Physical Features of Mountains

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PHYSICAL FEATURES OF MOUNTAINS

Physically, existing mountains have only slope and elevation in common, and the fact that all will ultimately be eroded into insignificance, while others will be created. 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.

Contents

Key Features of Mountains

Local variation

There is immense variation in the nature of mountain environments despite their common basic physical conditions of elevation and slope.

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.

Temperature

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.


Reference

  1. Mountain Watch Defining Mountain Regions [1]

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