High mountain dominion.
Howard Phillips Lovecraft
At the Mountains of Madness (1931)
1 Above it all
1. Upwards, towards the poles
1.1 The power of altitude
The areas of high mountain and high plateaux occupy a little more than one third of the world's land surface. These areas house only a small part of the world's population, but many more people depend in one way or another on their resources. These figures show the great importance of the mountain regions but undervalue their importance if other aspects are also taken into account. Thus, in many countries, and especially in more developed ones, wildlife reserves, national parks, and biosphere reserves tend to be located almost exclusively in steep and relatively inaccessible areas, where there are still landscapes relatively unaltered by humans. At the same time, mountains are rich in water, energy and forestry resources, and are thus very important in planning irrigation areas and the design of national energy plans.
The concept of mountain
For these reasons, and also because of the precarious survival of the forms of social organizations and cultures that have had to cope with the very harsh environmental conditions of mountain areas, mountain areas have been the subject of many geographical, geological, biological, and anthropological studies. Yet it is not possible to find a fully satisfactory definition of what a mountain is, because mountains vary greatly and can be analyzed from many perspectives.
True mountains should be distinguished from hilly lowland areas (because these are not so high and thus lack differentiated layers of vegetation), and also from highlands or platforms (because these have gentle slopes and may even be flat, despite their altitude). A good definition of a mountain should also include the special forms of land use, the complex adaptations of human groups to the heterogeneity, topography and the strong seasonal climatic contrasts found in mountain areas. A mountain is a mountain because its relief rises above the surrounding plain (at least 984-1,640 ft [300-500 m] higher), and it is high enough to have more than one vegetation layer and a wide range of morphoclimatic processes, with steep slopes (more than 20[degrees]) and the occasional presence of escarpments on these slopes, and soil use based on strong social cohesion, dominated by stockraising, because agricultural use is difficult for topographical and climatic reasons and is generally restricted to the middle and the lower slopes.
Mountains are the ultimate in three-dimensional spaces, the most complex sites on Earth's surface, as the landscape not only varies horizontally (for example, with latitude or distance from the sea) but also, and more importantly, vertically. High elevations and steep slopes are the most typical features of the mountain, and other equally important features are derived from them. Mountains thus show once more how, in nature, some factors have an overwhelming influence--in this case the relief, which moderates other abiotic factors, such as climate and hydrography, as well as the biotic complexes (vegetation, fauna, human population, and agricultural and livestock usages). Greater height, which leads to a decrease in temperature, makes it possible to distinguish different vegetation types, soil uses, and climatomorphological types. Climatomorphological criteria have consistently been used to separate the high mountain from the mid and lower mountain, as it is considered that geomorphological forms and processes reflect and bring together certain dynamic aspects that other criteria do not emphasise sufficiently.
One of the most important ecogeological problems is perhaps defining the vertical zonation observed in mountains and the way their organization changes with latitude and longitude. Several zones can be distinguished in the world's high mountains: a nival or snow zone, at the top, with a long snowfall period, snow patches that persist in the middle of summer, the occasional presence of glaciers, almost vertical slopes, and the domination by areas producing large quantities of sediments (the heads of avalanches), very sharp divides, and scattered vegetation adapted to the extreme climatic and geomorphological conditions. Below this layer is the Alpine subnival zone, which is divided into an upper layer, with screes, scattered pioneer vegetation, and structural soils, and a lower layer, with dense pastures and controlled solifluction. Below this layer is the subalpine zone, with scattered trees and shrubs, stunted and deformed by the harsh climatic conditions, especially wind and snow, and often remarkably altered by the development of stockraising that has led to the elimination of the forest areas at the highest elevations, so as to increase pastureland for summer grazing. Below the subalpine zone is the montane zone or forest zone.
Yet mountains are in reality more complex, and altitude alone does not describe the mountain landscape. There are enormous differences between mountains due to topographic and climatic factors, as well as on a single mountain. The materials they are formed of, their history, the climatic changes they have experienced, the organisms living there and the history of their colonization, and the vagaries of human occupation and resource exploitation, cannot be left out.
1.2 Geological and geomorphological complexity
The shape of a mountain is the result of its underlying geological history, but its actual appearance often depends much more on recent episodes related to phenomena of a superficial nature. The present-day topography of mountain systems is the result of both orogeny and the effects of erosion.
Large mountain ranges are the consequence of the movements of Earth's crust related to the movements of the internal magma. Thus, for example, it is known that the westward movement of the North American and South American plates caused them to collide violently with the Pacific plate, and this was the cause of the rising up of the mountain chains that run along the Americas from north to south. The opening of the Atlantic Ocean is another consequence of this westward movement, which created such great stresses that it left a chain of volcanoes running down the middle of the Atlantic, some of them still very active, such as in Iceland and the Canary Islands. Likewise, the Indian plate, which was initially joined to the southern hemisphere continent of Gondwana, started moving northwards until it collided violently with the southern part of Asia 60 million years ago, forming the Himalayas. The Pyrenees and the Baetic mountains are also the result of movement of the earth's crust, and were caused by the pressure of the African plate moving northwards (see volume 1, page 46). Because they originated as a result of movements on a planetary scale, the large mountain ranges are truly immense. For example, the Himalayas run for a length of 1,553 mi (2,500 km) and occupy an area of 193,050 [mi.sup.2] (500,000 [km.sup.2]), and there are also prolongations that extend into southeastern Asia. The Andes run for a length of 4,660 mi (7,500 km) from the southern tip of South America to the Caribbean, covering an area of almost 772,201 mi (2,000,000 [km.sup.2]). Other smaller, but still large, mountains ranges include the Atlas Range in North Africa, the Alps in Europe, the Carpathians, the Caucasus and the Australian Alps. These mountains, and other smaller ones, are the result of several orogenic (mountain formation) cycles that have superimposed very different forms and lithologies, like a sort of palimpsest that geologists try to decipher.
Yet the geological complexity of the large mountain ranges depends on the details of both their formation and their later development. When the mountains were formed, immense tectonic movements folded sediments and moved large fragments for tens of kilometers, forming faults and folds, that is, rock strata with origins far from their current location. This is why it is possible to talk of the autochthonous parts, those that have been deformed in situ, and the allochthonous parts, transported by the violence of the mountain formation process until they are on top of the autochthonous sectors, or even other folds, if this had happened before. The Alps and the Pyrenees clearly show this mixture of types. For example, on the southern face of the Pyrenees several folds, or nappes, have been identified that have been moved south by gravity, with a spectacular superimposition of overlying and cascading anticlines, the result of the movement of resistant materials over a relatively plastic and deformable plane. Thus, mountain ranges show great geological complexity, with folds of different sizes, that may be fractured, partially sunken, or locally crossed by trenches delimited by faults. This complexity is greatest in the areas corresponding to the most ancient mountain formation cycles, where in addition to the original deformations, there are the ones caused by more recent tectonic movements, which may so fragment the ancient craton (rigid, immobile region) that it makes the main lines of the original folds unrecognizable.
Furthermore, certain crystalline intrusions and different types of metamorphic rocks form a fundamental part of the mountain ranges, generally in the axial sectors. In the moments of most intense orogenic activity, basaltic and granitic batholiths totally or partially crossed through the sedimentary series and emerged at the surface, as a result of either extrusion during tectonic activity or later surface erosion. There is a ridge of contact metamorphism around these batholiths. Regional metamorphic activity, when it occurred as a result of the high temperatures reached in the sediments of the large geosynclinal trenches, is more extensive and gives rise to large outcrops of schists and slates. Volcanic activity on the fracture lines means volcanic cones form on top of the structure, and these may become the highest peaks in the range, such as Nevado del Ruiz, Chimborazo, Parinacocha, and Osorno in the Andes.
Rocky materials and the action of erosion
For all these reasons just discussed, the main feature of the large mountain ranges is their lithological diversity, dominated by materials of marine origin that occasionally include small sectors with continental sediments, corresponding in general to trenches filled with local inputs after the tectonic activity. Limestones, sandstones, loams, flysch (a thick deposit of sandstone), granites, basalts, slates, schists, quartzites, and several metamorphic rocks may alternate in the same area and offer widely varying resistance to erosive agents. This diversity is responsible for the differential erosion, which has clear effects on the relief at many different scales: the marked relief of calcareous rocks and sandstones with their steep slopes and large scarps is in sharp contrast to the solid, massive forms of granitic batholiths, while marls open up the wide corridors formed by the main river network, and the slates adopt slightly raised convex forms, susceptible to movements en masse.
Thus, the complex geological history of a mountain range must be considered when attempting to explain its highly heterogeneous landforms. It has already been pointed out that many mountains are the result of the superimposition of materials from several orogenic cycles of different types and lithology. The sectors corresponding to the most ancient cycles, for example, the Caledonian and Hercynian orogenies, have been worn away into erosion surfaces and have since been deformed and renewed by more recent cycles. The line of crests, however, retains traces of the former erosion surface and thus shows a topography without marked vertical irregularities, with broad divides at roughly similar altitudes. However, more recently formed mountain sectors, corresponding to the Alpine cycle (less than 60 million years old), show a very rugged line of crests, with sharp vertical irregularities, and are dominated by steep slopes.
Furthermore, there are also traces of the climate changes that periodically affect the planet, especially the most recent Quaternary ones. The different morphogenetic systems have left relatively clear imprints both in the form of the relief and in the deposits produced by erosion, which retain traces of identity of the different climates. In all the high mountain areas in the temperate regions, and even in the intertropical zone, there are deposits left by the former Pleistocene glaciers that mark the area covered by ice at the edges of glacial valleys or at the mouth of glacial cirques. Cirques are the most typical feature of the high peaks and were the source of thick tongues of ice that flowed slowly for tens of kilometers. Nowadays, the high mountain is still the refuge of the last valley glaciers and cirque glaciers on earth, and they, together with the ice caps found in Greenland and Antarctica, are reminders of the landscape that covered much of the planet during the Quaternary ice ages. The most recent of these crises, at its coldest between 18,000 and 40,000 years ago, has logically left the most marks on the actual landscape. The screes and stony debris covering many mountain slopes are thought to be the result of geomorphologic processes within the last 20,000 years.
Topographically, mountain ranges thus typically show both diversity and complexity. The influence of the structural support and the large contrasts in lithological resistance cause sudden changes in the slopes and great topographical diversity. The form adopted by a slope from the divide to the base conditions the water flow and largely explains the distribution of fertility and productive potential. The movement of water down this slope implies the redistribution of materials of different particle sizes, including nutrients.
The hydrology of a slope--shown by the amount of water that infiltrates the soil and is retained by evapotranspiration and by rates of surface runoff--depends on the slope's form and length. There are many other factors, especially soil depth and permeability. On ridges, however, many of these factors are controlled by the form and the length of the slope. It is generally accepted that convex slopes act as net exporters of water and sediments, while concave slopes act as receptors. Thus the divides, the convex surfaces that break the slope, together with outcrops half way up a slope, have difficulty retaining water and lose nutrients and sediments in suspension. Yet the bases of the slopes, concavities halfway up a slope, and terraced planes receive surplus water and nutrients from the higher exporting areas. Thus, the convex areas are said to be immature because in fact they invest a great deal of their energy in organizing and increasing the production of the concave areas, which can form complex systems, with deep soils and stable plant communities dominated by forests with great ecological inertia.
1.3 The climate in the mountain
Mountains are normally thought to have a harsher climate, with lower temperatures and higher rainfall, than that of the surrounding lowlands. The available information confirms this impression, although there are few weather stations in mountainous areas and they are generally sited in accessible areas, almost always in valley bottoms, and this is an obstacle to better knowledge of the spatial organization of the main climatic characteristics. It is, however, known that the montane climate is highly conditioned by altitude and topography, and that on a planetary scale, both latitude and continentality introduce very important changes. Altitude influences air temperature, radiation, atmospheric pressure, wind speed, and precipitation, all of them climatic features with direct effects on both morphodynamic systems and on ways of life in the mountain.
Temperatures and wind
It is well known that temperature diminishes with height at a rate of about 6.5[degrees]C for every 3,281 ft (1,000 m), and this is the explanation for vertical zonation of vegetation and of geomorphological processes and determines the increasingly unfavorable thermal regime of the highest parts of the mountain ranges, as well as the fact that the number of days with frost increases with height.
Temperatures at the peaks of the tallest ranges may be comparable with those of the poles all year round, although this may be partly compensated by the greater quantity of incoming radiation. Thus, moving to a higher altitude in a mountain range is, in terms of temperature, equivalent to moving to a higher latitude, that is, moving towards the poles, by about 621 mi (1,000 km) for every increase of 3,281 ft (1,000 m) in elevation.
All the world's mountains show this correlation between temperature and altitude, and in summer it may be very high. In winter, however, for some consecutive hours or even days decrease in temperatures with altitude may be reversed by cold night temperatures and the descent of the heavier cold air from higher areas, under anticyclonic conditions. The result is a very low temperature in the valley bottoms (temperature inversion), especially in the early morning, and if there is enough humidity, the consequent formation of fog. A few hundred meters uphill the mist has dissipated, the sun is shining, and temperatures are higher than in the lower areas.
This decrease in temperature with altitude also greatly influences the accumulation and melting of snow. Snow accumulation depends not only on the volume of precipitation but also on the location of the 32[degrees]F (0[degrees]C) isotherm in the cold season (from the end of autumn to the beginning of spring), as this acts as a limit above which precipitation falls in the form of snow, and this is unlikely to melt in the course of winter. On the southern slopes of the central Pyrenees, it has been estimated on the basis of correlations that in winter, the 32[degrees]F (0[degrees]C) isotherm is at an elevation of about 5,413-5,577 ft (1,650-1,700 m) above sea level, but in the Alps and in Scandinavia it is much lower, while in the Sierra Nevada and the Atlas it is considerably higher. Analogously, at similar latitudes, the higher the mountain, the later the snow melts. In the eastern Alps it has been shown that the snow at 6,562 ft (2,000 m) melts in late March, that at 8,202 ft (2,500 m) at the end of April, and the snow at 9,843 ft (3,000 m) in June, and this stepped melting influences the regime of most mountain rivers. Wind is another meteorological effect influenced by altitude. In middle and high latitudes, there is an increase in the velocity of the wind with altitude, especially close to the crest line, and to an even greater extent at the divides, where no topographical obstacles restrain the wind. Yet the factor exercising the greatest influence on the wind is not altitude but relief.
Precipitation: rainfall and snow
The effects of altitude on precipitations, both on the total volume and on its distribution in space and in time, are not as clear as in the case of temperatures, as precipitations do not increase linearly with altitude. It is often said that mountains are islands of humidity, especially in arid and semiarid regions. The orographic effect on precipitation is well known, since when air masses collide with a mountainous barrier they are forced to rise, causing them to cool and become saturated. Precipitation might logically be expected to increase proportionally with altitude, but in fact the vertical distribution of precipitation shows great variation within a single mountain and between different ranges, largely due to the fact that condensation level does not occur at a constant altitude but rises and falls in accordance with the general temperature conditions. The lack of weather stations in the high mountains does not help to solve this problem. The same thing is true of snowfall estimates, as the volume of snowfall is greatly influenced by wind and relief.
In general, mountains below about 8,200 ft (2,500 m) show a clear tendency to the progressively increase in precipitation from the base to the peak. This happens in the Apennines, in the Massif Central in France, in the Jura, in the Central Mountains and the Iberian Mountains in Spain, and in the Pyrenees, although when many localities are studied in more detail this tendency is not followed due to their position with respect to the dominant moisture-bearing winds (the rain shadow effect). Some of the highest mountains show a band of maximum rainfall at mid altitudes, above which the rainfall declines towards the peak. Thus in Hawaii, at about 2,297 ft (700 m) above sea level on the eastern face of Mauna Loa, annual rainfall is more than 217 in (5,500 mm), while at the peak, at 10,820 ft (3,298 m), it is only 17 in (440 mm) a year. A similar effect occurs in the Rocky Mountains, the Alps, and even the Himalayas. Evidently, this characteristic is related to the rapid decline in atmospheric moisture and to the fact that rising air masses increasingly lose their humidity and in the highest mountain ranges become very dry.
One undeniable fact is that the percentage of total precipitation that falls in the form of snow increases with altitude, owing to the lower temperatures. This relationship has been proved in the southern Pyrenees: at about 2,625 ft (800 m) solid precipitation represents 25% of the total, at about 3,773 ft (1,150 m) it represents 45%, at about 5,249 ft (1,600 m), 72%, and at about 5,413 ft (1,650 m), 80%. In high areas it snows often, even in summer, as happens in the Alps, where 65% of the precipitation in July and August is in the form of snow. The persistent low temperatures during the cold season maintain the snow layer, with brief periods of melting linked to the rise of warm air masses. In spring, the ice-melt front advances rapidly, at the same time as the snow layer becomes thinner, until all that is left is a few snow-filled depressions, where snow accumulates and may reach a thickness of several meters.
The presence of a continuous layer of snow throughout the winter has many important consequences. Thus the rate of accumulation and then melting of the snow has a profound effect on the flow regimes of the rivers. During the winter, the precipitation falls as snow, which does not flow down the river network, which means that the flow is very low and regular, almost without oscillations, although snowfalls may be very frequent. In spring, when the snow melts, it causes a surge in water supply to the rivers, which may double or triple their average flow over the year, especially if there are seasonal rains at the same time as the spring melt. The melting of the accumulated snow may last well into the summer, so that there is almost no drop in water levels in the summer due to lack of precipitation.
The hydrological importance of the snow varies greatly from one mountain range to another, as well as within a single range, as a function of the basin's elevation and its capacity to accept a large snow reserve during the winter. When the quantity of snow is not so great, they are called pluvio-nival (rain-snow) regimes, in which the changes in the volume of flow over the year are mainly related to the rainfall and only slightly to the snowfall. In this case, the flow of the rivers hardly decreases in winter, the melting finishes early, and as the accumulated snow melts quickly, the summer drop in water level may be very large. As the snow becomes more important there is a change to a pluvio-nival regime, a transitional nival (snow) regime, or nival regime, with a very simple curve that clearly shows the marked winter retention, the late start of the melt, the large volumes of flow in late spring and early summer, and the maintenance of high rates of flow at the height of the hot season. The most extreme case is that of the glacial regimes, in which the rivers may reach their peak flow in the second half of summer; however, these regimes occur in very few places.
Sunshine and shade
Although altitude is the main factor that affects the climate of mountains, exposure establishes differences within mountains, as it explains the balances of energy--with such important effects on evapotranspiration and for the organization of plant communities--and differences in rainfall between different slopes (see volume 1, pages 378-381). The people living in mountains are well aware of the differences between sunny places and shady ones. Shady places receive less energy in the form of sunshine, and the steepest slopes may receive no sunshine at all in winter and show lower evaporation and a smaller difference between daytime and night time temperatures. Sunny sites experience higher daily temperatures, much greater differences between the day and the night and show greater evaporation. Thus, in medium and high latitudes the sunny sites are rather assiduously cultivated, as the greater sunshine favors the earlier ripening of cereals and the meadows are more productive. Because of the difficulties of obtaining harvests in shady places, they are usually left as forest. For the same reasons, villages tend to be located on sunny slopes and, if possible, sheltered from the dominant winds.
In the temperate Mediterranean mountains, the situation is very different. Although the temperatures are lower than in the neighboring plains, there is no real heat deficit, and not only is it possible to obtain harvests in shady places, but as they show lower evapotranspiration, shady sites have many advantages in an environment where the lack of rain is perhaps the main climatic problem for agriculture. The Atlas Mountains and those of the south of the Iberian and the Italian Peninsulas are good examples of the widespread use of shady sites for the cultivation of cereals. Sunny places, which have been deforested for centuries, are widely dedicated to grazing for small livestock.
Exposure to the dominant winds: to windward and leeward
The orientation of the slopes with respect to the dominant winds gives rise to great differences in rainfall. It is well known that although precipitation tends to increase with altitude, there are great differences within a single mountain range, both in average rainfall values and in a single rainfall episode. Much of this variation is due to the relief and to the turbulences derived from it, but it is also partly explained by the slope's exposure to rainbearing winds. The windward slopes, exposed to the dominant winds, clearly show greater mist formation and rainfall than those that are not exposed, as the mountain forms an orographic barrier that the wind has to rise over. On the leeward side the opposite effect occurs, with dry winds that increase in temperature as they flow down. This phenomenon occurs normally or occasionally in many of the world's mountains, and is known as the fohn effect or fohn wind; there are other local names, especially in the United States where it is known as the chinook, a term that was originally applied to the wind that descends to the plains on the eastern slopes of the Rocky Mountains.
This fohn phenomenon occurs in many parts of the world. The Swiss Alps, where on some slopes vines can be cultivated with excellent yields; the Sierra Nevada in California and the ranges running parallel to the Rocky Mountains that intercept the moist winds from the Pacific; the Andes near Santiago de Chile and especially the Himalayas--all show a characteristic fohn effect with implications on a continental scale. The lush vegetation of the western slopes of the Sierra Nevada in California, with the giant redwoods (Sequoiadendron giganteum) in the Mariposa Grove forest and the magnificent forest in Yosemite National Park, is in total contrast to the parched slopes and valley bottoms on the eastern inland slopes, such as the famous Death Valley, or the less extreme deserts of Arizona or New Mexico. Likewise, the western face of the Andes, between Chile and Argentina, has taller and denser vegetation than the much drier eastern face. In the Himalayas, the monsoon winds blow from the southeast into the southern face, causing very intense, sometimes catastrophic, rains; on the other hand, the northern face is much drier. These data show a feature that is highly typical of all large mountains: the rapid reduction of the maritime influence as soon as the first orographic barriers are crossed. Inland valleys and tectonic depressions receive much less rainfall than do the mountain peaks, experience large variations in temperatures over the course of the day, and show a marked tendency to have a continental climate, even in mountains very close to the sea.
Not all descending winds are hot. Unlike the fohn, the bora is a cold dry wind that blows from the interior of some continental areas towards the coast. Although it takes its name from the wind that blows towards the Adriatic Sea from the mountains of Dalmatia and Slovenia to the north and northeast, other comparable winds blow on the northern coast of the Black Sea (the Crimean Peninsula) and in the fjords in northern Norway. They are thought to be the result of the turbulences generated in the upper troposphere because of the modifications of the upper circulation introduced by the mountains; these turbulences cause the descent of very cold air masses incapable of producing rainfall, because they are not only very dry but are also descending, especially in winter.
The topography and microclimates
When studying the mountain climate on a more detailed scale, the importance of the relief is clear. The form of the slopes, the local slopes, and the orientation of the different sectors of a single slope, all influence ground level temperature, radiation balance, wind speed, evaporation, the thickness of accumulated snow layer, and how long it lasts in the melt period.
In this way, there is an enormous diversity of microclimates in addition to the highly complex relief, and both contribute to the highly varied mosaic present in all mountain ranges. Thus, river gorges are subject to winds of high velocity that are channelled through the narrow valleys, and their desiccating effect leads to the presence of vegetation more typical of drier environments. Topographic depressions are the areas where most snow accumulates, because in winter the wind deposits the snow it has swept from the peaks in them. In these depressions, the snow takes longer to melt leading to an unusual flora with a very short growing season. Concave slopes show other special features: on windward slopes, they channel the rising moist air and favor mist formation, causing what is known as horizontal precipitation, the condensation of droplets of mist on the leaves, which explains the presence of some moist forests. The surface drainage concentrates greater soil moisture in these slopes, resulting in their being cooler environments.
Perhaps one of the best known effects of the relief is the day-night alternation of winds from the valley and winds from the mountain. During the day, the formation of small low-pressure areas due to the earlier and more intense sunshine falling on the higher slopes causes winds to rise from the valley bottom, a phenomenon that is especially clear in the summer. When night falls, the higher slopes rapidly cool and winds blow down to the valley bottom, especially in winter, although this also occurs in summer when a cool breeze blows in the afternoon, a characteristic that defines, better than any other, the summer climate of the mountains.
2. Incipient soils
2.1 Soil formation processes
High mountain soils are relatively homogeneous, as the vegetation and other factors affecting soil formation are not very variable. The climate, for example, is always cold, with important temperature differences between the seasons and over the course of the day. The time that has elapsed since the beginning of the soil formation process is relatively short and lies within a very short interval (between 10,000 and 12,000 years), the time elapsed since the retreat of the ice at the end of the last glaciation. In these conditions, the most important parameters explaining soil distribution and the changes in their nature are the parent materials, or the parent rock from which the soil has formed, and its geomorphology, or location in the landscape. The processes leading to soil formation and the differentiation of horizons are also very homogeneous, at least in the moist Alpine level, which is the most common one.
The physical fragmentation of the substrate
Physical breakdown of rocks is very rapid in the high mountain, unlike chemical breakdown, which is very slow because of the low temperatures. One of the most typical physical breakdown processes is congelifraction, which is determined by the porosity and the size distribution of the empty spaces within the rock. The porosity determines the maximum volume of water that can enter the rock and exert pressure within it on freezing; but if the diameter of the gaps is too small (as happens, for example, in gneiss), they do not let the water enter, and if they are too big (as in relatively uncemented sandstone) the water can leave through cryosuction when freezing occurs, without breaking the rock.
No matter how impermeable a rock is, when subject to repeated large changes in temperature, it will eventually break. This is why the high mountain climate, with major changes of temperature between the day and the night, favors the rapid formation of screes, which often leads to the presence of rocky debris under cornices, crags, and rocky areas.
Why rocks weather
The climate of the Alpine zone gradually dissolves the rocks, as rainfall is generally high and evapotranspiration is low. The resistance of the rocks also depends on the solubility of their components, which in turn depends on their composition, size, and mineralogical purity. Mountain massifs in which gypsum is abundant have many spectacular potholes and caves because gypsum rocks dissolve easily. Yet gypsum-rich soils do not cover a large area. Calcareous soils are, however, much more abundant in mountains. One of the main components of limestones, calcium carbonate, dissolves easily in moist environments in a process known as carbonation. It dissolves more readily at lower temperatures, and as a result high mountain calcareous rocks may show spectacular karstic formations, such as solution channels, potholes, sinkholes, and caves.
Soils are especially sensitive to carbonation because they contain small pieces of carbonated rock that are broken down more easily because of their high surface area/volume ratio. Thus, in wet environments, the smaller sand and limestone particles disappear rapidly from the soil. The most important consequence is that most mountain soils that form on limestone bedrock are not strictly calcareous but do not generally become acidic, because many of the calcium ions released by the breakdown of the calcium carbonate remain in the environment adsorbed onto the surface of clays and organic matter. Decalcification, or the export of soil calcium, is thus slow and difficult, so that most soils on limestone in the Alpine layer are only slightly acidic, or calcic. Under the influence of poor drainage and aggressive waters loaded with organic acids, all the calcium may be lost, and the pH may reach very low values. This is, however, relatively rare in the Alpine level.
The dry Alpine zone may lack water for two reasons: either because it does not rain enough (desert environment) or because the water is in the form of ice. In these conditions the dissociated ions are neither released nor washed out. This is why salt accumulations may occur in very dry high mountains.
The lateral circulation of water
After the rock fragments, the resulting flakes fall and accumulate as steep screes on top of the often impermeable rock masses of the mountain. This situation is highly favorable for the lateral circulation of the water through the soil, which has a variety of effects.
The first effect is the accumulation of water at the lower parts of the slopes, where it may cause the formation of peat bogs. Yet phenomena related to the reduction of iron (hydromorphism, gley formation) are not frequent, although they may sometimes be spectacular. In fact, soil-water saturation after the melt often leads to the formation of a light horizon, possibly whitish or bluish, near the surface of the soil, that is well differentiated but not very thick. This horizon, observed by many authors in soils under snowdrifts, is in reality very widespread. Although it may disappear totally in summer, it often remains relatively visible. Its color is mainly due to the migration of iron and has been considered to be a clay-depleted horizon linked to podzol formation (albic horizon), which is not that surprising as temporary hydromorphism and podzolization are closely related in these environments.
Water has other effects, above all as a vector for the transport of soluble ions. Thus calcium ions are exported from the higher parts of the landscape (ridges, divides, the heads of moraines, and solifluction tongue-shaped lobes) and transported to the lower points (valleys and gorges). The vegetation clearly reflects this phenomenon; thus, in the Chablais, the region of the Savoy Alps that dominates the southern shores of Lake Leman, the soils located in high areas, on calcilutites, are covered in an acidic turf, while in the lower areas the soils are calcic and have calcicolous vegetation.
The accumulation of organic matter
Soils covered with annual or biennial grasses, whether reaped or grazed, show good incorporation of humus into the soil, as carbon input occurs throughout the soil mass owing to the in situ decomposition of the roots. In this they differ from forest soils, where the carbon input is mainly in the form of leaf litter (the leaves, conifer needles, and twigs that fall on the ground), and the incorporation of the organic material into the soil requires agents to mix them.
Alpine soils, covered with grasses, show a homogeneous distribution of humus with depth, because the cold and the humidity do not favor the growth of decomposer organisms. Yet little carbon accumulates because at these altitudes plant production is low, and thus there is not much input of raw material for humus into the soil. However, the seasonal climatic contrasts (the melt, summer drought) cause alternating phases of saturation and shortage of water, as well as large changes in the oxygen concentration of the soil. These sharp changes are inimical to biological activity and favor the breakdown of the organic matter, mainly into carbon dioxide and nitrogen. In these conditions it is not surprising that the soils with the most organic matter, peat bogs (histosols), are uncommon in the Alpine level. They are found only in geomorphological locations where the environment is permanently saturated with water and there are no major oscillations in the water table.
2.2 Soil types
Despite the relative homogeneity of mountain soils, the geomorphological and geological conditions of the parent materials may introduce certain distinguishing features. A distinction has long been drawn between two different types of mountain soil, calcimorphic soils (also known as rendzinas) and acid soils (also known as rankers), a distinction that was still used in the FAO-UNESCO 1974 Soil Map of the World. This classification is now obsolete, and both soil types are now included within the category of leptosols.
Geomorphologically, in the high mountain it is possible to distinguish between surface soils overlying slabs and rock, soils on rock fragments, and soils on moraines. All three substrate types are mixed throughout the Alpine level, regardless of their parent materials, though all the soils that form over the same type of rock will have some features in common.
The surface soils on slabs and rock
Smooth rock platforms are common in the mountains, many of them scraped clean and polished by the action of the glaciers. Neither these slabs nor massive rocks are a priori favorable for soil development. But if the slope is not too steep, soils will eventually form. They probably initially develop as a result of wind-blown silt. In the high mountain, soil cover is often lacking and the fine particles resulting from this loss of cover are easily blown away, transported by the wind and sedimented in other places. Some authors consider that many mountain soils are enriched in silts and other particles of allochthonous origin.
Vegetation may develop on slabs and rocks where foreign particles (some of them organic) accumulate. But the underlying slab resists attack, as it is a hard material with the minimum surface in contact with the exterior, a single plane. The vegetation grows mainly by exploiting its own residues. The resulting soils are very rich in organic matter, consisting of a single dark horizon that is not very thick. Unless these horizons receive laterally circulating calcium-rich waters, these horizons are acidic on both calcareous and acid rocks. In fact, the contact between the slab and the humus layer is very limited, so the slab does not contribute enough calcium ions to compensate for the acidification caused by their export in rainwater. Soils on slabs are mainly classified as leptosols (shallow soils) because of their shallowness.
Soils on slabs and rocks lose their distinctive features if they increase in depth. A soil that is deeper than about 14 in (35 cm) is of necessity formed from a large quantity of fine mineral materials and is not so markedly organic as the soils described above. They are differentiated into horizons. The proportion of organic matter decreases, as the carbon is diluted because it is stored in a larger volume of soil. There is probably also an increase in mineralization; the presence of mineral particles enriches the medium with all sorts of ions and thus favors the activity of decomposer microorganisms.
Soils on screes
The screes in the Alpine zone are unstable environments. These slopes show, on the one hand, a continuous surface supply of rock fragments from higher areas, and on the other hand, the settling and redistribution of materials within the scree. This means that, although the soils are shallow (the organic matter is incorporated in the top few meters), there is little differentiation between the horizons, and their limits are not clear.
In these environments, there is much greater contact between the fine soil and the rock than on slabs. The chemical composition of the soil is thus largely dependent on that of the rock. In general, they are classified as cambisols when a Bw horizon has formed that has then undergone cambic alteration. Soils formed on acid rocks are classified as dystric cambisols and may have an epipedon consisting of an umbric horizon, rich in organic matter, but with a low saturation of the ion exchange complex. On limestone, the soil is generally calcic, and can be classified as a eutric cambisol, which may develop a mollic surface horizon that is also dark but shows greater saturation of the ion exchange complex.
In soils on fragments of calcareous rocks, acidification is often counteracted by two phenomena. On the one hand, the unstable environment causes friction between the larger fragments, leading to a continuous input of calcium carbonate to the fine fraction of the soil. On the other hand, the reserve of large calcareous fragments is constantly renewed with input from the higher parts of the slope. Thus, the fine fraction of the soil may continue to be calcareous, at least in not very wet climates (such as the Southern Alps). The corresponding soils are classified as calcareous cambisols.
Soils on moraines
Moraines, like screes, consist of fragmented materials that have not cemented and drain very freely. But unlike screes, moraines are stable environments, because the glaciers responsible for their formation have melted and they are no longer in movement. Moraine soils can thus develop and form distinct horizons. Very acid soils may form in moraines that are initially calcareous, if carbonation and decalcification have had long enough to act. In high valleys, however, where moraines are very recent, little material may have dissolved. This is the case of some valleys in the Chablais, where the moraines form a thin cover of calcareous materials in an Alpine layer where soils on plinths (slabs and underlying rock) became acidic a long time ago. The flora of moraines is highly calcicolous and greatly appreciated by livestock. Soils on moraines thus vary with age. The successive moraines aligned along the length of a valley, as for example in the Arpette on the Swiss face of Mont Blanc, are detailed chronological sequences that show how soils develop over the first ten centuries of their existence.
Variation due to bedrock
The alteration of the high mountain soils, except in some cases such as those already mentioned, is very limited and has not yet given rise to the large-scale appearance of mineral products that differ greatly from those contained by the original rock. It can thus be considered that, as a whole, mountain soils formed on a given type of bedrock share a number of characteristics.
Soils on calcareous rocks
If the limestone is pure, it leaves no residue when it dissolves. Limestone rock almost completely disappears when it is attacked by the carbonic acid contained in water. Thus, unless there are inputs of allochthonous materials, an organic-mineral soil cannot form and almost pure organic matter accumulates directly on the limestone (slabs or massive outcrops) or between calcareous gravels (rock fragments or slope colluvia). All now included as leptosols in the FAO-UNESCO Soil Map of the World, soils directly on rock are classified as eutric leptosols (or rendzinic leptosols, the former rendzinas) when the organic matter is saturated with calcium, as dystric leptosols when they have suffered decalcification and the soil is acidic (formerly known as ranker soils), or as intermediate type soils. In the case of dystric leptosols they are generally eutric cambisols, as decalcification is difficult in gravels, and a Bw alteration horizon is formed with the development of a good structure and with great accumulation of organic matter.
Finally, if there are mineral particle residues when the limestone dissolves, they will form the starting materials for the soil, which may increase with time owing to the accumulation of organic matter and the progressive inputs of residues as the rock dissolves. Finally, the site will be covered by a thick alteration layer, and it will require close observation to see that the environment was originally calcareous. Then a wider range of soils may appear as a function of the degree of desaturation, the organic matter content, and the degree of differentiation of the horizons.
The redistribution of carbonates within the landscape is a frequent process in slopes covered by rockfalls or calcareous colluvia. Subsurface runoff is responsible for the transport of these carbonates in the lower, more stable, parts of the slopes, where they accumulate. The resulting soils are called calcisols. This accumulation, which is clearer in the montane zone than in the Alpine zone, consists of subsurface calcareous crusts, often with the larger particles cemented into a petrocalcic horizon, or sometimes in calcic horizons in the form of subhorizontal lenses running parallel to the surface of slope, and showing irregular enrichment with carbonates.
Soils on mica-rich rocks
Schists are metamorphic rocks, including slates and shales, that are rich in phyllosilicates (sheet silicates) or micas; when they contain carbonates they are known as calcoschists. In moist climates, calcium carbonate, if there is any, rapidly disappears from the rock, as do other minerals that break down easily. When these components migrate, water can flow through the fine pores left in the rock, thus favoring the progressive and continuous alteration of the schist. In extreme cases, the rock may break down over a thickness of several meters, providing an abundant micaceous residue that is rapidly transformed by the progressive breakdown of the particles into loam and then into clay. The soils are then classified as more or less desaturated Cambisols, as they develop a subsurface cambic alteration horizon. The following edaphic process would be the washing of this clay and its accumulation in deeper horizons, which would give rise, if it were on a large enough scale, to luvisols. Anyway, these soils are very rare in the Alpine layer, showing that the illuviation of clays to form differentiated accumulation horizons--argic (FAO) or argillic (ST) horizons--takes more than 10,000 years.
Breakdown materials from schists are prone to erosion because of their small particle size. Curiously, this is rarely erosion by surface runoff, even on steep slopes. The soil's thickness and structure allows great infiltration and absorption of rainwater, even intense downpours. Yet in spring the water derived from the melting snow saturates the soil, which becomes doughy and flows down the slope due to solifluction. On a human scale the phenomenon is not very obvious, but when it occurs it is very dangerous, because thousands or millions of tons of mud start moving and may damage houses or entire villages. On the scale of thousands of years, solifluction is common enough to modify completely the topography of a site, deforming the slopes with disorderly gaps (the scars left by large movements) and protuberances (mud lobes) The water circulates in these slopes mainly in the form of subsurface runoff below the soils (and on the unaltered rock) but often emerges from the cracks in the lobe fields, leading to hydromorphic environments even when they are still calcic.
The soils on granulated rocks rich in quartz
When the rocks contain large resistant grains of quartz, the soils that develop in the high mountain are sandy and acidic, and often shallow. In principle, these environments are ideal for podzol formation (the leaching to deep layers of the organic matter and iron in an acid medium), but for this to occur there has to be acidifying vegetation, such as heathers or coniferous forests. Thus the crystalline-rock high mountain, whether lacking plant cover or covered with grass, is not very favorable for this phenomenon of extreme acidification. At most, these quartzitic materials have acidified soils. On slabs they form lithic leptosols or dystric leptosols, that are black, rich in organic matter and shallow. On screes, where the organic matter is in closer contact with the minerals, the soils are lighter in color and deeper. They are humic cambisols (the former Alpine ranker), when the Bw alteration horizon fulfills the conditions for classification as Cambic.
Soils on rocks rich in vitreous volcanic materials
In some places in the world, such as Japan, the mountains are essentially volcanic, and in other mountainous areas, such as the Rocky Mountains and the Andes, the volcanoes are so abundant that their emissions are deposited in the neighboring soils. In regions with a wet climate, such as the peaks of the mountains on Reunion Island, the New Hebrides, and Japan, volcanic rocks rich in vitreous materials, or amorphous or noncrystallized minerals, break down quickly. They break down into a range of mineral residues: amorphous (not crystallized), incompletely crystallized or formed of special crystallites (allophane, protoimmogolites and immogolites). These substances show a high affinity for organic matter, with which they form complexes. The dark soils that develop are andosols, (in Japanese, an means dark and do, soil). Andosols show a great affinity for water and can absorb up to three times their own weight in water. They also retain a lot of phosphorus, and so it is not uncommon for plants to have problems absorbing this nutrient.
3. The world's high mountains
3.1 The high mountain space
When speaking of the world's high mountains, it should be noted that there are large environmental and cultural differences between them. The latitude, the distance from oceanic influences, the altitude, and the main orientation of the major axes of folding are some of the factors explaining the world's great diversity of mountains.
The origin of the mountain ranges
The large mountain ranges, such as the Andes, the Rocky Mountains, the Himalayas, and the mountains of central Europe and the Mediterranean are the result of tectonic movements that deformed large sedimentary accumulations. Other mountains, however, are the consequence of the raising of crystalline and/or metamorphic blocks that could not fold because of their rigidity. Normally, these fractures are associated with intense volcanic activity that raised large volcanic cones high above the general relief. This is how tropical mountains formed (apart from the tropical Andes), and they are usually located in areas dominated by crystalline plinths (bases) and volcanic corridors; there is, however, no sensation of a true mountain range, as there are no folds, or at least none from the more recent orogenic phases.
The world's great mountain systems
When considering the planet's most important mountain ranges, one may distinguish between mountain ranges that essentially run east-west, such as the Himalayas and the other ranges of central Asia, and the north-south alignment of the mountain range running along the western coastline of North and South America from Alaska to Tierra del Fuego. Both mountain ranges are on a planetary scale and contain the highest places on earth. Furthermore, there are other mountain ranges of regional dimensions that represent models of certain types of environment, including the Alps, the mountains surrounding the Mediterranean Sea and the tropical mountains, especially those in eastern Africa and those in the Indo-Chinese arc, although these are in fact an eastward
prolongation of the Himalayas. Other mountains cross and deform the earth's crust, but most cannot be included in a classification of the world's highest areas. This is the case of the Australian Alps, the Urals, the ranges in northeast Siberia, or those in northeast Brazil. Others are not included here as they are scarcely of continental size, such as the Drakensberg Escarpment in South Africa or the mountainous relief of Japan or the Philippines.
3.2 The high tropical mountains
The African tropical mountains rise above a pre-Cambrian plinth, a highly resistant shield that was fractured by Tertiary orogenic processes. Most are, in fact, plateaux divided to a greater or lesser extent by the river network with rounded peaks and monotonous relief, and are the remains of ancient erosion surfaces raised to more than about 6,562 ft (2,000 m), but with some sectors raised even more owing to the vertical movement of the plinth. This is the case of the Mitumba Mountains in southeast Democratic Republic of Congo, the Bie Plateau in Angola, and the Abyssinian Plateau. A similar situation prevails in other tropical mountains. For example, the Indian Peninsula consists basically of the Deccan Plateau, a pre-Cambrian plinth that was raised and fractured during the Alpine folding. The edges of this plateau are profoundly deformed into sunken and emerged blocks, forming the Western Ghats and the Eastern Ghats. Together they form an immense slope that rises from the sea.
The orogenic process
As a consequence of these deformations, great elongated north-south valleys opened up in Africa, the eastern and western troughs of the East African Rift Valley, which extend north through the Red Sea and east through the Gulf of Aden. Both troughs contain a string of very deep lakes, especially Lake Nyassa (or Lake Malawi), Lake Tanganyika, Lake Edward, Lake Albert (or Lake Mobuto Sese Seko), and Lake Turkana. Along these troughs and other fractures, there is an unbroken chain of many active volcanoes, including some of the highest sites in Africa: Mount Kilimanjaro (19,341 ft [5,895 m]), Mount Kenya (17,000 ft [5,200 m]), Mount Ruwenzori (16,761 ft [5,109 m]) and Mount Meru (14,984 ft [4,567 m]). The activity of these volcanoes has given rise, in some cases since the Jurassic, to the enormous accumulations of lava over 3,281 ft (1,000 m) thick that cover most of Ethiopia. This phenomenon is similar to that shown by the Cameroon-Tibesti Rift in the Gulf of Guinea, where the volcano Mount Cameroon reaches 13,353 ft (4,070 m) only a few kilometers from the coast, and the peak of Mount Basile, which rises to more than 9,843 ft (3,000 m) in the island of Bioko.
In southeast Asia, the mountainous relief is a mixture of large volcanoes and plateaux crossed by deep valleys. There are over 100 active volcanoes, which have been responsible for many catastrophes, such as the explosion of Mount Krakatoa, in 1883, or the 1991 eruptions of Mount Pinatubo. Despite its immediate negative effects, volcanic activity has repercussions that can be considered positive. Fertile soils develop on the ashes and lava, and are renewed with relative regularity when compared with other tropical soils, which are generally very poor because of the intense washing away of nutrients by the high rainfall. Yet the relief in this area contains some more recent sedimentary rocks, the most important being limestones, which are responsible for a very active karstic landscape, shown spectacularly in some mountains in southern China and northern Indochina.
The climate of the tropical mountains is usually defined as moist subtropical with a hot summer. The average temperatures are roughly the same as those of the temperate zone, although they lack their seasonality. However, there are major contrasts between the day and the night, especially on the highest peaks, which may even experience nighttime frosts. The rainfall is abundant, and shows a tendency to diminish in areas farther from the equator.
There are always isolated moist areas, as can clearly be seen in the Abyssinian Plateau, but the winter dry season is very harsh in the tropics. This contrast is also evident in the mountains surrounding the Indian Plateau, as they are linked to the rhythm of the monsoon rainfall that supplies large volumes of water to the reliefs that rise abruptly above the coastline. In Indonesia, however, it rains a lot throughout the year, which corresponds to its equatorial climate.
3.3 The Himalayas and the high mountains and plateaux of central Asia
Between the great Siberian plain and the Deccan Peninsula there is a complex set of mountains that basically run east-west, the most important of which are the Himalayas and their prolongation to the northwest, the Karakoram Range. With many peaks over 22,966 ft (7,000 m) and even 26,247 ft (8,000 m), the Himalayas contain the highest points on earth, topped by Mount Everest at 29,028 ft (8,847 m). In the north, other ranges often exceed 5,000 m, such as the Altai Mountains in Siberia, the Pamirs and the Hindu Kush in northern Afghanistan, the Kunlun Shan Mountains and the Altin Tagh in Tibet, and the Tian Shan Mountains in northwest China. Between these mountain ranges there are rugged plateaux with relatively steep relief formed as a result of erosion by the river network and with inland plains subject to very harsh climates. Some of Asia's great rivers are born in this great group of mountains, such as: the Syr-Dar'ya and the Amu-Dar'ya (Oxus), whose sources are in the Tian Shan and the Pamirs respectively; the Ganges on the southern face of the Himalayas; the Indus and the Brahmaputra on the northern, or Trans-Himalayan, face; the Irrawaddy, Salween and Mekong, in the Tibetan Plateau; and the Yellow River in the Kunlun.
The mountain formation process
The formation of the Himalayas and the other mountain ranges of central Asia was long and complex and included some of the most important geological events in the earth's history. The entire zone was originally an immense subsiding trough--it was gradually sinking as it filled with sediments, from the Paleozoic until the end of the Tertiary. The total thickness of sediments has been estimated at 67 mi (108 km), 37.2 mi (60 km) of them deposited in the Paleozoic, 16.1 mi (26 km) in the Mesozoic and 14 mi (22 km) in the Tertiary. Several indirect data allow us to deduce that the Caledonian and Hercynian folds raised large mountain ranges in the current location of the Himalayas, which were reduced to virtual peneplains at the end of the Primary era. Later there was a long phase of accumulation of sediments in the trough that opened between the continents of Angara (Siberia) and Gondwana (the present-day Deccan or Indian Plateau) in the Tethys Sea. India's collision with Asia led to the folding of the sediments accumulated in the Tethys trench and the disappearance of the ancient sea (see pages 40-41 and 46 in volume 1). The Alpine tectonic cycle started in the Jurassic, with volcanic eruptions and phenomena of metamorphism at depth. It was, however, in the upper Cretaceous, in what is known as the Thorung phase, when the first part of the definitive rise of the Himalayas took place. In the Miocene, the region was affected by the southward displacement of enormous layers of rock, (displacement phase), coinciding with the rise of large granitic masses that are a major part of the range's structure. Finally, post-Miocene movements raised Everest, the highest point in the range, and provided the range's definitive structure. During this phase the Gangetic Sea was still present at the southern base of the Himalayas, separating them from the Indian subcontinent and the site of the accumulation of debris that formed the Siwalik, or preHimalayan, Range. The folding of this alignment confirms the existence of mountain formation movements until the Pleistocene--movements that still continue, as shown by the earthquakes that still affect the entire Himalayan arch, the existence of terraces that have been deformed and raised tens of meters, and fault planes with signs of very recent activity.
Several units can be distinguished within the Himalayas on the basis of their geological development and the characteristics of the current relief. From south to north, from the Gangetic Plain to Tibet, first there are the gentle foothills that rise to the first mountain spurs of the Siwalik (pre-Himalayan) Range, formed by detritic Pleistocene materials (coarse-grained sandstones, conglomerates, sands, and clays) and presenting a geological structure dominated by south-facing isoclinal folds, with frequent inverse faults and the beginnings of displacements. A first set of longitudinal valleys ("dunns") separates this range, which reaches elevations of 6,562-9,843 ft (2,000-3,000 m), from the Mid-Himalayas, characterized by the presence of large nappes superimposed on top of each other that had originated to the north in the Great Himalayas. Large displacements also occur, although autochthonous materials also emerged, consisting of metamorphic sediments and granitic batholiths. This part of the range contains the highest points on earth, with many peaks higher than 26,247 ft (8,000 m). On the edge of Tibet are the Trans-Himalayas, formed by the folding of the marine materials accumulated in the Sea of Tibet between the Cambrian and the Cretaceous; it is a large platform with very simple geological structures, raised to an elevation of more than 16,404 ft (5,000 m) and with closed basins occupied by lakes. Further north, between the Kunlun Shan and the Tian Shan, there is a large depression with smooth relief, the Takla Makan Desert, a westward prolongation of the Gobi Desert.
The climatic conditions
The Himalayas are at a latitude similar to that of the hot deserts, and there are very arid regions right at their base, such as the Thar Desert. Yet the monsoon winds bring high rainfall to the entire southern face of the range, although the northern face is, except for a few exceptions, dominated by the cold continental deserts of central Asia. The great height of the Himalayas means the climate varies from the tropical monsoon environment to the perpetual snows of the highest peaks. The lowlands below 8,202 ft (2,500 m) have a tropical climate with hot and very rainy summers, and cold, temperate winters, and snow and frosts are rare. In the highlands, the climate is colder with long winters, frosts are frequent and snow is common. Above 8,202 ft (2,500 m), the climate changes from sub-Alpine to Alpine and then to the perpetual snows, where glaciers alternate with bare rock and active screes, the product of the accumulation of blocks that have fragmented from escarpments because of repeated freezing and melting.
In addition to the differences due to altitude, there are important climatic contrasts. The southern slopes are highly influenced by the monsoon winds, which increase the amount of precipitation as a result of the orographic effect of the first mountainous foothills. The northern face of the Himalayas is much drier, and the winds that blow down to the Tibetan Plateau from the highest peaks clearly show the fohn effect as they have discharged most of their moisture on the southern slopes. In much of Tibet, despite its great altitude, average precipitation is only around 4 in (100 mm) per year, except in the valley of the Cangbo at the head of the Brahmaputra, where the monsoon is responsible for an average annual rainfall of up to 39 in (1,000 mm). In northern Tibet, the mountain ranges are like relatively moist islands surrounded by continental deserts on the nearby plains. In addition to the dry conditions, there are large seasonal differences in temperature and harsh winters.
From east to west there are also notable differences in the precipitation regime. The eastern Himalayas have a very clear monsoon regime, with precipitation of more than 157 in (4,000 mm) in many regions. In the central Himalayas, there is still a monsoon regime, but in winter there are some rains and the total precipitation is about 98 in (2,500 mm). In the western Himalayas, the winter and spring rains are heavier than the monsoon rains, with an annual total of around 59 in (1,500 mm). These differences are also shown in the size of the current glaciers; thus, in the eastern Himalayas, where the precipitation is greatest, the glaciers reach lower elevations than in any other site in the whole range. The large glaciers of the western tip (the Karakoram Range and the Pamirs) are the longest glaciers outside the polar regions, as precipitation falls mainly in winter and the latitude is higher. The glaciers in the central area are much shorter; thus the Khumbu glacier, the longest on Everest, is only 9 mi (15 km) long.
3.4 Mediterranean and Alpine high mountains
The center and south of Europe include a very heterogeneous group of mountains. Individually, none is comparable in size with the Himalayas or the ranges running down the west of the Americas, but the Alps, for example, are considered as typical and are used as a point of reference because of their location and the fact that they have been very thoroughly studied. Normally, the European mountains are analyzed as separate unconnected ranges, as they are separated by wide corridors or depressions and form part of different political spaces. Although each has its own identity, they share many common features, especially geological ones, and especially in the case of the Mediterranean mountains, climatic ones.
The orogenic process
The central and southern mountains of Europe have a very complex history, although all of them were affected by the Alpine folding and incorporate greater or lesser fragments of the Hercynian plinth that usually forms the central core. Both the Alps and the Pyrenees have a Paleozoic crystalline and metamorphic axis that has been highly fractured and enveloped by more recent materials, from the Mesozoic and Tertiary, folded during the Alpine orogeny. In the Alps, it is therefore possible to distinguish in the first place a central part at great elevation, often above 13,123 ft (4,000 m) with very hard rocks (granites and gneiss) that are weathered very slowly, and have formed steep slopes with sharp ridges. To the north and to the south of this basement, there are Paleozoic sedimentary rocks, with easily weathered materials that allow the development of wide valleys between the mountains, such as those in the Rhone, the Rhine, the Inn, and the Salzach. Further north and further south, the calcareous rocks give rise to vertical scarps and high partially karstic platforms. Finally, in the outermost area, dominance by flysch leads to more gentle reliefs and intense erosive processes.
In the Pyrenees, it is also possible to distinguish between a Paleozoic axis in which some granitic batholiths (Maladeta, Panticosa, etc.), with solid, heavy forms occupy the highest reliefs and contrast with the smoothness of the large outcrops of slates and schists, affected by massive movements at depth. In the north and the south, the outer ranges form ranges of secondary calcareous rocks and sandstones with very steep relief. Further outwards, the Eocene flysch, loams, and moraines of the last phases of the folding help to confirm the idea that there is a ring of progressively younger materials from the central nucleus outwards. In both cases, terms such as the Pre-Alps or Pre-Pyrenees are used for the outer sectors around the central module.
Analogously, the Apennines also have schist and crystalline massifs corresponding to ancient tectonic cycles that have been included within the Alpine orogeny. The Carpathian Mountains occasionally show the effects of both the Hercynian and the Alpine orogenies, with some crystalline blocks split by faults and uplifting, with the remains of level surfaces, together with typical Alpine reliefs. The Sierra Nevada in Andalusia, in the Baetic Mountains in the south of the Iberian Peninsula, is a large Paleozoic block surrounded by more recent materials. Other lesser ranges, such as the Iberian Mountains, have some small ancient massifs included within materials belonging to the Alpine cycle.
One common characteristic of all the Mediterranean and Alpine mountains is the violence of the deformations, especially in the Alps, where displacements and nappes have been thoroughly studied. One part of the Alps can be considered to be autochthonous, that is to say, it is still located in its geological region of origin. The rest, however, is allochthonous, having separated and moved several kilometers from its origin and been superimposed on autochthonous materials or other nappes. The same thing happens in the Pyrenees, in the Baetic Mountains, in the Balkans and in the Carpathians. The explanation for these nappes is still not sufficiently clear, but in the case of the Pyrenees it has been suggested that there was an uplifting of the ancient massifs (axial Pyrenees) that caused the sedimentary covering to separate, which then slid downwards, using the Permo-Triassic materials in contact with the shear plane. There have been attempts to apply this explanation to the Alps, but it is not valid for the entire range.
As a consequence of their lithological diversity, their generally recent origin, and their proximity to the sea in some cases, the Mediterranean and the Alpine mountains have very steep relief, with very few plains. There are deep river gorges running between vertical walls. The slopes are very steep throughout the area, especially in calcareous and crystalline massifs. The valleys are always very narrow, except in areas where the action of glaciers has formed wide valley bottoms. The erosive processes act intensely on the soft materials (loams, slates, moraine deposits and to a lesser extent flysch), and where there are limestone outcrops, the ranges are scattered with impressive forms related to the karstic relief, most spectacularly in the Dinaric Alps in the Balkans.
Although the cold periods of the Quaternary left their mark on all these mountain ranges, the best examples of glacial and periglacial relief are in the Alps, followed by the Pyrenees. The glaciers in the Alps now occupy about 1,544 [mi.sup.2] (4,000 [km.sup.2]), split between the different massifs, the most important being Mont Blanc and the Swiss Oberland. During the Quaternary cold crises, however, they occupied a large area that reached Lyon and part of the plain of the River Po. In the Alps, there are many large cirques, broad U-shaped valleys, crossed by deep bands of moraine, excavation basins, lakes and all the phenomena related to glacial action. Glacial action was much less intense in the Pyrenees, although on the northern face piedmont glaciers formed in the Aquitaine plain, while on the southern face there were valley glaciers more than 19 mi (30 km) long.
Mediterranean mountains have many climatic characteristics in common that they do not share with Alpine mountains. Mediterranean mountains are influenced by the presence of a warm sea and by the rainfall regime typical of the Mediterranean climate. The summers are dry and mild, and even hot during the day, and the winters are cold and moist. Mediterranean mountains with precipitation of more than 39 in (1,000 mm), much greater than the 16-20 in (400-500 mm) of the surrounding plains, are like moist islands. They do, however, show some variation in space and time: the rains are a little more abundant in the northern Mediterranean mountains (the Provencal Alps, the northern Apennines and the Pyrenees), where the summer dry period may be broken by the passing of westerly fronts. More rain tends to fall at the equinoxes in the mountains of the western Mediterranean, whereas in the eastern ones more rain falls in winter, meaning that the mountains in the eastern Mediterranean have a longer dry period.
The rainfall is intense, and this very important feature shared by the entire Mediterranean Basin is even more intense in the mountains due to the orographic effect (see vol. 5, pages 21-26). The high temperatures of the seawater at the end of summer favor the development of low pressures (leading to cold drizzle), which bear very moist masses of air towards the coasts. These masses rise rapidly when they collide with the steep mountain foothills, causing intense rainfall for several days (many sites have recorded rainfall greater than 24 in [600 mm] in 24 hours), leading to catastrophic flooding and large-scale loss of soil.
In the mountains of central Europe there is a sharp transition from the western part to the eastern part, as the oceanic influences decline and continental ones increase. In the western Alps, it rains throughout the year, as corresponds to a mountainous environment that is highly exposed to the dominant winds blowing from the west and northwest. Further east, the winter is dry and the rains fall mainly in summer, following the model of a continental climate. The temperature contrasts, which in the western Alps are great, increase towards the east and are greatest in the Carpathians and in the northern Balkans, where the winters are very harsh. This means that the difference between sunny sites and shady ones is of great significance for human activities, as are the intensity of winter temperature inversion in situations of calm and the greater or lesser frequency of drying winds (fohn), especially in some Swiss and Austrian valleys.
3.5 The high mountain in the Americas
On the western faces of North and South America there are long mountain ranges running north-south. In North America, these alignments are a set of highlands that do not form a single mountain range, as there are several depressions between them. In South America, however, the relief is much more consistent, as the Andes form a huge mountain range running 4,660 mi (7,500 km) along the Pacific coastline. The Andes also shows major internal differences, with deep valleys between the very high ranges and the very high plateaux.
The orogenic process
The formation of the high mountain areas in the Americas started in the pre-Cambrian and has lasted almost until the present day. In both the North American ranges and the Andes, the oldest regions are in their interiors. When the distribution of the continents was very different from what it is today, the current location of these mountain ranges was occupied by a deep geosyncline in which thousands of meters of sediment accumulated.
In the late Paleozoic and early Mesozoic, the continent of Pangaea began to split up, and what are now North and South America moved westwards, causing the gradual raising and deformation of the materials of the geosyncline. By the early Cretaceous, Europe and Africa had already separated. The westernmost ranges formed more recently as a consequence of the subduction of the oceanic plate under the continental plate. During the Tertiary, the major uprisings coincided with intense erosion that filled the inland depressions. Tectonic movements still continue in both North and South America, with frequent earthquakes associated with volcanic activity. The intense tectonic movements explain the coexistence of deformed and fractured sedimentary rocks with metamorphic rocks and granitic batholiths that emerged at different moments of geological history.
The North and Meso-American Systems
The highlands of western North America run for a length of more than 3,107 mi (5,000 km), from the Bering Sea to Meso-America, with a maximum width of 994 mi (1,600 km) at a latitude of about 40[degrees]N, more or less between San Francisco and Denver. Some of the tallest peaks exceed 19,685 ft (6,000 m), such as Mount McKinley 20,299 ft (6,187 m) in Alaska, and Mount Logan 19,849 ft (6,050 m), between Alaska and the neighboring Canadian territory of Yukon. Citlaltepetl (or Orizaba), at 18,697 ft (5,699 m), is the highest peak in the southern sector of the North American ranges.
In North America, the most important mountains are the Rocky Mountains and the Pacific Coastal Range. The Rockies form a series of alignments with a very steep relief but with few contrasts in the line of crests, due to the dominance of erosion forms derived from ancient peneplains. Some mountains have a granitic or metamorphic core with a sedimentary covering and are dominated by structural forms. Continuing tectonic activity is shown by the geysers in Yellowstone Park. The Rockies stretch south as the Sierra Madre Oriental until this range then merges into the volcanic chain at the south of the Mexican Plateau, with the Pacific Coastal Range and the Sierra Madre Occidental. The Pacific Coastal Range is more recent and is divided into two alignments: On the one hand, there are the Oregon-California Coastal Range, the Olympic Mountains, the islands of Vancouver, and the steep coastline of the Gulf of Alaska; on the other hand, there are the Californian Sierra Nevada, the Cascade Range, and the Canadian Pacific coastal ranges. Between them is a wide corridor that gradually descends towards the south forming the Gulf of California, whereas on the mainland it is well represented by the Central Valley of California. To the south, the Californian Coastal Mountains project into the sea as the Baja California peninsula, while the inland ones continue as the Sierra Madre Occidental as far as the volcanic mountain range in the center of Mexico. Even further south, the Sierra Madre del Sur and the mountain ranges of the Central American isthmus connect with the Northern Andes. The Oregon-California coastal range is not very high, rarely exceeding 4,921 ft (1,500 m). One of its most important features is the San Andreas Fault, which is still active, as shown by the repeated occurance of earthquakes. Along this fault, the arrangement of the river network and a variety of geomorphological phenomena show the intensity of the displacements. The mountain ranges in the interior, however, exceed 13,123 ft (4,000 m), with a very steep relief, granitic batholiths (such as the one in Yosemite National Park) and a Quaternary glacial activity that formed large U-shaped valleys and even fjords where the range juts into the Pacific (British Columbia and Alaska) and the glacial valleys are invaded by ocean. There was a surprisingly large amount of Quaternary glacial activity on the eastern face of the California Sierra Nevada, with ice extending as far as Owens Valley, in an environment of intense aridity. In the inland valleys, the mountainous fronts are occupied by a continuous succession of alluvial cones that are the starting points of extensive glacis (gentle slopes).
Between the Rocky Mountains (and the Sierra Madre Oriental) and the Pacific Coastal Range (and the Sierra Madre Occidental) there is a region of depressions at an elevation of 6,562 ft (2,000 m). They show little folding and have been intensely excavated by the river systems, which in the case of the Grand Canyon in Colorado, reach down more than 4,921 ft (1,500 m) to the pre-Cambrian plinth. Tableland reliefs occupy a large part of the area, but they are sometimes covered by volcanic deposits and deformed by mountainous reliefs. In the state of Nevada, the presence of emerged and sunken blocks occasionally gives rise to small inland depressions below sea level (Death Valley, -279 ft [-85 m], Salton Sea, -240 ft [-73 m]).
The Andean system
The Andes are organized in long alignments that run almost parallel to the coast and are separated sometimes by highlands and sometimes by deep valleys. They are divided into the northern, central, and southern Andes. The northern Andes run from the Cordillera de Merida in Venezuela, through Colombia and Ecuador, to Cajamarca in Peru. In northern Venezuela, the east-west Venezuelan coastal chain runs southwest towards the Cordillera de Merida, and finally in Colombia it turns and runs almost north-south. In Colombia, the Andes form three ranges (eastern, central, and western) that run almost parallel and are separated by deep valleys that converge until they join further to the south. The eastern range is separated from the central range by the valley of the Magdalena River, while the Cauca Valley runs between the central and the western ranges. When the three ranges enter Ecuador they join into a single range, known as the Cordillera Real, with peaks that often exceed 16,404 ft (5,000 m), and many of them almost permanently active volcanoes, the most prominent of which is Chimborazo (20,577 ft [6,272 m]). The central Andes run from Cajamarca (Peru) in the north to Antofagasta (on the Chilean side) and Catamarca (on the Argentinean side), in the south. The central Andes are clearly divided into two ranges, the eastern and the western ranges, separated by large plateaux, most of them at elevations of more than 11,483 ft (3,500 m). Both ranges are very high, and the western range has one of the highest concentrations of volcanoes in the world, with peaks that usually exceed 18,045 ft (5,500 m) and often exceed 19,685 ft (6,000 m). The inland basins have either joined the Amazon river network or retain remains of former endorheic drainage systems, the best example of which is Lake Titicaca. Other enclosed basins developed into extensive salt flats as a result of the arid conditions, such as Lake Poopo, the Uyuni salt flats, and the Atacama salt flats. After the eastern and the western ranges join in the north of Chile, the southern Andes form a single large chain and there are no more plateaux between them. There is a also smaller range, running parallel to the coast, which gradually decreases in height until it finally disappears into the sea at Puerto Montt, giving rise to an uninterrupted series of islands; between this minor range and the main range of the Andes there is a large valley running north-south that contains many of Chile's main towns and cities, including Santiago. After Aconcagua (22,831 ft [6,959 m]), the highest peak in the Americas, the Andes rapidly decrease in elevation to the south, although at the southern tip of Chile and Argentina they again reach heights of more than 9,842 ft (3,000 m).
The climatic conditions
The complex relief and the enormous length of the American ranges mean that their climate varies greatly. It has to be borne in mind that the two tips of the ranges almost reach the circumpolar regions, while the Andes are largely tropical. There is thus no specific climate corresponding to the North American mountains or the Andes, as they are enjoy zonal, or regional, climatic conditions. Exposure to moist winds is also very important and explains the contrasts in rainfall at similar latitudes.
The North American and Meso-American montane climates
In the North American ranges, the temperatures clearly increase from north to south on both the coastal and the inland faces. The lowest temperatures are found in the interior of Alaska, where temperatures below -76[degrees]F (60[degrees]C) have been recorded. The highest temperatures are found in the south of the inland face of the Rocky Mountains, with an absolute maximum temperature of 129[degrees]F (54[degrees]C) in Death Valley. Precipitation, however, has a much more irregular distribution.
Schematically it is possible to distinguish (a) an oceanic mountain climate that to the north, in Alaska and the Canadian Yukon Territory, clearly becomes polar, with very low temperatures and generally abundant precipitations, although limited to the western faces of the mountain ranges, the ones that are most exposed to winds from the west; (b) a sub-Mediterranean or mountain Mediterranean climate on the western face of the mountains in California, with precipitation that diminishes towards the south as subtropical conditions come to dominate; (c) a large diversity of climate in the interior sharing a common denominator, namely the sharp temperature contrasts with increasing distance from the oceanic influence (the valleys and plateaux between the mountains experience very low temperatures in winter and very high ones in summer, showing their highly continental nature--much of the precipitation may fall in summer, the total volume of rain is perceptibly less than the volume on the coast, and the total volume is only large on the highest sectors exposed to the west, where it may reach 59 in [1,500 mm] per year); (d) an equally great variety of subtropical climates in the Mexican and Central American mountains, where depending on the altitude and average temperatures, it is necessary to distinguish between Tierras calientes (hot lands) (up to 3,281 ft [1,000 m] elevation and average temperatures of more than 68[degrees]F [20[degrees]C]), Tierras templadas (warm lands) (between about 3,280 and 6,562 ft [1,000 and 2,000 m]), and average temperatures between 61 and 68[degrees]F (16 and 20[degrees]C), and Tierras frias (cold lands) (above 6,562 ft [2,000 m] and average temperatures less than 61[degrees]F [16[degrees]C]).
The Andean mountain climate
In the Andes, it is also possible to establish a simple climatic classification closely related to the latitude. Thus, the northern Andes are in the equatorial (or wet tropical) zone, with abundant rainfall that decreases towards the east, where there is a clear dry season. The high areas enjoy a temperate climate, but without seasonal contrasts, although there is a sharp contrast between the very hot days and the very cold nights. Above 11,483 ft (3,500 m) the mountain is again dominant: the low temperatures are accompanied by intense winds and snowfall in the highest areas, where some zones of perpetual snows persist. The central Andes are characterized by the great aridity of the lower areas due to the influence of the cold Humboldt Current, although this does give rise to abundant coastal fogs. Precipitations increase slightly with altitude, but the inland altiplanos (high plateaux or plains) are dry as well as cold. The eastern range receives more rain because of the frequent penetration of tropical influences. Snow is present above 13,123 ft (4,000 m) for much of the year. The southern Andes begin with a desert zone, perhaps the harshest desert on earth (the Atacama Desert), in the inland plateau. But the nearby mountains are not very moist either. Around 30-35[degrees]S, the desert climate is gradually replaced by a semiarid climate, which is clearly Mediterranean in the region around Santiago, with hot, dry summers and relatively moist winters. Further south it is replaced by a moister climate, with oceanic features, that is cold and rainy at the southern tip, with winds from the west that may cause annual precipitations of more than 393 in (10,000 mm). Here, the highest areas have polar climates similar to those in Alaska, allowing the presence of large glaciers.
111 The tranquillity of the mountain landscape, shown here in the Paine National Park in the Andes, has even affected this guanaco (Lama guanicoe), and tends to make one forget the great importance of mountains to the earth's surface. Mountains not only break the monotony of the landscape, but, because they create new habitats different from those of the surrounding lowlands, also modify the distribution of the vegetation and the fauna and create a natural theater of extreme complexity and heterogeneity.
[Photo: Gunter Ziesler / Jacana]
112 The levels at different elevations are clearly shown by this autumn view of mountains in Colorado. The trembling aspen (Populus tremuloides) turns yellow in the autumn, emphasizing the difference between the deciduous and evergreen plant communities.
[Photo: John Shaw / Auscape International]
113 The action of erosion has carved Mount Fitzroy and the surrounding peaks in the Los Glaciares National Park in Patagonia, Argentina. As soon as a mountain massif forms, even before the mountain formation processes cease, erosive agents start to act and gradually change the physiognomy of the landscape. The speed at which mountains erode is determined by the site's landform, which may be rough to some degree; by the presence or absence of a protective plant cover; and by the intensity and the frequency of the action of erosive agents, which in turn depends on climatic conditions and on the nature of the geological materials.
[Photo: Ted Mead / ANT / NHPA]
114 The highest slopes of mountains are often in contact with mist and clouds, as can be seen in this photo of a valley on the western coast of the Saddle Aspiring National Park in New Zealand. Mountains are areas of high atmospheric instability, and precipitation (as rain or snow) in the mountians is greater than in the surrounding lowlands. The presence of clouds and mist near the soil increases the levels of humidity; much of the humidity in high mountain regions is due to dripping mist and hoarfrost.
[Photo: N. Groves / Natural Science Photos]
115 The global distribution of high mountain, and climograms of eight mountain localities, representing different climatic situations (in the climogram for Cuzco the year runs from July to June to make comparison with the sites in the northern hemisphere easier). Compare with the climograms in vol. 1, page 327. There are mountains on all continents and at all latitudes, so the ecological and climatic conditions of the mountain environments in different parts of the world are not identical. The main climatic difference between the tropical mountains and those at midlatitudes is due to the presence or absence of seasonality. In mountainous areas at midlatitudes, there are variations in temperature over the course of the year (large variations in the cases of Erzurum and Ely) that do not occur in tropical mountains, where the temperature is almost constant throughout the year (as at Addis Ababa, Cuzco, and Equator Station). However, in tropical mountains there is a difference between a dry season (from October to February in the African stations and from May to September in Cuzco in the southern hemisphere) and a rainy season: In temperate zones the rainfall is much more constant over the course of the year. (Ely is the clearest example.) Note the Qamdo station, which is at an intermediate location, and thus shows characteristics intermediate between the tropical and truly temperate areas (Qamdo, or Qabdo in the Pinyin transcription of the Tibetan locality of Chab-mdo, traditionally written as Chamdo, or Ch'angtu in the Wade system). These and other differences are enough to prevent plants from tropical mountain regions from growing in temperate mountains (and vice versa), and so the environmental differences between them are even sharper.
[Drawing: Editronica, from several sources]
116 The lower limit of perpetual snow is governed by temperature, latitude, and precipitations. With increasing latitude, the snow reaches lower altitudes, and in the polar regions may reach sea level. In the tropics, however, the lowest perpetual snow is at altitudes of 16,404-19,685 ft (5,000-6,000 m). Yet temperature and precipitation also play a role, and thus on the equator, the lower limit of the snow descends slightly because of the rain and cloudiness at these latitudes. In general, at a given latitude the snow limit is much lower in areas with high rainfall (such as mountains near the coast) and higher in mountains where it rains little (such as continental mountains).
[Drawing: Jordi Corbera, based on Price, 1981]
117 The fohn effect occurs when warm air collides with a mountainous obstacle and has to rise to pass it. This causes precipitations and a general year-round increase in cloudiness on the windward side. When the air mass reaches the top of the mountain, it starts to descend on the other side and as it sinks it gets warmer. Now, however, it is a dry wind because it has lost most of its humidity as rainfall or mist on its ascent. Thus, the leeward side has higher temperatures and appreciably lower precipitations than the windward side (see also figures 126 and 128).
[Drawing: Editronica, from several sources]
118 Polygonal soils are typical of all the high mountain environments and resemble those of the Arctic regions (see photos 7 and 37). They form as a consequence of the cracking of soil by ice, on gentle slopes and flat areas dominated by sandy materials and with relatively high humidity. The photograph shows polygonal soils in the mountain pass of Varrados, in the Vall d'Aran (central Pyrenees). Note the polygonal distribution of the plants growing on these geometric soils.
[Photo: Josep M. Barres]
119 The Vig Mole grottoes in Switzerland were formed as rainwater dissolved the rocks, enlarging the small pores through which the water enters. The formation of spectacular structures such as gorges, eddies, or grottos is easier in soluble rocks such as gypsum, but also depends on the arrangement of the materials and the specific conditions of each site, among other factors.
[Photo: Josep M. Barres]
120 The umbric Leptosols that occur in the sub-Alpine layers, for example at an altitude of 5,905 ft (1,800 m) in Courcheval (Haute Savoie, France), are poorly developed acid humus-bearing soils, as they are limited by the presence of calcareous blocks of moraine origin. The surface horizon, or epipedon, is rich in organic matter, which give it its dark color, but poor in carbonates. The vegetation of rhododendrons (Rhododendron ferrugineum) growing on this soil shows that it is acidic.
[Photo: Jean-Paul Legros]
121 The mechanical breakdown of the rock substrate and the accumulation of the fragments at the base of scarps, where they form large screes that are generally very unstable, are stimulated by the large temperature differences over the course of the day and of the year in the higher areas. The calcareous debris in the photo, taken in Flaine (Haute Savoie, France) at an altitude of 7,874 ft (2,400 m), have been fragmented by the action of ice. The presence of active screes decreases with height. The lengthening of the vegetative cycle and the increase in the summer temperatures allows soils to form, and locally they may be very deep.
[Photo: Jean-Paul Legros]
122 Solifluction terraces or steps, shown in the photo in the central Pyrenean valley of Baqueira, form on slopes because of the action of ice, the drag of snow, and the saturation of soil with water during the spring. The constant trampling by livestock, which use the flatter sites to circulate, gradually compacts the soil and also contributes to the formation of this stepped microrelief.
[Photo: Josep M. Barres]
123 Glaciers generate valleys with typical forms, and favor the divagation of rivers, that is to say, changes in their beds and direction of flow, and this leads to undermining of the banks and the formation of steep reliefs. The ice-polished gneiss outcrop forming this half-dome in Yosemite National Park, California, is a spectacular example of how powerful an erosive force ice is.
[Photo: Earshal Long / WWF / Still Pictures]
124 The southern beech (Nothofagus) forests in Waimakariri Valley in Arthur's Pass National Park, in New Zealand, are botanical reminders of when this island was joined to South America. The distribution of the genus Nothofagus and other plants and animals in both New Zealand and South America is a further proof of continental drift. The distribution of Nothofagus is especially significant because it cannot disperse over long distances. In New Zealand, the trees of this genus form a belt below the conifers and may account for most of the tree cover when they are scattered.
[Photo: N. Groves / Natural Science Photos]
125 The Karakoram Range, an extension of the Himalayas towards Kashmir, contains some of the highest peaks on earth (some over 22,296 ft [7,000 m]). This range and the other central Asian mountain ranges formed when the Indian subcontinent collided with Asia and the earth's crust buckled, giving rise to very high mountain chains. The photo is a panoramic view of the Muztagh Tower (Chinese Karakoram) at sunset from the Chongtar.
[Photo: Colin Monteath / Auscape International]
126 The northern slopes of the Himalayas are very dry, as can be seen in this photo of the Chinese plateaux in Tingri, in Xegar (Tibet), as the winds that have arrived from the higher zones have lost almost all their humidity on the southern face due to the fohn effect (see figure 117). Thus, this mountain massif is responsible for the different climates on the windward and the leeward slopes. All mountain chains, in one way or another, act as a barrier, but the effects they have on the climate are not identical, as they depend on the range's size, form, orientation and geographical location.
[Photo: Roland Seitre / Bios / Still Pictures]
127 Deformations are common in the Mediterranean and Alpine high mountain. The deep Anisclo canyon in the Ordesa National Park (Spanish Pyrenees) has very steep walls and is the result of intense deformations that were caused by the formation of the Pyrenees and its later modelling by geological forces.
[Photo: Ramon Torres]
128 Rainfall and vegetation section along a transect following the parallel 39oN from the Pacific Ocean. The warm, moist ocean air rises because of the Coastal Ranges and the Sierra Nevada, cooling as it rises, so that its humidity condenses as rain. This is known as the fohn effect (see figure 117). To the east of the Sierra Nevada the climate becomes drier and drier. The Great Basin is thus in a rainfall shadow area, and the rainfall does not exceed 20 ft (500 mm) per year. The Rocky Mountains, also show the fohn effect. The vegetation changes not only with altitude but also with rainfall. Thus, Central Valley in California and the great plains are covered in herbaceous formations, whereas in the lower parts of the Great basin the subdesert scrub known as sagebrush develops.
[Drawing: Editronica, from several sources]
129 The salt lakes of the Andean high mountain, such as the Salar de Surire (Surire salt flat) in Chile, have developed into enclosed basins owing to endorheism (inward-flowing drainage) and aridity. The photo shows an Andean flamingo (Phoenicopterus jamesi) feeding. All the rivers and streams that make up the region's hydrographic network flow into the lowest areas, forming lakes and wetlands. The aridity of the climate in this area, with low precipitation and high evaporation, means that these still waters become salty.
[Photo: Xavier Ferrer and Adolf de Sostoa]