Page 556 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

520 part III The Earth–Atmosphere Interface
swell and shrink clay-sized particles. The gravel frag- ments are gradually lifted to surface positions to form the pavement (Figure 16.30c).
Desert pavements are so common that many provin- cial names are used for them—for example, gibber plain in Australia; gobi in China; and in Africa, lag gravels or serir, or reg desert if some fine particles remain. Most desert pavements are strong enough to support human weight, and some can support motor vehicles; but in general these surfaces are fragile. They are also of criti- cal importance, since they protect underlying sediment from further deflation and water erosion.
Eolian Deposition
The smallest features shaped by the movement of windblown sand are ripples, which form in crests and troughs, positioned transversely (at a right angle) to the direction of the wind. Larger deposits of sand grains form dunes, defined as wind-sculpted, transient ridges or hills of sand. An extensive area of windblown sand (usually larger than 125 km2) is an erg (after the Arabic word for “dune field”), or a sand sea.
The Grand Erg Oriental in the central Sahara, active for more than 1.3 million years, exceeds 1200 m in depth and covers 192000 km2. Similar sand seas, such as the Grand Ar Rub’al Khaˉlıˉ Erg, are active in Saudi Arabia. In eastern Algeria, the Issaouane Erg covers 38000 km2 of the Sahara Desert (Figure 16.31a). Extensive dune fields characterise sand seas, which are also present in semi- arid regions such as the Great Sand Hills in southwestern Saskatchewan and the Great Plains of the United States (Figure 16.31b), as well as on the planet Mars (see photos on the MasteringGeography website).
Dune Formation and Movement When saltating sand grains encounter small patches of sand, their kinetic energy (motion) is dissipated and they accumulate. Once the height of such accumulations increases above 30 cm, a slipface and characteristic dune features form. Geosystems in Action, Figure GIA 16, illustrates a dune profile and various dune forms.
A dune usually is asymmetrical in one or more di- rections. Winds characteristically create a gently sloping windward side (stoss side), with a more steeply sloped slipface on the leeward side (Figure GIA 16.1). The angle of a slipface is the steepest angle at which loose material is stable—its angle of repose. Thus, the constant flow of new material makes a slipface a type of avalanche slope: Sand builds up as it moves over the crest of the dune to the brink; then it avalanches, falling and cascading as the slipface continually adjusts, seeking its angle of repose (usually 30° to 34°). In this way, a dune migrates down- wind, in the direction in which effective—that is, sand- transporting—winds are blowing, as suggested by the successive dune profiles in GIA 16.1. (Stronger seasonal winds or winds from a passing storm may prove more effective in this regard than average prevailing winds.)
(a) A typical desert pavement.
Deflation
Wind removes dust
Concentration of larger pebbles
Desert pavement
Time
(b) The deflation hypothesis: Wind removes fine particles,
leaving larger pebbles, gravels, and rocks that become consolidated into desert pavement.
Deflation
Rain washes dust downward
Gravel displacement
upward
Desert pavement
(c) The sediment-accumulation hypothesis: Wind delivers fine particles that settle and wash downward as cycles of swelling and shrinking cause gravels to migrate upward, forming desert pavement.
▲Figure 16.30 Desert pavement. [(a) Bobbé Christopherson.]
Dunes have many wind-produced shapes that make classification difficult. Scientists generally classify dunes according to three general shapes—crescentic (crescent, curved shape), linear (straight form), and massive star dunes. Figure GIA 16.2 shows eight types of dunes that fall within these classes or are a complex mix of these general shapes. The crescentic class in- cludes barchan, transverse, parabolic, and barchanoid