Mountain

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A mountain is a landform that extends above the surrounding terrain in a limited area. A mountain is generally higher and steeper than a hill, but there is considerable overlap, and usage often depends on local custom. Some authorities define a mountain as a peak with a topographic prominence over a defined value: for example, the Encyclopædia Britannica requires a prominence of 610 m (2,000 ft).

Mountains cover 52% 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. All the world's major 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 the things associated with them.

Heights

Heights of 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, 8,848 m, set in the world's most significant mountain range, the Himalaya. Other definitions of height are possible. The peak that is farthest from the centre of the Earth is Chimborazo in Ecuador. At 6,272 m above sea level it is not even the tallest peak in the Andes, but because the Earth bulges at the equator and Chimborazo is very close to the equator, it is 2,150 m further away from the Earth's centre 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.

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.

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.

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 topography. 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 topography, 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 other, 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. 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

United States

In the United States, a mountain is 1,000 feet (304.4 metres) or more in height from bottom to summit. A hill is 500 (152.4 metres) to 999 (304 metres) feet. A discernible hill that is less than 500 feet high is a knoll. A series of knolls constitutes a rolling plain. A plain is generally considered flat if it has no significant prominences (e.g., "hills" less than 20 feet high, though the range of height varies for a plain to be considered flat).

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 (609.6 m). ^* 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, as well as many lower hills which are distinctive or mountainous. In the United Kingdom the term "hill" is commonly used for all hills and mountains, regardless of height.

External links

Pics from the Tatra Mountains - Poland
Mount Everest - Nepal
Canadian Mountain Encyclopedia - an exhaustive index of North American peaks, including thousands of unnamed ones. Includes the United States and Mexico as well as Canada.

References

Fraknoi, A., Morrison, D., & Wolff, S. (2004). Voyages to the Planets. 3rd Ed. Belmont: Thomson Books/Cole.

Landforms | Mountains | Mountaineering | Geomorphology

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