A mountain is a large landform that stretches above the surrounding land in a limited area, usually in the form of a peak. A mountain is generally steeper than a hill. Mountains are formed through tectonic forces or volcanism. These forces can locally raise the surface of the earth. Mountains erode slowly through the action of rivers, weather conditions, and glaciers. A few mountains are isolated summits, but most occur in huge mountain ranges.
High elevations on mountains produce colder climates than at sea level. 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 and recreation, such as mountain climbing.
The highest mountain on Earth is Mount Everest in the Himalayas of Asia, whose summit is above mean sea level. The highest known mountain on any planet in the Solar System is Olympus Mons on Mars at .
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. The highest point in San Francisco, California, is called Mount Davidson, notwithstanding its height of , which makes it twenty feet short of the minimum for a mountain by American designations. Similarly, Mount Scott outside Lawton, Oklahoma is only from its base to its highest point. Whittow's ''Dictionary of Physical Geography'' states ''"Some authorities regard eminences above 600 m (2,000 ft) as mountains, those below being referred to as hills."''
In the United Kingdom and the Irish Republic, a mountain is usually defined as any summit at least 2,000 feet (or 610 metres) high,〔(【引用サイトリンク】title=A Mountain is a Mountain – isn't it? )〕〔(【引用サイトリンク】title=mountain )〕 whilst the official United Kingdom government's definition of a mountain, for the purposes of access, is a summit of 600 metres or higher.〔(【引用サイトリンク】title=What is a "Mountain"? Mynydd Graig Goch and all that... )〕 In addition, some definitions also include a topographical prominence requirement, typically . For a while, the US defined a mountain as being or taller. Any similar landform lower than this height was considered a hill. However, today, the United States Geological Survey (USGS) concludes that these terms do not have technical definitions in the US.〔(【引用サイトリンク】title=What is the difference between lake and pond; mountain and hill; or river and creek? )〕
The UN Environmental Programme's definition of "mountainous environment" includes any of the following:
*Elevation of at least ;
*Elevation of at least , with a slope greater than 2 degrees;
*Elevation of at least , with a slope greater than 5 degrees;
*Elevation of at least , with a elevation range within .
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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』