2]
- Height over base of at least 2,500 m (8,202 ft).
- Height over base of 1,500 m (4,921 ft).–2,500 m (8,202 ft). with a slope greater than 2 degrees
- Height over base of 1,000 m (3,281 ft).–1,500 m (4,921 ft). with a slope greater than 5 degrees
- Local (radius 7,000 m (22,966 ft). elevation greater than 300 m (984 ft)., or 300 m (984 ft)–1,000 m (3,281 ft). if local (radius 7,000 m (22,966 ft). elevation is greater than 300 m (984 ft).
By 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 and 10% of people live in mountainous regions.[3] Most of the world's rivers are fed from mountain sources, and more than half of humanity depends on mountains for water.[4][5]
Characteristics
High mountains, as well as those 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 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.
Tall mountains have different climatic conditions at the top than at the base, and will thus have different life zones at different altitudes. At the highest elevations, trees cannot grow, and whatever life may be present will be of the alpine type, resembling tundra.[6] Just below the tree line, one may find subalpine forests of needleleaf trees, which can withstand cold, dry conditions.[7] In regions with dry climates, the tendency of mountains to have higher precipitation as well as lower temperatures also provides for varying conditions, which in turn lead to differing flora and fauna.[6][8] Some plants and animals found in these zones tend to become isolated since the conditions above and below a particular zone will be inhospitable and thus constrain their movements or dispersal. On the other hand, birds, being capable of flight, may take advantage of montane habitats and migrate into a region that would otherwise not provide appropriate habitat.[9] These isolated ecological systems, or microclimates, are known as sky islands.[10]
The reason mountains are colder than lowlands has to do with how the sun heats the surface of the earth. Practically all the heat at the surface of the Earth comes from the sun, in the form of solar energy. The sun's radiation is absorbed by land and sea, whence the heat is transferred into the air. Air is an insulator, so conduction of heat from the ground to the atmosphere is negligible. Heat is mainly transferred into the atmosphere through convection and radiation. Warm air rises because of its buoyancy, leading to convective circulation, in the form of thermals, within the lowest layer of the atmosphere, the troposphere. When heat radiates from the surface of the earth, it is released as long-wave radiation, which does not travel through the air efficiently. This radiant heat is absorbed temporarily by gasses in the atmosphere, such as carbon dioxide and water vapor. Thus, the lower portion of the troposphere—more than 50% of all air lies below the altitude of the summit of Mt. Everest—forms a blanket of air keeping the surface warm. This is the Greenhouse Effect. The higher one goes in altitude, the less of this blanket there is to keep in the heat. Thus, higher elevations, such as mountains, are colder than surrounding lowlands.[11] Air temperature in the lowest layer of the atmosphere, the troposphere, decreases with gains in altitude. The rate at which the temperature drops with elevation, called the environmental lapse rate, is not constant (it can fluctuate throughout the day or seasonally and also regionally), but a normal lapse rate is 5.5°C per 1,000 m (3°F per 3,000 ft).[12][13] The temperature continues to drop up to a height of about 9–16 km, where it does not decrease further. However, this is higher than the highest mountaintop.
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 ft).
Many mountains and mountain ranges throughout 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. Some mountains offer spectacular views from their summits, while others are densely wooded. Summit accessibility is affected by height, steepness, latitude, terrain, weather. Roads, lifts, or tramways affect accessibility. Hiking, backpacking, mountaineering, rock climbing, ice climbing, downhill skiing, and snowboarding are recreational activities enjoyed on mountains. Mountains that support heavy recreational use (especially downhill skiing) are often the locations of mountain resorts.
Mountains are made up of earth and rock materials. The outermost layer of the Earth or the Earth's crust is composed of six plates. When two plates move or collide each other, vast land areas are uplifted, resulting in the formation of mountains. Depending upon the geological process, as to how the mountains are formed and the mountain characteristics, there are five major types of mountains.
Fold Mountains: Fold mountains are the most common type of mountains. Examples of fold mountains are the Himalayas (Asia), the Alps (Europe). They are formed due to collision of two plates, causing folding of the Earth's crust. The fold that descends on both sides is called anticline; whereas, the fold that ascends from a common low point (on both sides) is called syncline.
Fault-Block Mountains: As the name suggests, fault mountains or fault-block mountains are formed when blocks of rock materials slide along faults in the Earth's crust. There are two types of block mountains, namely the lifted and tilted. In the former type, the mountain has two steep sides; whereas, the tilted type has one steep side and gentle sloping side. Example of fault-block mountain is the Sierra Nevada mountains (North America).
Volcanic Mountains: Volcanic mountains are formed due to volcanic eruptions, for e.g. Mount Fuji (Japan). They are formed when volcanic magma erupts and piles up on the surface of the Earth.
Dome Mountains: Dome mountains are formed when the hot magma rises from the mantle and uplifts the overlying sedimentary layer of the Earth's crust. In the process, the magma is not erupted, but it cools down and forms the core of the mountain. Example of dome mountain is the Navajo Mountain in Utah. They are called dome mountains due to their appearance that resembles dome shape.
Plateau Mountains: Plateau mountains are pseudo mountains that are formed because of erosion. An example of plateau mountain is the Catskill Mountains (New York). They usually occur near the fold mountain ranges.
There are also some mountains that are formed as a result of many forces of the Earth. Though the Rockies in North America is formed due to folding, there are mountains in the same range that are formed by faulting and doming. In nature, there is a continuous process of glaciation, soil erosion, and mechanical and chemical weathering, which altogether play a major role in altering the shape and characteristics of mountains.
Geology
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.
Compressional forces in continental collisions may cause the compressed region to thicken, so the upper surface is forced upward. 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.
Rock that does not fault may fold, 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.
Mountain Climbing
Mountain climbing is a popular sport in which people hike and climb up mountains. The main goal is to reach the summit of the mountain. Ropes and other equipment are often used to aid climbers. Mountain climbing is a dangerous sport as people are often injured or killed as they try to climb a mountain. Mountain climbers usually travel in a small group as climbing alone is considered very dangerous.
Gallery
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Mount Kilimanjaro, 5,895 metres (19,341 ft), Tanzania. -
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Table Mountain Cape Town, South Africa -
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The Durmitor, the highest mountain in Montenegro -
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See also
- List of mountains
- List of mountain types
- Category:Lists of mountains
- Mountain range
- List of mountain ranges
- List of highest mountains
- List of peaks by prominence
- Latin names of mountains
- List of ski areas and resorts
- Mountaineering
- Mountain building
Notes
- ^ a b Gerrard, A. J. 1990. Mountain Environments
- ^ Blyth, S., Groombridge, B., Lysenko, I., Miles, L. & Newton, A. (2002). "Mountain Watch" (PDF). UNEP World Conservation Monitoring Centre, Cambridge, UK. Retrieved 2009-02-17.
{{cite web}}: CS1 maint: multiple names: authors list (link) - ^ Panos (2002). "High Stakes". Retrieved 2009-02-17.
- ^ "International Year of Freshwater 2003". Retrieved 2006-12-07.
- ^ "The Mountain Institute". Retrieved 2006-12-07.
- ^ a b "Biotic Communities of the Colorado Plateau: C. Hart Merriam and the Life Zones Concept". Retrieved 30 January 2010.
- ^ "Tree". Microsoft Encarta Reference Library 2003. Microsoft Corporation. 1993–2002. 60210-442-1635445-74407.
{{cite encyclopedia}}:|access-date=requires|url=(help)CS1 maint: date format (link) - ^ "Mountain Environments" (PDF). United Nations Environment Programme World Conservation Monitoring Centre. Retrieved 30 January 2010.
- ^ Taylor, Richard Cachor (2005). A Birder's Guide to Southeastern Arizona. American Birding Association. pp. 2–4. ISBN 1-878788-22-1.
- ^ Tweit, Susan J. (1992). The Great Southwest Nature Factbook. Alaska Northwest Books. pp. 138–141. ISBN 0-88240-434-2.
- ^ Lutgens, Frederick K.; Tarbuck, Edward J. (1998). The Atmosphere: An Introduction to Meteorology. Prentice Hall. pp. 15–17, 30–35, 38–40. ISBN 0-13-742974-6.
- ^ "Temperature". Microsoft Encarta Reference Library 2003. Microsoft Corporation. 1993–2002. 60210-442-1635445-74407.
{{cite encyclopedia}}:|access-date=requires|url=(help)CS1 maint: date format (link) - ^ "Atmosphere". Microsoft Encarta Reference Library 2003. Microsoft Corporation. 1993–2002. 60210-442-1635445-74407.
{{cite encyclopedia}}:|access-date=requires|url=(help)CS1 maint: date format (link)
Further reading
- Fraknoi, A., Morrison, D., & Wolff, S. (2004). Voyages to the Planets. 3rd Ed. Belmont: Thomson Books/Cole.
External links
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