Mountain

A mountain is an elevated portion of the surface of a planet, generally with steep sides that show significant exposed bedrock. Although there are no universally accepted definitions, a mountain is usually considered higher than a hill. By definition, mountains are often thought of as being a landform which is higher than 2,000 feet (610 m). It may either have a limited summit area or be a smaller plateau with high elevation and steep sides. 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). The highest mountain on Earth from base to peak is Mauna Kea in Hawaii, which rises 9,330 m (30,610 ft) from its base at the bottom of the ocean; some scientists consider it to be the tallest on Earth.

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." Conversely, some lower mountain ranges with an average elevation between 1,500 metres (4,921 ft) and 2,000 metres (6,562 ft) may instead be called hills, such as the Sivalik Hills in the Himalayas and the Black Hills in the United States. 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 United States 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 United States 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:

Class 1: Elevation greater than 4,500 m (14,764 ft). Class 2: Elevation between 3,500 and 4,500 m (11,483 and 14,764 ft). Class 3: Elevation between 2,500 and 3,500 m (8,202 and 11,483 ft). Class 4: Elevation between 1,500 and 2,500 m (4,921 and 8,202 ft), with a slope greater than 2 degrees. Class 5: Elevation between 1,000 and 1,500 m (3,281 and 4,921 ft), with a slope greater than 5 degrees or 300 m (984 ft) elevation range within 7 km (4.3 mi). Class 6: Elevation between 300 and 1,000 m (984 and 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 mi2) in area that are completely surrounded by Class 1 to 6 mountains, but do not themselves meet criteria for Class 1 to 6 mountains. Using these definitions, mountains cover 33% of Eurasia, 19% of South America, 24% of North America, and 14% of Africa. As a whole, 24% of the Earth's land mass is mountainous.

Geology

There are three main types of mountains: volcanic, fold, and block. All three types are formed from plate tectonics: when portions of the Earth's crust move, crumple, and dive. 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. Major mountains tend to occur in long linear arcs, indicating tectonic plate boundaries and activity.

Volcanoes

Volcanoes are formed when a plate is pushed below another plate, or at a mid-ocean ridge or hotspot. 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. When the magma reaches the surface, it often builds a volcanic mountain, such as a shield volcano or a stratovolcano. Examples of volcanoes include Mount Fuji in Japan and Mount Pinatubo in the Philippines. The magma does not have to reach the surface in order to create a mountain: magma that solidifies below ground can still form dome mountains, such as Navajo Mountain in the United States.

Fold mountains

Fold mountains occur when two plates collide: shortening occurs along thrust faults and the crust is overthickened. Since the less dense continental crust "floats" on the denser mantle rocks beneath, the weight of any crustal material forced upward to form hills, plateaus or mountains must be balanced by the buoyancy force of a much greater volume forced downward into the mantle. Thus the continental crust is normally much thicker under mountains, compared to lower lying areas. Rock can 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 Balkan Mountains and the Jura Mountains are examples of fold mountains.

Block mountains

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