..............hill, mound, or ridge of sand or other loose material that is formed by wind action. The existence of dunes is a direct function of the ability of wind to transport unconsolidated material. Dunes are commonly associated with desert regions where windblown sand occupies extensive areas. It has been estimated, for example, that sand deposits in the Sahara (desert) cover about 7,000,000 square km (2,700,000 square miles). In the recent geological past, desert areas may have been even larger during dry periods in the Pleistocene glaciation. At that time great areas of loess (windblown silt) were deposited across North America, Europe, and Asia. Dunes are also associated with coasts where beach sands may be reworked by the wind.
A brief treatment of sand dunes follows. For full treatment, see Continental Landforms: Sand Dunes.
The term desert can also apply to very cold regions. Thus large areas of dunes are found in nonglacial Antarctica. Although sand grains are the primary constituents of dunes, the wind can transport and deposit clay particles or other unconsolidated materials, should the right conditions prevail.
Dune sand is moved in two ways. First, by the process of saltation, the wind lifts up sand grains and carries them for a short distance before dropping them. If the sand is being blown across a stony surface, the grains may bounce up to a height of a few metres. Otherwise they move only a few centimetres above the ground surface. The second mode of movement occurs when the saltating grains hit the ground again and by the force of their impact push other grains forward. This movement is termed surface creep.
The simplest way in which a dune forms is when an obstacle, be it a rock or a plant, impedes airflow and causes sand to pile up on its downwind side, rather in the way that a snowdrift forms. Gradually the dune grows, presenting even more of an obstruction to the wind-bearing sand and catching saltating grains in the leeward wind shadow.
As the dune becomes larger it begins to move slowly downwind and adopts a more asymmetrical shape. As the dune intrudes more and more into the airflow, the wind speed is actually increased on the dune's windward side, and saltating grains are moved upward and over the dune crest, where they fall on the upper portion of the leeward slope, creating a steeper slip face. Dune sand grains tend to have diameters of less than l mm (0.04 inch) and an angle of repose of about 35 degrees. When the steepening upper section of the slip face reaches or passes this angle, the slope gradually becomes unstable. Eventually the sand slides down the slip face and the dune advances.
The initiation of dunes by obstacles does not explain how dunes form on smooth, level surfaces and build up sand seas consisting of regular mounds. One suggestion is that such dune formation results from frictional drag between the air and the ground and that the dunes form in much the same way that sand ripples do on a riverbed or beach.
A basic distinction is made between barchan and seif dunes. The barchan is one of the classic desert landforms. It is a crescent-shaped dune with the horns of the crescent stretching out in the leeward direction. Barchan dunes may reach more than 27 m (90 feet) in height. It has been suggested that barchans approach an equilibrium form in which the length and width are equal and height is approximately a tenth of this dimension.
Seif dunes are long ridges of sand. In general they are aligned in the direction of the prevailing wind. The slip face of such dunes are probably formed by eddies. The depressions between seif dune ridges are swept clear of sand by the winds. The ridges run for long distances, sometimes several kilometres. Their height appears to be about one-sixth of the width of the dune's base.
Although barchan and seif dunes are the main types, other forms of dune are recognized. In areas where there is plenty of sand available, barchans may coalesce to form a "sea" of transverse dunes, in which the crescentic ridges are not so obvious. Parabolic dunes form where vegetation cover has been broken and the wind hollows out a depression. Although they are roughly crescentic in plan, the horns trail to windward and the slip face is on the outside of the crescent. As they move downwind they create a hairpin shape. A final dune form, noted in the Middle East and the Sahara, is the star, or pyramidal, dune. Its shape in plan is of a many-pointed star with steep ridges rising from the points to the central summit. It has been suggested that such sand mountains do not move and that they have come to serve as landmarks for generations of desert travellers.
· Sands.
· Winds.
· FORMATION AND GROWTH OF DUNES
· FIXED DUNES IN SEMIARID REGIONS
Sand dunes are accumulations of sand grains shaped into mounds or ridges by the wind under the influence of gravity. They are comparable to other forms that appear when a fluid moves over a loose bed, such as subaqueous "dunes" on the beds of rivers and tidal estuaries and sand waves on the continental shelves beneath shallow seas. Dunes are found wherever loose sand is windblown: in deserts, on beaches, and even on some eroded and abandoned farm fields in semiarid regions, such as northwest India and parts of the southwestern United States. Images of Mars returned by the U.S. Mariner 9 and Viking spacecrafts have shown that dunes are widely distributed on that planet both in craters and in a sand sea surrounding the north polar ice cap.
True dunes must be distinguished from dunes formed in conjunction with vegetation. The latter cover relatively small areas on quiet humid coastlands (see below Beaches and Coastal Dunes) and also occur on the semiarid margins of deserts. True dunes cover much more extensive areas--up to several hundred square kilometres--primarily in great sand seas (ergs), some of which are as big as France or Texas. However, they also occur as small isolated dunes on hard desert surfaces, covering an area of as little as 10 square metres (107 square feet). Areas of gently undulating sandy surfaces with low relief are classified as sand sheets. They commonly have a nearly flat or rippled surface of coarse sand grains and are only a few centimetres to metres thick. Minor sand sheets cover only a few square kilometres around the margins of dune fields. A few, such as the Selima Sand Sheet in southwestern Egypt and the northwestern Sudan, are probably almost as extensive as some of the great sand seas.
During the last 2,000,000 years or so the conditions of very low rainfall under which true dunes form expanded beyond the margins of the Sahara and other present-day arid regions into areas that are now more humid. The best evidence for these changes is the presence of sand seas that are immobilized by vegetation. Dunes formed under similar climates in the geologic past and at certain times occupied deserts as extensive as modern ones. Rocks formed by the solidification of ancient sand seas occur, for example, in the walls of the Grand Canyon in the southwestern United States; in the west Midlands of England; and in southern Brazil.
An understanding of sand dunes requires a basic knowledge of their sands, the winds, and the interactions of these main elements. These factors will be treated in turn in the following sections.
Dunes are almost invariably built of particles of sand size. Clay particles are not usually picked up by the wind because of their mutual coherence, and if they are picked up they tend to be lifted high into the air. Only where clays are aggregated into particles of sand size, as on the Gulf Coast of Texas, will they be formed into dunes. Silt is more easily picked up by the wind but is carried away faster than sand, and there are few signs of dunelike bed forms where silt is deposited, for instance as sheets of loess. Particles coarser than sands, such as small pebbles, only form dunelike features when there are strong and persistent winds, as in coastal Peru, and these coarse-grained features are generally known as granule ripples rather than dunes. Larger particles, such as small boulders, can be moved by the wind only on slippery surfaces (e.g., ice or wet saline mud) and never form into dunes.
Common dune sands have median grain diameters between 0.02 and 0.04 centimetre (0.008 and 0.016 inch). The maximum common range is between 0.01 and 0.07 centimetre. Most dune sands are well sorted, and a sample of sand from a dune will usually have particles all of very similar size. The sand on sand sheets, however, is poorly sorted and often bimodal--i.e., it is a mixture of coarse sands, often about 0.06 centimetre in diameter, and much finer sands, as well as particles of intermediate size. Windblown sands, especially the coarser particles, are often rounded and minutely pitted, the latter giving the grains a frosted appearance when seen under a microscope.
Most windblown sand on the Earth is composed of quartz. Quartz exists in large quantities in many igneous and metamorphic rocks in crystals of sand size. It tends to accumulate when these rocks are weathered away because it resists chemical breakdown better than most minerals, which are taken away in solution. Most of the great sand seas occur in continental interiors that have been losing soluble material for millions of years; as a consequence, quartzose sandstones are common. These sandstones are eroded by rainwash and stream runoff, processes that are spasmodic but violent in deserts. The eroded products are transported to great interior basins where they are deposited. Such alluvial deposits are the sources of most windblown sand. Quartz also predominates in most coastal dune sands, but there usually are considerable mixtures of other minerals in dunes of this kind.
Dune sands not composed of quartz are rarer but not unknown. Near volcanic eruptions in Hawaii, some western states of the continental United States, and Tanzania, for example, dunes are built of volcanic ash particles. In many arid areas, gypsum crystals of sand size are deposited on the floors of ephemeral lakes as the water dries out; they are then blown like sand to form gypsum dunes. Gypsum dunes occur in the White Sands National Monument in New Mexico, as well as in northern Algeria and southwestern Australia.
Winds have three sources of variation that are important--namely, direction, velocity, and turbulence. Most of the great deserts are found in the subtropical areas of high atmospheric pressure, where the winds circulate in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. The high-pressure systems tend to dip down to the east so that winds are stronger there, a pattern mirrored by the dunes. Poleward of these circulation systems are the zones of eastward moving depressions in which there are generally westerly winds that mold the dunes of the North American and Central Asian deserts and of the northern Sahara. The boundaries between these two circulation systems migrate back and forth seasonally, so that complicated dune patterns are found in the zones of overlap. Only a few deserts, notably the Thar Desert of India and the Sonoran Desert of the American Southwest, are affected by monsoonal wind systems. Some dunes are built by sea breezes and local winds, as in coastal Peru.
The direction of the wind at any one place in the desert is affected by a number of local factors. Winds are particularly channeled around topographical features, such as the Tibesti Massif in the Sahara, so that dunes are affected by different winds on different sides of the obstruction. Winds also can be channeled around the dunes themselves, thereby developing patterns of secondary flow that modify the shapes of the dunes.
The pattern of wind velocity also is important. Like many natural phenomena, wind velocities have a log-normal distribution: there are a large number of moderate breezes and a diminishing number of increasingly more violent winds. The greatest volumes of sand are probably moved by unusually strong winds, because the amount of sand moved by wind is a power function (exponential factor) of the wind speed. For example, a 10-kilometre-per-hour wind carries 13 grams per hour (0.39 ounce per hour), a 20-kilometre-per-hour wind carries 274 grams per hour, and a 30-kilometre-per-hour wind carries 1,179 grams per hour. A wind of a particular velocity will move fewer larger than smaller grains. Strong winds often blow from a particular direction, as in the southern Sahara, where the intense winds of sandstorms come predominantly from one direction. Such winds are responsible for the undulations of the sand sheets, because they alone can move coarse sands. Lighter winds blow from several different directions, and the dunes, being of finer sand, are therefore affected by several winds.
The wind is retarded near the surface by friction. Above the ground the wind velocity increases rapidly. The near-surface velocity must rise above a certain threshold value before sand will be picked up, the value depending on the size of the sand grains; for example, a wind of 12 kilometres per hour measured at a height of 10 metres is required to move sands 0.02 centimetre in diameter, and a 21-kilometre-per-hour wind is required to move 0.06-centimetre sands. Once sand movement has been initiated by wind of such velocity, it can be maintained by winds blowing at lower speeds. Because instantaneous wind speeds in eddies can rise well above the average velocity, turbulence also is important, but it is difficult to measure.
The dune-forming process is complex, particularly where many thousands of dunes have grown side-by-side in sand seas. Yet, an introductory account can be given based on the example of a single dune on a hard desert surface.
Most of the sand carried by the wind moves as a mass of jumping (saltating) grains; coarser particles move slowly along the surface as creep and are kept in motion partly by the bombardment of the saltating grains. Saltating sand bounces more easily off hard surfaces than off soft ones, with the result that more sand can be moved over a pebbly desert surface than over a smooth or soft one. Slight hollows or smoother patches reduce the amount of sand that the wind can carry, and a small sand patch will be initiated. If it is large enough, this patch will attract more sand.
The wind adjusts its velocity gradient on reaching the sand patch; winds above a certain speed decrease their near-surface velocity and deposit sand on the patch. This adjustment takes place over several metres, the sand being deposited over this distance, and a dune is built up. The growth of this dune cannot continue indefinitely. The windward slope is eventually adjusted, so that there is an increase in the near-surface velocity up its face to compensate for the drag imposed by the sandy surface. When this happens, the dune stops growing and there is no net gain or loss of sand.
As the dune grows, the smooth leeward slope steepens until the wind cannot be deflected down sharply enough to follow it. The wind then separates from the surface leaving a "dead zone" in the lee into which falls the sand brought up the windward slope. When this depositional slope is steepened to the angle of repose of dry sand (about 32 degrees), this angle is maintained and the added sand slips down the slope or slip face. When this happens, the dune form is in equilibrium, and the dune moves forward as a whole, sand being eroded from the windward side and deposited on the lee.
If the regional rate of sand flow can be calculated from measurements of wind speed and direction, and if it is assumed that the dune has a simple cross section that migrates forward without change of form, a formula for the rate of movement of a dune that agrees with actual measurements can be derived. In Peru dunes have been observed to move at 30 metres per year; in California rates of 25 metres per year have been measured; and in the al-Kharijah Oases (or Kharga Depression) in southern Egypt dunes have been reported to move 20 to 100 metres per year, depending on dune size (in general, small dunes move faster than large dunes because their smaller cross-sectional area requires less sand to be transported to reconstitute their form one dune-length downwind).
If the wind were a homogeneous stream of air blowing from one constant direction, long straight dune ridges oriented at right angles to the wind would result. Most dunes, however, are neither straight nor at right angles to the wind, and this indicates that the winds are not a uniform stream or that they blow from different directions. The fairly uniform geometric shapes of several basic types of dunes can be recognized from desert to desert on Earth, and some of the same types have been identified on Mars as well.
Barchan dunes are common to both the Earth and Mars. These small crescent-shaped sand bodies occur in areas where the regional wind blows consistently from one direction (Figure 29). Their crescentic shape must be due to spatial variations in wind velocity, and the regular repetition of dune shapes and spacings when they are close together indicate that the variations in the wind are also regular. This is a property common to all bed forms. It is thought that the flow of a fluid arranges itself in long spiral vortices parallel to the direction of flow, which, with zones of faster and slower velocities arranged transverse to the flow, gives a regular sinuous pattern on the bed.
Where there is a continuous sand cover, a varied dune pattern results from the pattern of flow. The main forms are transverse ridges composed of alternating crescentic elements, like barchans, facing downwind, and other crescentic elements facing upwind. These enclose between them a regular pattern of small hollows. Superimposed on this are small straight ridges parallel with the flow. These elements form a network pattern that is extremely common in the great sand seas. The dunes commonly reach a height of nearly 200 metres and are spaced hundreds of metres to more than two kilometres apart.
One of the important features of sandy terrains is that their forms occur in a number of distinct sizes. Large features are covered with smaller ones, and the smaller ones are covered with ripples (Figure 30). In most of the larger sand seas there is usually a network pattern of very large dunes known as compound dunes, mega-dunes, or draa. These are sometimes arranged parallel to the apparent flow, in long ridges, and occasionally transverse to it in great sand waves. The compound dunes are usually covered with a smaller, secondary dune pattern, and the smaller dunes with ordinary sand ripples in most cases. Within each of the size groups of the hierarchy (ripples, dunes, or compound dunes) there are variations in size depending on the grain size of the sand and wind velocity; for example, whereas most ripples are spaced only a few centimetres apart, "mega-ripples," built in very coarse sand, are spaced almost as far apart as small dunes; and whereas most dunes are about 100 metres apart, the low undulations of coarse sand on sand sheets are up to 500 metres apart. The relation between sand grain size and the shape of a dune is not, however, one of simple cause and effect, for the relation is not constant in all dunes of a given shape or in all localities.
Some dune forms can be related to variations in the overall wind direction, usually on a seasonal cycle. In some areas, winds from opposed directions blow during different seasons, so that "reversing dunes" are formed, in which the slip faces face first in one direction and then in the other. Distinct dunes are formed around topographic obstructions and in sheltered zones on the lee of small hills into which the sand migrates. If the wind meets a high scarp or large hill massif, a so-called echo dune is deposited on the upwind side separated from the scarp by a rolling eddy of air that keeps a corridor free of sand. Many oases and routeways are found in this kind of corridor. Echo dunes are among the largest dunes in the desert, sometimes reaching a height of more than 400 metres.
Dunes also form around plants in the desert where groundwater is available for vegetation. The usual dune forms that occur in such instances are isolated mounds around individual plants. These forms are known as coppice dunes, or nebkha. Further, in many regions that are now subhumid or humid, one finds areas of older dunes fixed by vegetation, providing undeniable evidence that these regions were once more arid than they are today. On the North American high plains, in Hungary, and in Mongolia, the fixed sands have a cover of rich grassland. In Poland they are covered with coniferous forests. The dune patterns on these fixed sands bear a close resemblance to those in active sand seas, except that their forms are rounded and subdued. (A.Wa./W.J.Br./C.S.Br.)