Page 554 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

518 part III The earth–atmosphere interface
▲Figure 16.26 Dust storm engulfing Phoenix, Arizona. a massive dust storm known as a haboob passes through Phoenix, arizona, in July 2011. in dry regions such a wall of dust frequently precedes a thunderstorm, with winds travelling in the opposite direction of the oncoming storm. [ross D. Franklin/aP images.]
move approximately one-half tonne of sand per day over a 1-m-wide section of dune.
Eolian Erosion
Erosion of the ground surface resulting from the lifting and removal of individ- ual particles by wind is deflation. Wher- ever wind encounters loose sediment, deflation may remove enough material to form depressions in the landscape ranging in size from small indentations less than a metre wide up to areas hun- dreds of metres wide and many metres deep. The smallest of these are known as deflation hollows, or blowouts. They commonly occur in dune environments where winds remove sand from specific areas, often in conjunction with the re- moval of stabilizing vegetation (possibly by fire, by grazing, or from drought). Large depressions in the Sahara Desert
associated with thunderstorms can cause dramatic dust storms (Figure 16.26) consisting of fine particles that in- filtrate even the smallest cracks of homes and businesses.
Eolian processes transport particles larger than about 0.2 mm along the ground by saltation, the bounc- ing and skipping action. About 80% of wind transport of particles is accomplished by saltation (Figure 16.25). In fluvial transport, saltation is accomplished by hydraulic lift; in eolian transport, saltation occurs by aerodynamic lift, elastic bounce, and impact with other particles (com- pare Figure 16.25a with Figure 15.13).
Particles too large for saltation slide and roll along the ground surface, a type of movement called surface creep. Saltating particles may collide with sliding and rolling particles, knocking them loose and forward in this process, which affects about 20% of the material trans- ported by wind. In a desert or along a beach, sometimes you can hear a slight hissing sound, almost like steam es- caping, produced by the myriad saltating grains of sand as they bounce along and collide with surface particles. Once particles are set in motion, the wind velocity need not be as high to keep them moving.
British Army Major Ralph Bagnold, an engineering officer stationed in Egypt in 1925, pioneered studies of wind transport and authored a classic work in geomor- phology, The Physics of Blown Sand and Desert Dunes, published in 1941. One result of Bagnold’s work was a graph showing the rate at which the amount of trans- ported sand over a dune surface increases with wind speed (Figure 16.27). The graph shows that at lower wind speeds, sand moves only in small amounts; however, be- yond a wind speed of about 30 km · h−1, the amount of sand moved increases rapidly. A steady wind of 50 km · h−1 can
are at least partially formed by deflation but are also af- fected by large-scale tectonic processes. The enormous Munkhafad el Qattâra (Qattâra Depression), which cov- ers 18000 km2 just inland from the Mediterranean Sea in the Western Desert of Egypt, is now about 130 m below sea level at its lowest point.
The grinding and shaping of rock surfaces by the “sandblasting” action of particles captured in the air
1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
010 20 30 40 50 60 70 Wind speed (km·h–1)
▲Figure 16.27 Sand movement and wind velocity. Sand move- ment relative to wind velocity, as measured over a metre cross section of ground surface. [Created from data in The Physics of Blown Sand and Desert Dunes, by r.a. Bagnold, © 1941, 1954 (Methuen and Co., 1954).]
Rate of sand movement
(tons per day per metre of dune cross section)