Characteristics
Tall mountains reach into the colder layers of the atmosphere. They are consequently subject to glaciation, and erosion through frost action. Such processes produce the peak shape. Some mountains have glacial lakes, created by melting glaciers; for example, there are an estimated 3,000 glacial lakes in Bhutan. Mountains can be eroded and weathered, altering their characteristics over time.
Tall mountains have different climatic conditions at the top than at the base, and will thus have altitudinal zonation of ecosystems. At the highest elevations, trees cannot grow, and whatever life may be present will be of the alpine type, resembling tundra. Just below the tree line, one may find subalpine forests of needleleaf trees, which can withstand cold, dry conditions. In regions with dry climates, the tendency of mountains to have higher precipitation as well as lower temperatures also provides for varying conditions, which in turn leads to differing flora and fauna. Some plants and animals found in these zones tend to become isolated since the conditions above and below a particular zone will be inhospitable and thus constrain their movements or dispersal. On the other hand, birds, being capable of flight, may take advantage of montane habitats and migrate into a region that would otherwise not provide appropriate habitat. These isolated ecological systems, or microclimates, are known as sky islands.
Mountains are generally colder than their surrounding lowlands due to the way that the sun heats the surface of the Earth. Practically all the heat at the surface of the Earth comes from the sun, in the form of solar energy. The sun's radiation is absorbed by land and sea, whence the heat is transferred into the air. Static air is a poor conductor of heat, so conduction of heat from the ground to the atmosphere is negligible. Heat is mainly transferred into the atmosphere through convection and radiation. The air immediately adjacent to the warmed surface will rise due to its buoyancy, leading to convective circulation, in the form of thermals, within the lowest layer of the atmosphere, the troposphere. When heat radiates from the surface of the earth, it is released as long-wave radiation, which can move freely through gases composed of diatomic molecules (such as the atmosphere's oxygen and nitrogen), but is readily absorbed by triatomic molecules, such as carbon dioxide and water vapor. Since most of the atmosphere's quantity of such triatomic gases is contained within the troposphere, this portion of the atmosphere is readily heated by the earth's radiation. The tropopause forms a blanket of air keeping the surface warm. This is the Greenhouse Effect. The higher the altitude, the less of this blanket there is to keep in the heat. Thus, higher elevations, such as mountains, are colder than surrounding lowlands. Air temperature in the troposphere decreases with gains in altitude. The rate at which the temperature drops with elevation, called the environmental lapse rate, is not constant (it can fluctuate throughout the day or seasonally and also regionally), but a normal lapse rate is 5.5°C per 1,000 m (3.57°F per 1,000 ft). The temperature continues to drop with increasing altitude, until the tropopause (11,000m or 36,089 ft in the U.S. Standard Atmosphere, where it does not decrease further. However, this is higher than the highest mountaintop.
Mountains are generally less preferable for human habitation than lowlands; the weather is often harsher, and there is little level ground suitable for agriculture. The decreasing atmospheric pressure means that less oxygen is available for breathing, and there is 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 metres (11,480 ft).
Many mountains and mountain ranges throughout 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. Some mountains offer spectacular views from their summits, while others are densely wooded. Summit accessibility is affected by height, steepness, latitude, terrain, weather. Roads, ski lifts, or aerial tramways allow access. Hiking, backpacking, mountaineering, rock climbing, ice climbing, downhill skiing, and snowboarding are recreational activities enjoyed on mountains. Mountains that support heavy recreational use (especially downhill skiing) are often the locations of mountain resorts.
Mountains are made up of earth and rock materials. The outermost layer of the Earth or the Earth's crust is composed of seven primary plates. When two plates move or collide with each other, vast land areas are uplifted, forming mountains.
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