A mountain is a landform that extends above the surrounding terrain in a limited area. A mountain is generally steeper than a hill, but there is no universally accepted standard definition for the height of a mountain or a hill although a mountain usually has an identifiable summit. Mountains cover 54% 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. 10% of people live in mountainous regions. Most of the world's 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 things associated with them.
The height of a mountain is measured as the elevation of its summit above mean sea level. The Himalayas average 5 km above sea level, while the Andes average 4 km. The highest mountain on land is Everest, 8,848 metres (29,030 ft) in the Himalayas.
Other definitions of height are possible. The peak that is farthest from the center of the Earth is Chimborazo in Ecuador. At 6,267 meters (20,560 ft) above sea level it is not even the tallest peak in the Andes, but because Chimborazo is very close to the equator and the Earth bulges at the equator, it is 2,150 meters (7,100 ft) further away from the Earth's center than Everest. The peak that rises farthest from its base is Mauna Kea on Hawaii, whose peak is 10,200 metres (33,500 ft) above its base on the floor of the Pacific Ocean.
Even though Everest is the highest mountain on Earth today, there have been much taller mountains in the past. During the Precambrian era, the Canadian Shield once had mountains 12,000 metres (40,000 ft) in height that are now eroded down into rolling hills. These formed by the collision of tectonic plates much like the Himalaya and the Rocky Mountains.
At 26 kilometres (85,000 ft) (Fraknoi et al., 2004), the tallest known mountain in the solar system is Olympus Mons, located on Mars and is an ancient volcano. Volcanoes have been known to erupt on other planets and moons in our solar system in our life-times (volcanoes on Venus for example, constantly erupt) and some of them erupt ice instead of lava. Several years ago, the Hale telescope recorded the first known live images of a volcano erupting on a moon in our solar system.
High mountains, and mountains located closer to the Earth's poles, have elevations that exist in colder layers of the atmosphere. They are consequently often subject to glaciation and erosion through frost action. Such processes produce the popularly recognizable mountain peak shape. Some of these mountains have glacial lakes, created by melting glaciers; for example, there are an estimated 3,000 glacial lakes 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. The flora and fauna found in these zones tend to become isolated since the conditions above and below a particular zone will be inhospitable to those organisms. These isolated ecological systems are known as sky islands and/or microclimates. Tree forests are forests on mountain sides which attract moisture from the trees, creating a unique ecosystem. Very tall mountains may be covered in ice or snow.
Mountains are colder than lower ground, because the Sun heats Earth from the ground up. The Sun's radiation travels though the atmosphere to the ground, where Earth absorbs the heat. Air closest to the Earth's surface is, in general, warmest (see lapse rate for details). Air as high as a mountain is poorly warmed and, therefore, cold. Air temperature normally drops 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit) for each 300 meters (1000 feet) of altitude.
Mountains are generally less preferable for human habitation than lowlands; the weather is often harsher, and there is little level ground suitable for agriculture. 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 meters (11,483 feet).
A number of mountains and mountain ranges of the world have been left in their natural state, and are today primarily used for recreation, while others are used for logging, mining, grazing, or see little use of any sort at all. Some mountains offer spectacular views from their summits, while others are densely wooded. Summit accessibility ranges from mountain to mountain; height, steepness, latitude, terrain, weather, and the presence or lack thereof of roads, lifts, or tramways are all factors that affect accessibility. Hiking, backpacking, mountaineering, rock climbing, ice climbing, downhill skiing, and snowboarding are recreational activities typically enjoyed on mountains. Mountains that support heavy recreational use (especially downhill skiing) are often the locations of mountain resorts.
If you climb from base to peak of a big mountain, you will pass through a number of different types of plant community. The vegetation on the lower slopes largely depends on which climate zone the mountain is in. The foothills may be covered in broadleaved forests. On the upper slopes, needleleaf trees (coniferous trees) like spruce and pines. As you climb higher it gets colder and colder and the trees eventually thin out and disappear.
The highest parts of the mountain support only sparse grasses and low-growing Alpine flowers which can withstand the harsh conditions. If the mountain is high enough even this vegetation disappears and the peak is bare and rocky and perhaps covered in snow and ice.
Mountains make up about one-fifth of the world's landscape,
The world's highest peak on land is Mount Everest in the Himalayas. It is 8,850.1728 m ( 29,036 ft ) tall.
The world's highest mountain, from its base on the ocean floor, is Mauna Kea, on Hawaii. It is 10,203 m ( 33,474 ft ) high but only 4,205 m ( 13,796 ft ) is above sea level.
Ben Nevis is also the highest mountain in Great Britain.
The tallest known mountain in the solar system is Olympus Mons, located on Mars.
There are mountains under the surface of the sea!
Mountains occur more often in oceans than on land; some islands are the peaks of mountains coming out of the water.
About 80 per cent of our planet's fresh water originates in the mountains.
All mountain ecosystems have one major characteristic in common - rapid changes in altitude, climate, soil, and vegetation over very short distances.
Plants that may be found on mountains include conifers, oak, chestnut, maple, junipers, stonecrops, campions, mosses, ferns and climbers.
The highest 14 mountains in the world are all found in the Himalayas
In some mountainous areas the rivers are permanently frozen. These are called glaciers.
The Alps are the most densely populated mountain area in the world.