Physical geology of principal mountain systems
- The great extent of Earth’s two newest continental orogens is noted and attributed to recent and continuing tectonic activity.
- American Cenozoic orogens include the type-cordilleran Andes, Rocky Mountain and Coast Range subsystems. They began to form in pre-Cenozoic times, at and behind the B-subduction zone of Pacific and adjacent plates, at the leading edge of the American continental plates.
- The North and South American continents contain not one but many cordilleran elements and the relative simplicity of the Andes compression contrasts with more complex ranges and displaced terranes in North America, including large areas of crustal extension related to continental override of ocean plate.
- Many parts of the cordilleran system are still tectonically active, promote vigorous geomorphic processes and are dotted throughout their length by active stratovolcanoes, earthquakes, alpine glaciers and small icefields.
- They barrier zonal climatic and oceanic circulation and are high enough to stimulate migrating waves in mid–upper troposphere circulation.
- Quasi-continuous Eurasian orogens extend from the Pyrenees through the Alps to the Himalayas and beyond. They also comprise a complex assemblage of terranes and ages but generally represent subduction and intercontinental suturing, marking closure of the Tethys Sea.
- More complex Africa–Europe collision began earlier although it is still incomplete but the Karakoram–Himalaya–Tibetan Plateau region is the most extensively active orogen, uplifted where India not only collided with but continues to indent Asia.
- Mediterranean B-subduction feeds stratovolcanoes on Europe’s southern fringe but they are absent from the A-subduction of India–Asia intercontinental collision. The Tibetan plateau’s youth leaves it poorly connected to intense denudation systems of neighbouring orogens.
- The orogens inhibit meridional weather circulation but the Tibetan plateau in particular has an impact on global weather. Its very recent Pliocene–Pleistocene uplift is instrumental in the creation and character of the Asian monsoon.
- New Zealand occupies a fascinating microplate position with east-to-west subduction and stratovolcanoes in North Island, contrasting with west-to-east subduction and orogenesis of the Southern Alps in South Island, which drives intense denudation there.
Mountain meteorology and climate
- Mountain meteorology shows a systematic decline in temperature with altitude and a tendency towards higher humidity, cloud cover and precipitation which can obscure a complex mosaic of rapid seasonal, spatial and synoptic variation.
- The presence of a landsurface capable of exchanging radiation, heat and moisture with the free atmosphere at high elevations complicates standard atmospheric changes, in temperature etc., with height.
- On balance, mountains may act as high-altitude heat sources and they orographically enhance precipitation, not merely in terms of amount but in terms of intensity. They also encourage azonal snow and ice cover and its meteorologically sensitive surfaces where mountains project above regional potential snowlines.
- Mountains which penetrate the mid–upper troposphere generate major atmospheric disturbances which influence global climate. On a regional scale, they create airflow perturbations through thermal and mechanical effects, which set up a variety of mountain winds, including lee waves, rotors and fohn disturbances and anabat–katabat and cold air drainage circulations.
- The mountain climate can generate marked local variations and departures from altitudinal norms through intense differences in shade, albedo, moisture, materials and other components.
- Mountain geoecology – climate, geomorphology, pedology and ecology – is more integrated than in most terrestrial ecosystems, owing to the high variability over short distances and mutual sensitivity to instability and environmental change. • Within the short vertical distances and space of the mountains there can exist montane forest, alpine tundra and their ecotones and a cryonival system of permanent snow, glaciers and permafrost.
- Montane forest and its timberline are generally defined by summer temperatures but drought may be the limiting factor in rain-shadow areas. Occasionally, forest girdle and cloud forest above the regional timberline reflect the importance of mountain surfaces as a moisture source and moist topographically induced updraughts respectively.
- Steep and high coastal orogens such as the equatorial Andes can show a full range of vegetation belts from tropical forest to arctic–alpine in five or six vertical kilometres.
- Modern timberlines and Pleistocene snowlines are separated vertically by < 1 km in tropical and arid mountains but are virtually identical elsewhere. Modern timberlines and snowlines constrain the arctic–alpine zone, which is therefore of Holocene age. Its marked edaphic and microclimatic variations makes for a fluctuating forest tundra ecotone.
- Alpine tundra, with dwarf shrubs, herbs, mosses and lichens, shows strong taxonomic similarities with arctic tundra, with similar adaptations in life forms, growth and reproductive behaviour. However, it is noted that patterns of radiation receipt and seasonality range from polar to equatorial compared with the far more restricted patterns in the Arctic proper.
- Highly adapted plant forms, which support a similar food web, and their distributions suggest that mountains act as plant refugia during Quaternary cold stages and arctic–alpine flora may have evolved from low-latitude communities during poleward tectonic drift since the Cretaceous.
The alpine landsystem
- The alpine landsystem is a model which integrates glacial, cryonival, slope and fluvial elements of the landscapes of high mountains with mountain climates and ecosystems.
- The landsystem is partially isolated by the timberline and glacially excavated lake basins, which buffer sediment and water transfer to fluvial systems below. It may also be tectonically active and, for all these reasons, is one of Earth’s most geomorphically active environments.
- The landsystem is best developed in areas of intense Pleistocene glaciation, where excavated deep troughs with oversteepened rock walls allow large spreads of unstable debris (talus or scree).
- The continuing presence of glaciers in some mountains ‘insulates’ subglacial surfaces from other alpine processes but promotes them at their margins and intensifies the mosaic of microclimates and materials.
- A cryonival belt of snow and ice occurs in all alpine mountains irrespective of a permanent snowline and/or glaciers. Nivation or snow-bed weathering and mass wasting may operate alongside frost weathering, cryoturbation and solifluction above sporadic permafrost.
- High potential energy, enhanced by continuing uplift in some mountains, permits the reworking of landforms and sediments of the alpine landsystem. High rates and variable styles of mass wasting in small areas are characteristic of the system.
- In particular, this promotes the progressive large-scale destruction of glaciated rock walls and attendant debris slope landforms, incorporated in a model alpine landsystem.
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