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Chapter 15 The Work of Wind and Deserts Introduction What is desertification? Desertification is the expansion of deserts into formerly productive lands. These expansions destroy croplands and rangelands. Can cause massive starvation and even force hundreds of thousands of people from their homelands Introduction By understanding how desert processes operate people can take steps to reduce the spread of desertification and its effects Overgrazing and improper cultivation destroys vegetation and soils, and can result in desertification. Desertification is also closely tied to global warming. We will also be able to better understand and deal with present-day environmental changes. Sediment Transport by Wind Sediment may be transported by winds as: bed load (by the process of saltation) or suspended load Fig. 15.1, p. 367 Sediment Transport by Wind Bed load is the material that is too large or heavy to be carried in suspension by water or wind. Particles move along the surface by saltation, rolling, or sliding. Fig. 15.1, p. 367 Sediment Transport by Wind Suspended load - material that can be carried in suspension by water or wind. Suspended load is composed of silt- and clay-sized particles They can be carried thousands of kilometers Wind Erosion Material is eroded by wind either through abrasion or deflation. Abrasion is the impact of saltating sand grains on an object. Its effect is similar to sandblasting. Fig. 15.2 p. 367 Wind Erosion Ventifacts are rocks whose surfaces have been polished, pitted, grooved, or faceted by the wind abrasion. Fig. 15.3 p. 368 Wind Erosion Deflation is the removal of loose surface material by wind. Desert pavement and deflation hollows are common features of deserts resulting from differential erosion by deflation. Fig. 15.5, p.369 Wind Erosion Deflation The formation of desert pavement prevents further deflation. Fig. 15.3b, p. 368 Fig. 15.6, p. 369 Wind Deposits The wind is responsible for two important desert deposits, sand dunes and loess. The Formation and Migration of Dunes Fig. 15.7, p. 370 Wind Deposits The Formation and Migration of Dunes Dunes are mounds or ridges of wind-deposited sand. They form when wind flows over and around an obstruction, resulting in the deposition of sand grains, which accumulate and build up a deposit of sand. Fig. 15.7, p. 370 Wind Deposits The Formation and Migration of Dunes Most dunes have an asymmetric profile, with a: gentle windward slope steeper downwind, or leeward, slope that is inclined in the direction of the prevailing wind Fig. 15.7, p. 370 Wind Deposits The Formation and Migration of Dunes Dunes migrate by sand moving up and over the gentle windward slope by saltation and accumulating and sliding down the windward side. Fig. 15.8, p. 370 Wind Wind Direction of dune migration Sand moves by saltation Windward side a. Profile of a sand dune. Leeward slope b. Dunes migrate when sand moves up the windward side and slides down the leeward slope. Such movement of the sand grains produces a series of crossbeds that slope in the direction of wind movement. Stepped Art Fig. 15-8, p. 370 Wind Deposits Dune Types The five major dune types are: Barchan Longitudinal Transverse Parabolic Star Wind Deposits Dune Types Barchan Dunes - Crescentshaped dunes whose tips point downwind Fig. 15.10, p. 371 Wind Deposits Dune Types Barchan Dunes form in the areas that have a generally flat, dry surface with: little vegetation a limited supply of sand a nearly constant wind direction Fig. 15.10b, p. 371 Wind Deposits Dune Types Longitudinal Dunes - Long, parallel ridges of sand aligned generally parallel to the direction of the prevailing winds They form where the sand supply is somewhat limited. Fig. 15.11, p. 372 Wind Deposits Dune Types Transverse Dunes - Form long ridges perpendicular to the prevailing wind direction Fig. 15.12, p. 372 Wind Deposits Dune Types Transverse Dunes - Found in areas that have: abundant sand little or no vegetation Fig. 15.12, p. 372 Wind Deposits Dune Types Parabolic areas Dunes - Common in coastal Characteristics: abundant sand strong onshore winds partial cover of vegetation Fig. 15.13, p. 373 Wind Deposits Dune Types Star Dunes - huge pyramidal hills of sand Characteristics: variable wind direction do not migrate, form desert landmarks used by the nomadic people for centuries Fig. 15.14, p. 373 Wind Deposits Loess - is wind-blown silt and clay deposits composed of angular quartz grains, feldspar, micas, and calcite Loess is derived from deserts, Pleistocene glacial outwash deposits, and river floodplains in semiarid regions. Loess covers approximately 10% of Earth’s land surface and weathers to a rich, productive soil. Fig. 15.15, p. 373 Air-Pressure Belts and Global Wind Patterns The global wind patterns are determined by air-pressure belts and the Coriolis effect. Fig. 15.16, p. 374 Air-Pressure Belts and Global Wind Patterns Air flows from high-pressure to low-pressure zones. Moisture air rises at the equator, in the doldrums, producing equatorial rains as it rises and condenses. Fig. 15.16, p. 374 Air-Pressure Belts and Global Wind Patterns The doldrums and the horse latitudes are areas of very little to no wind because air is moving vertically. Fig. 15.16, p. 374 Air-Pressure Belts and Global Wind Patterns Dry air falls at the horse latitudes and returns to the equator. As the air returns to the equator, it moves horizontally, producing wind, the tradewinds. Fig. 15.16, p. 374 Air-Pressure Belts and Global Wind Patterns The winds are deflected as the earth rotates due to the Coriolis effect. Fig. 15.16, p. 374 Air-Pressure Belts and Global Wind Patterns The winds are deflected clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Fig. 15.16, p. 374 The Distribution of Deserts Dry climates occur in the low-middle latitudes. They occur where the loss of water by evaporation is greater than the annual precipitation. Dry climates cover 30% of the Earth’s land surface. Dry climates are subdivided into semiarid and arid regions. Semiarid regions receive more precipitation, yet they are moderately dry and support grasslands. The Distribution of Deserts The majority of the world’s deserts are located in the between 20 and 30 degrees north and south latitudes. Their dry climate results from a high-pressure belt of descending dry air. Examples: Southwest U.S. and Mexico, Sahara, Arabian Peninsula, Australia, Atacama (Chile and Peru) Fig. 15.17, p. 375 The Distribution of Deserts The remaining deserts are in the middle latitudes, where their distribution is related to the rainshadow effect, and in the dry polar regions. Examples: Gobi and Central Eurasian, Great Basin (U.S.) Fig. 15.17, p. 375 The Distribution of Deserts Rain shadow effect Many middle and high latitude deserts are located far from the coasts. Often, mountain ranges block the flow of moist marine air creating a “rainshadow”. The Himalayas block the Gobi desert, and the Sierra Nevadas block the Great Basin. Fig. 15.18, p. 375 Moist marine air Warm dry air Rainshadow desert Stepped Art Fig. 15-18, p. 375 Characteristics of Deserts Temperature, Precipitation, and Vegetation Most deserts are characterized by: high temperatures (except Polar) little precipitation and sparse plant cover Fig. 15.19, p. 376 Characteristics of Deserts Weathering and Soils Mechanical weathering is the dominant form of weathering and, coupled with slow rates of chemical weathering, results in poorly developed soils. Fig. 15.20, p. 377 Characteristics of Deserts Mass Wasting, Streams, and Groundwater Running water is the major agent of erosion in deserts, with most streams being poorly integrated and flowing intermittently. Characteristics of Deserts Wind Wind, though secondary to water as an erosional agent in deserts, is still capable of producing a variety of distinctive erosional and depositional features. Wind forms sand deposits Important alternate energy source Fig. 15.4, p. 368 Desert Landforms Major landforms of deserts Important desert landforms include Playa lakes and playas Alluvial fans and bajadas Pediments Inselbergs Buttes and mesas Desert Landforms Playas are dry lakebeds characterized by mud cracks and precipitated salt crystals. When temporarily filled with water following a rainstorm, they are known as playa lakes. Fig. 15.21, p. 380 Desert Landforms Alluvial fans are fan-shaped sedimentary deposits. They form when sediment-laden streams flow out from mountain fronts and deposit their load on the relatively flat desert floor. Coalescing alluvial fans form bajadas Fig. 15.22, p. 380 Desert Landforms Pediments are erosional bedrock surfaces Low relief Slope gently away from mountain bases Fig. 15.23, p. 381 Desert Landforms Inselbergs Isolated, steep-sided erosional remnants that rise above desert plains Uluru (Ayers Rock) of Australia Fig. 15.24, p. 381 Desert Landforms Buttes and mesas - flat-topped erosional remnants with steep slopes Buttes – pillar-like Mesas – table-like Fig. 15.25, p. 382 End of Chapter 15
