SAND DUNE

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. 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. 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. any accumulation of sand grains shaped into a mound or ridge by the wind under the influence of gravity. Sand dunes 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 and also occur on the semiarid margins of deserts. True dunes cover much more extensive areasup to several hundred square kilometresprimarily in great sand seas (ergs), some of which are as big as France or Texas. They also occur, however, 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. Additional reading Descriptions and geographic distribution of forms produced by wind deposition are provided in a still-useful classic text, R.A. Bagnold, The Physics of Blown Sand and Desert Dunes (1941, reprinted 1984). More information is given in K.W. Glennie, Desert Sedimentary Environments (1970); Ronald U. Cooke and Andrew Warren, Geomorphology in Deserts (1973); Edwin D. McKee, Carol S. Breed, and Steven G. Fryberger, "Desert Sand Seas, ch. 2 in Skylab Explores the Earth (1977), pp. 548, NASA document sp 380; Carol S. Breed, Maurice J. Grolier, and John F. McCauley, Morphology and Distribution of Common Sand Dunes on Mars: Comparison with the Earth, Journal of Geophysical Research, 84(B14):81838204 (Dec. 30, 1979); Edwin D. McKee (ed.), A Study of Global Sand Seas (1979); Carol S. Breed et al., Eolian (Wind-Formed) Landforms, in Terah L. Smiley et al. (eds.), Landscapes of Arizona: The Geological Story (1984), pp. 359413; and Ronald Greeley and James D. Iversen, Wind as a Geological Process: On Earth, Mars, Venus, and Titan (1985). William J. Breed C.S. Breed