Mountain
A mountain is a landform that extends above the surrounding terrain in a limited area. 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 54% 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. Also, 1 in 10 people live in mountainous regions. Most of the world's major rivers are fed from mountain sources,and more than half of humanity depends on mountains for water.[1][2]
The adjective montane is used to describe mountainous areas and the things associated with them.
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Heights
Mountains are generally given as heights above mean sea level. The Himalayas average 5 km above sea level, whilst the Andes average 4 km. Most other mountain ranges average 2 – 2.5 km. The highest mountain on Earth is Everest, 8,850 m (29,028 feet), set in the world's most significant mountain range, the Himalayas.
Other definitions of height are possible. The peak that is farthest from the center of the Earth is Chimborazo in Ecuador. At 6,267 m 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 2,150 m 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 over 9,000 m above its base on the floor of the Pacific Ocean.
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 enormous mountains 12,000 m in height that are now eroded down into rolling hills. These enormous mountains formed by the collision of plate tectonics much like the Himalaya and the Rocky Mountains.
At 26 km (Fraknoi et al., 2004), the tallest known mountain in the solar system is Olympus Mons, located on Mars.
Characteristics
The altitude of mountains means that the tops exist in higher cold layers of the atmosphere. They are consequently often subject to glaciation and erosion through frost action. This produces the classic mountain peak shape. Some mountains have glacial lakes, created by melting glaciers; for example, there are an estimated 3,000 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 on their slopes. The plants and animals of a zone are somewhat isolated when the zones above and below are inhospitable, and many unique species occur on mountainsides as a result. Extreme cases are known as sky islands. Cloud forests are forests on mountain sides which attract moisture from the air, creating a unique ecosystem. Very tall mountains may be covered in ice or snow.
Mountains are not generally favored for human habitation; the weather is harsher, less food is available, and there is little level ground suitable for farming. 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 metres. Despite some biological adaptation by peoples who have lived on mountains for hundreds or thousands of years, babies' average birthweight is reduced by 100 grams for every 1,000-metre gain in altitude. Because of the dangers faced by those visiting high altitudes, stores lining the mountain ranges known to cause these sicknesses provide oxygen tanks and higher SPF sunscreens.
Most mountains of the world have been left in their natural state, and are today primarily used for recreation. Some mountains are very difficult to climb, and offer spectacular views. Some people therefore enjoy the sport of mountaineering. Mountains are also the site for the sport of downhill skiing. People engaging in these activities often stay at mountain resorts built for the purpose.
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 upwards, 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. Mountain creation tends to occur in discrete periods, each referred to as an orogeny. The orogeny may last millions of years, and the uplifted region is being eroded away, producing valley-and-peak terrain, even while the uplift is taking place. 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 upwards. In order to balance the weight, much of the compressed rock is forced downwards, producing deep "mountain roots". Mountains therefore form downwards as well as upwards (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.
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.
Local definitions
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 metres)".[3] The Encyclopædia Britannica, on the other hand, does not prescribe any height, merely stating that "the term has no standardized geological meaning".[4]
United Kingdom
In England and Wales the Department for Environment, Food and Rural Affairs has defined "mountain" (as a mass noun) as all land over 600 metres, for the purposes of right to roam legislation. This is a close metric equivalent of 2,000 feet (610 meters).[5] The Land Reform (Scotland) Act 2003 does not appear to draw this distinction, and in Scotland the term "mountain" is more subjective, often being used for hills exceeding 3,000 feet (914.4 m) listed as Munros. In the United Kingdom the term "hill" is commonly used for all hills and mountains, regardless of height.
References
- ^ International Year of Freshwater 2003. Retrieved on 2006-12-07.
- ^ The Mountain Institute. Retrieved on 2006-12-07.
- ^ Mountain -- Britannica Student Encyclopedia. Retrieved on 2007-01-08.
- ^ Mountain -- Britannica Concise Encyclopedia - The online encyclopedia you can trust!. Retrieved on 2007-01-08.
- ^ http://www.defra.gov.uk/wildlife-countryside/consult/access/statut.htm (Link broken)
Sources
- Fraknoi, A., Morrison, D., & Wolff, S. (2004). Voyages to the Planets. 3rd Ed. Belmont: Thomson Books/Cole.
