mountain geomorphology

mountain geomorphology Mountains have been defined in various ways. Peattie in his classic book Mountain geography, published in 1936, suggested several subjective criteria by which to define a mountain. He believed that mountains should be impressive, they should enter into the imagination of the people that live in their shadow, and that they should have individuality. He cites Mount Etna and Mount Fuji (Fujiyama) as examples. A more objective and common definition is based on altitude. In Britain and America, for example, elevations greater than 300 m above sea level (a.s.l.) are frequently considered as mountains, whereas geographers in continental Europe believe that mountains should be at least 900 m a.s.l. If the former definition is used, we would have to include such regions as the Great Plains of North America, and the plains of central Asia and eastern and southern Africa, which are above 1000 m in elevation. These regions can hardly be described as mountainous. If we use the 900 m definition, however, only the Alps, Pyrenees, Caucasus, Himalayas, and Andes would qualify to be called mountains. In stark contrast, there are regions such as parts of highland Scotland which rise from sea level to only a few hundred metres, but with their steep slopes, glacially carved valleys, snow-capped peaks, severe weather conditions, and sparse vegetation clearly have the appearance of high mountains. Clearly, there are problems with defining what constitutes a mountain. Most geomorphologists would generally agree that a mountain is a topographic high with a large relative relief (e.g. greater than 300 m); that is, the difference between the highest and lowest elevation in a region. A mountain has steep slopes (greater than 10–30°) and has great climatological, vegetational, geological, and hydrological contrast within short distances. Furthermore, most mountains are glaciated or have been glaciated during the recent past, and they contain many landforms that have been formed by ice and snow. Mountains inspire the imagination of the people who live and work in them, and they are usually very beautiful.

Although a mountain can be a single isolated feature (for example, Mount Kilimanjaro in equatorial Africa), most mountains occur as a succession of mountains or narrowly spaced mountain ridges: a mountain range. The Transverse Ranges of southern California are a classic example. A mountain range may form part of a mountain chain which persists for hundred or thousands of miles. The Mediterranean mountain chain of southern Europe is a good example. A group of mountains of irregular shape may also exist; such a group is known as a mountain mass. The Tibetan Plateau and bordering mountains constitute the world's largest mountain mass. When mountain ranges, chains, and masses span continents they are called mountain systems: the Alpine–Himalayan system and the Andes–Western Cordillera of North America are the best examples.

Geomorphologically, mountains are very complex and varied. Geomorphologists require a knowledge of geology, climatology, biology, hydrology, and anthropology to understand the dynamics of the Earth-surface processes and the formation of landforms in mountain regions. We shall consider each of these in turn.

The underlying geology and tectonic setting is the ultimate control on the creation of positive relief and the formation of mountains; as mountains form they are subsequently shaped by the erosive power of Earth-surface processes. Most young mountains (less than 200 million years old) are globally distributed along zones that are generally a reflection of the tectonic setting. They occur most commonly along the collision zones between tectonic plates. These collision zones are known as orogenic belts, and the mountains are referred to as orogens. Mountain belts produced by the collision of oceanic and continental plates are known as Cordilleran-type orogenic belts. These include the Western Cordillera and Andean mountain belt, which formed and are still forming as a consequence of the oceanic Pacific plate colliding with, and being subducted under, the continental plates of North and South America, respectively. As oceans close, the oceanic–continental collision may progress to become an interaction between two continental masses. This results in mountain belts of the Alpine type. These include the Alpine–Himalayan belt, where the African and Indian continental plates are colliding with the Eurasian continental plate. Where an oceanic plate is being subducted under another oceanic plate, as for example, in the western Pacific on the margins of east Asia, volcanic island arcs are forming. Mount Fuji is a classic example of such a mountain that has formed within a volcanic island arc. Isolated mountain masses also occur on hot-spots, where anomalously hot magma rising from the Earth's interior towards its surface causes volcanic activity that produces high volcanic mountains. Mount Kilimanjaro is a classic example, rising to 5895 m from a high plain of a little over 1000 m above sea level. Other mountains may be uplifted along zones of compression in major strike-slip fault systems. The Transverse Ranges of southern California along the San Andreas fault system and the Gobi Altai Mountains in Mongolia along the Gobi-Tien Shan fault system are examples of these transpressive-type mountains.

Young mountains are seismically active and may also be volcanically active. These active tectonic processes may also influence the geomorphology of the mountain. Earthquakes, for example, may initiate landslides, and volcanoes extrude lava or may collapse in on themselves, or fail by landsliding, to produce new landforms almost instantaneously.

Other mountain ranges are remnants of former orogenic belts. These are usually more than 200 million years old. They include mountain ranges such as the Appalachians of the United States, the Caledonides of Ireland, Britain, and Scandinavia, and the Urals in Russia.

Figure 1 shows a model of a composite orogenic belt that can be used as a mechanism for organizing observations of real mountain belts and comparing them with others. This model assumes that an orogen is the final result of the collision of two continents. It is most similar to the ancient Appalachian–Caledonide that stretches along eastern North America, Scotland, and Norway, and the younger Alpine– Himalayan belt. The orogen is characterized by:(1) an outer foredeep or foreland basin;(2) a foreland fold-and-thrust belt;(3) a crystalline core zone that includes: sedimentary rocks and their basement; volcanic and igneous rocks and associated sediments; metamorphosed ocean crust (ophiolites); gneissic terranes with abundant ultramafic bodies; and granitic batholiths;(4) rectilinear (high-angle) fault zones.The tectonics and geology described above essentially control the distribution of elevations within mountains. This, in turn, controls the development of drainage patterns, and it influences climate, which, in turn, controls vegetation and soil development. The altitudinal distribution of Earth-surface processes is controlled by all these factors. Figure 2 illustrates the linkage between these factors by showing the altitudinal distribution of environments and processes in a typical high mountain.

Mountains control climate on global to microclimatic scales. On a global scale, the highest and largest mountain chains may block or deflect global atmospheric systems. The Alpine–Himalayan chain, for example, deflects the mid-latitude westerly jetstreams, and the Andes block maritime air masses from reaching the easternmost ranges, which creates the hyperarid mountain environments of the Atacama Desert. Mountains may also help to drive monsoonal systems by thermally induced pressure gradients. The South Asian summer monsoon is produced in this way, air masses being forced by the heating effect of the Tibetan Plateau. On smaller scales, climatic contrasts exist because of altitudinal differences. Temperature decreases with increasing altitude, by approximately 6.5 °C km⁻¹. Pressure also decreases with altitude, which further influences meteorological processes. The decrease in temperature with elevation allows permanent snow fields and glaciers to exist in high mountains. The altitude at which these forms is also a function of latitude; for example, the snow line at latitude 60° S is at sea level and rises to a maximum altitude of 6000 m at latitude 23° S. The size of the mountain system also affects the snow-line altitude. Generally, the larger the mountain mass, the higher the snow line. Precipitation also varies altitudinally, becoming progressively greater with rising altitude until the rising air masses have lost their moisture, which then results in a decrease in precipitation with altitude. On even smaller scales, slope aspect may affect the amount of solar radiation a slope receives; for example, south-facing slopes are warmer than north-facing slopes. On north-facing slopes, this may increase the persistence of snow fields into the spring or summer, and it allows glaciers to become larger than on the equivalent south-facing slopes. Proximity to a lake or glacier may have a cooling effect on, or may increase the windiness of, a particular valley. Furthermore, the world's climate has changed drastically over geological times, altering the distribution of climatic belts both altitudinally and geographically through all the world's mountains. The most dramatic climate changes have occurred during the past 2 million years when the world entered the present ice age. During this time the world's climate has fluctuated rapidly from glacial times, when ice sheets were extensive and sea ice and permafrost were widespread in middle and high latitudes, to interglacial periods, as at the present time, when ice sheets, sea ice, and permafrost are much less extensive.

All these climatic contrasts, on their various scales, help to control the nature and distribution of Earth-surface processes throughout mountainous regions. They also control the distribution of animal and plant life, soils, and even human occupancy. These, in turn, affect the distribution of Earth-surface processes that ultimately help to shape mountain landforms. Given this framework, it is easy to appreciate the complexity of mountain landscapes and the processes that operate in them and shape their landforms. Nevertheless, it is possible to make some generalizations about the nature of geomorphological processes and the landforms that characterize mountain environments. Glacial, periglacial, fluvial, and mass-movement processes dominate in mountain environments. Other processes that are, however, important on local scales include aeolian (wind), lacustrine (lake), and pedogenic (soil) processes.

In the highest mountains, where average annual temperatures are well below 0 °C, permanent snow fields and glaciers exist. Glaciers may be of many different types, ranging from ice caps (e.g. the Agassiz ice cap on Ellesmere Island in the Canadian Arctic) and long valley glaciers (the Khumbu glacier flowing from the slopes of Everest; Fig. 3), to cirque glaciers that fill small basins in high mountain slopes (e.g. Cirque Mountain in Labrador, Canada). There is much debate about the amount of erosion occurring as a result of glacial activity in mountainous regions. However, given the steep slopes, and the generally high snow falls and rapid ice formation, as well as the high ablation rates, glacial movement in mountainous regions is fast and erosion rates are probably among the highest of all glacial environments. Mountain glaciers also tend to be major transporters of debris and sediments. The sediment falls on the glacier surface by avalanche processes and is transported as the glacier flows, eventually to be deposited when the sediment reaches the edge of the glacier or when the glacier melts. These sediments form impressive landforms called moraines (Fig. 3). In many mountain areas, multiple series of moraines are present, recording past changes in climate, from glacial to interglacial times.

Periglacial processes, conditions, climates, and landforms (those associated with the immediate margins of former and existing glaciers and ice sheets) are influenced by the low temperature of the ice. Frost action is an important factor in a periglacial climate beyond the periphery of glaciers. Periglacial processes in high mountain regions are dominated by cold-process weathering (cryogenic weathering), which leads to the disintegration of rock as a result of pressures exerted within cracks in the rock as interstitial water (water inside the rock) freezes with falling atmospheric temperatures. This leads to the production of extensive areas of broken rock and large scree slopes. In many high mountains regions, permafrost may be present. This is ground that has a temperature below 0 °C for 2 or more years. The upper layer of permafrost (0.1–1 m thick), the active layer, warms in summer and cools in winter. This allows water to change to freeze and thaw seasonally, creating stresses within the active layer that may lead to sorting of stones of different sizes within the substrate, to the formation of ‘patterned ground’. Gelifluction (the slow movement of debris downslope) is associated with frost action and is particularly common in mountains where permafrost exists.

Mass movement and fluvial processes dominate in mountain regions that are not high enough for glacial and periglacial processes to operate, or at lower altitudes of high mountains. Many such areas are forested, and human activity is generally more prevalent. In these regions, anthropogenic processes are most active in shaping the landscapes and influencing natural Earth-surface processes. There is much current concern about the environmental impact that human activity has in many mountain regions, and there is pressure for active conservation in threatened areas.

Mass movement is extremely prevalent in mountain environments because of the very steep and long slopes. It can take many forms, including free fall as rockfalls and avalanches, sliding down discrete surfaces as rockslides or mudslides or flowing as debris flows, and the extremely rapid movements (faster than several hundred km h⁻¹) known as flowslides or sturzstroms. Many geomorphologists believe that low-frequency, mass-movement events of large magnitude are probably among the most important factors in shaping mountain landscapes. Other geomorphologists, however, believe that the higher-frequency, smaller processes, such as weathering and fluvial action, are more important in shaping mountains. It is also likely that mass movement is much more common during periods when glaciers are retreating; large moraines and steep valley sides are then unstable because they are no longer supported by the glacier. The landscape then readjusts itself, by mass movement and erosional processes, to a new, geomorphologically more stable, environment.

Many of the world's most important rivers drain from high mountain regions. For example, the Ganges and Indus rivers from the Himalayas, the Huanghe and Yangste from Tibet, and the Amazon from the Andes. Not only do these rivers support vast populations, but they erode the mountains intensely and carry vast amounts of sediment from the mountains to their forelands. Rivers also help to induce other geomorphological processes. For example, undercutting of river banks may induce slope failure. Major floods are also common along many mountain rivers. These may be induced by heavy rainfalls, particularly in monsoon climates, or when rivers have been blocked by landslides or advancing glaciers, producing lakes that have the potential to be breached. When these natural dams burst, the result is often catastrophic. The floods not only kill people and livestock, destroy property and farmland; they also destroy and create new landforms. Many geomorphologists believe that large flood events are are among the most important formative (landform-forming) processes in mountainous environments.

Tectonics, climate, vegetation, hydrology, and geomorphological processes are intricately linked, and understanding the relative importance of each set of factors in the evolution of mountain landscapes and the dynamics of mountain processes is one of the chief goals of most mountain geomorphologists. However, only in relatively recent years have detailed geomorphological studies of individual catchments in mountains begun to elucidate the relative roles of each of these sets of factors. Unfortunately, it is difficult and scientifically unsafe to extrapolate these studies to other areas because of the complexity of mountain environments: no two mountain environments are identical or have the same geomorphological history. Nevertheless, geomorphologists are beginning to understand some of the dynamics of mountain environments.

Lewis A. Owen

Bibliography

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