Mountain
A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock.
Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher than a hill, typically rising at least 300 metres (980 ft) above the surrounding land.
[1] Mountains are formed through tectonic forces, erosion, or volcanism,[1] which act on time scales of up to tens of millions of years.
[3] High elevations on mountains produce colder climates than at sea level at similar latitude.
The highest mountain on Earth is Mount Everest in the Himalayas of Asia, whose summit is 8,850 m (29,035 ft) above mean sea level.
John Whittow's Dictionary of Physical Geography[6] states "Some authorities regard eminences above 600 metres (1,969 ft) as mountains, those below being referred to as hills."
[12] In addition, some definitions also include a topographical prominence requirement, such as that the mountain rises 300 metres (984 ft) above the surrounding terrain.
[1] At one time, the United States Board on Geographic Names defined a mountain as being 1,000 feet (305 m) or taller,[13] but has abandoned the definition since the 1970s.
Major mountains tend to occur in long linear arcs, indicating tectonic plate boundaries and activity.
[18] At a depth of around 100 km (60 mi), melting occurs in rock above the slab (due to the addition of water), and forms magma that reaches the surface.
The intervening dropped blocks are termed graben: these can be small or form extensive rift valley systems.
This kind of landscape can be seen in East Africa,[25] the Vosges and Rhine valley,[26] and the Basin and Range Province of Western North America.
[28]: 160 Glacial processes produce characteristic landforms, such as pyramidal peaks, knife-edge arêtes, and bowl-shaped cirques that can contain lakes.
[15]: 12 The effect of the climate on the ecology at an elevation can be largely captured through a combination of amount of precipitation, and the biotemperature, as described by Leslie Holdridge in 1947.
Mountain environments are particularly sensitive to anthropogenic climate change and are currently undergoing alterations unprecedented in last 10,000 years.
Observational studies show that highlands are warming faster than nearby lowlands, but when compared globally, the effect disappears.
Many mid-latitude mountains act as cold climate refugia, with the ecosystems occupying small environmental niches.
[42] In regions with dry climates, the tendency of mountains to have higher precipitation as well as lower temperatures also provides for varying conditions, which enhances zonation.
[46] At very high altitudes, the decreasing atmospheric pressure means that less oxygen is available for breathing, and there is less protection against solar radiation (UV).
Mountains are generally less preferable for human habitation than lowlands, because of harsh weather and little level ground suitable for agriculture.
[51] A counterexample is El Alto, Bolivia, at 4,150 metres (13,620 ft), which has a highly diverse service and manufacturing economy and a population of nearly 1 million.
[52] Traditional mountain societies rely on agriculture, with higher risk of crop failure than at lower elevations.
More recently, tourism has become more important to the economies of mountain communities, with developments focused around attractions such as national parks and ski resorts.
[16] Most of the world's rivers are fed from mountain sources, with snow acting as a storage mechanism for downstream users.
[69] Mount Kailash, in the Tibet Autonomous Region of China, is considered to be sacred in four religions: Hinduism, Bon, Buddhism, and Jainism.
Mount Ararat is a sacred mountain, as it is believed to be the biblical landing place of Noah's Ark.
[73] There are at least 100 mountains with heights of over 7,200 metres (23,622 ft) above sea level, all of which are located in central and southern Asia.
There is no precise definition of surrounding base, but Denali,[74] Mount Kilimanjaro and Nanga Parbat are possible candidates for the tallest mountain on land by this measure.
