Physical Features of Mountains
PHYSICAL FEATURES OF MOUNTAINS
They may be formed by uplift of extensive blocks of land around major faultlines, or by folding of rock strata, both of which result from continental movements, or by volcanic activity often associated with both faulting and folding.
Any given segment of land may well have been affected by all three processes over the course of Earth history, and so, with the exception of volcanic cones, mountain ranges will often be composed of a variety of igneous, sedimentary and metamorphic rock types. Accordingly, there is wide variation in features that depend on rock type, such as erosion potential, slope stability and soil.
Mountains vary widely in age. One of the better known episodes of ancient folding affected rocks now within northwest Europe around 400 million years ago; geological evidence for this early mountain-building has been largely obscured by later earth movements and the levelling effects of erosion. Much of the folding involved in uplift of the Alpine-Himalayan chains took place around 35 million years ago, and these tend to retain the sharp peaks and ridges typical of younger mountain ranges.
The Earth’s very youngest peaks are volcanic in origin. Paricutin in Mexico, for example, had built a cinder cone about 500 m high within a year of its eruption in 1943 (total elevation about 2 770 m).
With the present configuration of continents, more than two-thirds of the world land surface is located in the northern hemisphere, and the area of land north of the Tropic of Cancer slightly exceeds that in the rest of the world put together. This in part explains why the northern temperate belt contains a far greater mountain area than any other zone.
The Antarctic region comes a distant second in total mountain area, but owing to the immense extent and thickness of its icecap, it has the highest proportion of overall area defined as mountainous and the greatest surface area above 2500 m.
Dividing the world’s land by continental groups, rather than by latitude, shows unsurprisingly that the enormous Eurasian landmass has by far the greatest mountain area Eurasia also has the most extensive inhabited land area above 2 500 m elevation, in the Tibet (Xizang) Plateau and adjacent ranges.
All of the world’s mountains above 7,000 m in height are in Asia, and all the 14 peaks above 8,000 m are situated in the Greater Himalaya range extending along the southern rim of the Tibet Plateau.
After Eurasia, and excluding Antarctica, South America has the second most extensive area of high elevation land, formed by the mountains and basins of the Central Andes.
The world’s highest individual peak outside Asia is Aconcagua, which reaches an elevation of around 6,959 m in the southern Andes.
A major part of Greenland is above 2,500 m, and this region resembles Antarctica in that much of the surface is composed of a deep icesheet; in both cases most of the very small human population is restricted to the coast.
 Key Features of Mountains also see Landform Gallery
 Local variation
Much of this variation arises from differences in temperature and precipitation regimes associated with position on the Earth’s surface – whether at high or low latitudes, whether deep within a continental landmass or subject to oceanic influence along the margin of a landmass. Mountains guide approaching air masses upward, and as temperature falls, the air is able to hold less water vapour, leading to increased rainfall on the windward side and a reduction on the lee side (the ‘rain shadow’ effect). More locally, conditions vary greatly according to aspect of slope (northfacing or southfacing), soil and local topography.
 High energy, high erosion
Mountains are typically high energy environments, subject to strong winds, frequent freeze-thaw cycles at higher elevations, accumulation and melting of snow masses in some parts and heavy rainfall in others.
Collectively, these agents speed up the process of weathering, while altitude and slope hasten the loss of erosional debris. Slope, thin soils, and the general absence of a permanently frozen subsoil, mean that water is similarly lost rapidly downslope, and mountain plants are often well adapted to drought conditions. The need to reduce erosion while improving soil and water conditions for crop plants is a key factor behind the widespread adoption of terracing by mountain agriculturalists. If wind velocity doubles, the force exerted increases fourfold; this has a direct physical impact on humans and other species (leading to the prostrate or cushion-like growth form of many high mountain plants), as well as a desiccating effect that adds to the risk of water stress.
Air temperature on average decreases by about 6.5° C for every 1 000 m increase in altitude; in mid latitudes this is equivalent to moving poleward about 800 km. The dry dust-free air at altitude retains little heat energy, leading to marked extremes of temperature between day and night.
In seasonal climates, daytime temperatures can rise sharply in sunlit mountain areas. In tropical climates, the sun is high overhead throughout the season, so that tropical mountains tend to have high temperatures and sometimes high rainfall throughout the year. Temperature is one factor determining the natural upper limit of tree growth (the ‘treeline’), which varies locally and with latitude, from around 5 000 m in parts of the tropics to near sea level at high latitudes.
Air pressure and oxygen availability As a consequence of decreasing air pressure, the partial pressure of oxygen falls with increasing altitude (partial pressure is the constant 21 per cent concentration of oxygen multiplied by the barometric pressure). At 1 500 m the partial pressure of oxygen is about 84 per cent of the value at sea level, falling to 75 per cent at 2500 m and 63 per cent at 3500 m (with minor variation with latitude and season).
The consequence of this for humans and other animals is that with increasing altitude, less oxygen is obtained per volume of air inspired, and fewer oxygen molecules diffuse into the bloodstream to maintain cell function and support physical activity.
Mountaineers and other temporary residents at high altitude can achieve limited acclimatization to oxygen shortage (hypoxia) over a period of days or weeks. Populations that live permanently at high altitude are subject to life-long hypoxic stress, and have in some instances evolved the metabolic capacity to maintain physical activity. Nevertheless, in human populations hypoxia has demonstrable adverse effects on birthweight and reproductive success.
Mountains occur on all continents, in all latitude zones, and within all the world’s principal biome types – from hyperarid hot desert and tropical moist forest to arid polar icecaps – and support a correspondingly wide variety of ecosystems.
Mountain ecosystems tend to be important for biological diversity, particularly in the tropics and warmer temperate latitudes.
Isolated mountain blocks are often rich in endemics.
Polar mountains may be entirely without vegetation; at other high latitude sites, mountains may bear only sparse tundra-like scrub. On low elevation mountains at lower latitudes, vegetation may be broadly similar to that of surrounding lowlands, often with coniferous or broadleaf forest. With increasing elevation, the effects of temperature, precipitation and wind combine to induce an altitude-related zoning in vegetation. As elevation increases, the availability of moisture – as rain or condensation from cloud or fog – tends to increase (up to a level that varies with latitude and between continents).
In arid regions such as the Horn of Africa, this can allow tree growth near the top of mid elevation mountains that emerge from treeless semi-desert plains. In more humid regions, shortstature epiphyte-rich evergreen forest (cloud forest) may flourish above more seasonal forest types.
Ultimately, temperature and moisture availability decrease, and windspeed increases, to a point where tree growth cannot be sustained.
Above this point, low herbaceous vegetation, often including tussock grassland, takes over, to be succeeded by largely bare rock or snow. Such montane grasslands are often important for livestock grazing, as exemplified by the páramo zone of the northern Andes. This is an extensive tract of grass and shrub, lying between the upper limit of cultivation (around 3,250 m) and the high summits (> 4,000 m).
Distinctive giant forms of groundsel and lobelia (whose widespread relatives are small herbaceous plants) occur above the treeline on high mountains in tropical Africa, while giant bromeliads and large composites occur on the Andean páramo. In many hill and mountain regions the present treeline has been pushed downslope from its potential level by burning and agricultural activity.
The vegetation zones encountered with increasing elevation on an idealized tropical mountain tend to resemble the biome types found with increasing latitude. Vegetation types similar to those that succeed one another through more than 80° of latitude and 3 000 km distance – tropical moist forest, deciduous forest, coniferous forest, shrub and grassland, or ice – may be compressed onto the slopes of a mountain perhaps 5,000 m high.
Despite superficial resemblance in vegetation, there are fundamental differences between elevational gradients in the tropics and latitudinal gradients. In tropical regions, the sun is high overhead throughout the year, whereas seasonality increases with increasing latitude. At high arctic latitudes, permafrost is common and there is little shortage of water during the short growing season, whereas alpine environments are less seasonal, with high light levels and daytime warming through much of the year.
 Also See
- Mountains and Mountain Forests Global Statistical Summary
- Rain shadow
- Defining Mountain Regions
- Sea level
- Mountain Glossary
- Mountain Watch Defining Mountain Regions