One of the characteristic landforms of deserts is sand dunes, geometrically regular mounds or ridges of sand found in several geological environments, including deserts and beaches. Most people think of deserts as covered with numerous big sand dunes and continual swirling winds of dust storms. Dunes and dust storms are not as common as depicted in popular movies, and rocky deserts are more common than sandy deserts; for instance, only about 20 percent of the Sahara is covered by sand, and the rest is covered by rocky, pebbly, or gravel surfaces. sand dunes are locally important in deserts, and wind is one of the most significant processes in shaping deserts worldwide. The Institute of Desert Research in Lanzhou, China has recently estimated that in China alone, 950 square miles (2,500 km2) are encroached on by migrating sand dunes from the Gobi Desert each year, costing the country $6.7 billion per year and affecting the lives of 400 million.
Wind moves sand by saltation—an arching path in a series of bounces or jumps. One can see this often by looking close to the surface in dunes on beaches or deserts. Wind sorts different sizes of sedimentary particles, forming elongate small ridges known as sand ripples, similar to ripples found in streams. Sand dunes are larger than ripples (up to 1,500 feet high, or almost 0.5 km), and are composed of mounds or ridges of sand deposited by wind. These may form where an obstacle distorts or obstructs the flow of air, or they may move freely across much of a desert surface. Dunes have many different forms, but all are asymmetrical. They have a gentle slope that faces into the wind and a steep slope that faces away from the wind. Sand particles move by saltation up the windward side, and fall out near the top where the pocket of low-velocity air cannot hold the sand anymore. The sand avalanches, or slips, down the leeward slope, known as the slip face. This keeps the slope at 30°-34°, the angle of repose. The asymmetry of old dunes is used to determine the directions ancient winds blew.
The steady movement of sand from one side of the dune to the other causes the whole dune to migrate slowly downwind (typically about 80-100 feet per year, or 24-30 m/yr), burying houses, farmlands, temples, and towns. Rates of dune migration of up to 350 feet per year (107 m/yr) have been measured in the Western Desert of Egypt and the Ningxia Autonomous Region of China.
A combination of many different factors leads to the formation of very different types of dunes, each with a distinctive shape, potential for movement, and hazards. The main variables that determine a dune's shape are the amount of sand available for transportation, the strength (and directional uniformity) of the wind, and the amount of vegetation that covers the surface. If there is a lot of vegetation and little wind, no dunes will form. In contrast, if there is little vegetation, a lot of sand, and moderate wind strength (conditions that might be found on a beach), then a group of dunes known as transverse dunes form, with their crests aligned at right angles to the dominant wind direction.
Barchan dunes have crescent shapes and horns pointing downwind; they form on flat deserts with steady winds and a limited sand supply. Parabolic dunes have a U-shape with the U facing upwind. These form where there is significant vegetation that pins the tails of migrating transverse dunes, with the dune being warped into a wide U-shape. These dunes look broadly similar to barchans, except the tails point in the opposite direction. They can be distinguished because in both cases, the steep side of the dune points away from the dominant wind direction. Linear dunes are long, straight, and ridge-shaped, elongated parallel to the wind direction. These occur in deserts with little sand supply and strong, slightly variable winds. Star dunes form isolated or irregular hills where the wind directions are irregular.
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