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 (1,000 feet) above the surrounding land. A few mountains are isolated summits, but most occur in mountain ranges.Mountains are formed through tectonic forces, erosion, or volcanism, which act on time scales of up to tens of millions of years.
Once mountain building ceases, mountains are slowly leveled through the action of weathering, through slumping and other forms of mass wasting, as well as through erosion by rivers and glaciers.High elevations on mountains produce colder climates than at sea level at similar latitude. These colder climates strongly affect the ecosystems of mountains: different elevations have different plants and animals. Because of the less hospitable terrain and climate, mountains tend to be used less for agriculture and more for resource extraction, such as mining and logging, along with recreation, such as mountain climbing and skiing.

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. The highest known mountain on any planet in the Solar System is Olympus Mons on 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 have been used as criteria for defining a mountain. 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."Whether a landform is called a mountain may depend on local usage. John Whittow's Dictionary of Physical Geography states "Some authorities regard eminences above 600 metres (1,969 ft) as mountains, those below being referred to as hills."

In the United Kingdom and the Republic of Ireland, a mountain is usually defined as any summit at least 2,000 feet (610 m) high, which accords with the official UK government's definition that a mountain, for the purposes of access, is a summit of 2,000 feet (610 m) or higher. In addition, some definitions also include a topographical prominence requirement, such as that the mountain rises 300 metres (984 ft) above the surrounding terrain. At one time the US Board on Geographic Names defined a mountain as being 1,000 feet (305 m) or taller, but has abandoned the definition since the 1970s. Any similar landform lower than this height was considered a hill. However, today, the US Geological Survey concludes that these terms do not have technical definitions in the US.The UN Environmental Programme's definition of "mountainous environment" includes any of the following:: 74 

Class 1: Elevation greater than 4,500 m (14,764 ft).

Class 2: Elevation between 3,500 m (11,483 ft) and 4,500 m (14,764 ft).

Class 3: Elevation between 2,500 m (8,202 ft) and 3,500 m (11,483 ft).

Class 4: Elevation between 1,500 m (4,921 ft) and 2,500 m (8,202 ft), with a slope greater than 2 degrees.

Class 5: Elevation between 1,000 m (3,281 ft) and 1,500 m (4,921 ft), with a slope greater than 5 degrees and/or 300 m (984 ft) elevation range within 7 km (4.3 mi).

Class 6: Elevation between 300 m (984 ft) and 1,000 m (3,281 ft), with a 300 m (984 ft) elevation range within 7 km (4.3 mi).

Class 7: Isolated inner basins and plateaus less than 25 km2 (9.7 sq mi) in area that are completely surrounded by Class 1 to 6 mountains, but do not themselves meet criteria for Class 1 to 6 mountains.
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