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Deserts and Wind Deserts and Desertification • Deserts – regions with limited precipitation, people, and vegetation. (< 10cm annual preciptaion) • Causes of Natural Deserts – General circulation pattern • 30o Latitude (north or south – air is sinking • dry descending and warm air masses • Warm air can hold abundant water but evaporation rates are high – Topography and prevailing wind patterns can establish rain shadow; moisture extracted on windward slopes of mountain ranges • Air mass is cool and dry at maintain tops, it warms as it descends on leeward side of mountain – Coastal deserts – western coasts of Africa and S. America • Cold ocean currents adjacent to high temperature land areas • Causes of Desertification – Rapid development of desert-like conditions caused by human activity – Major and repeated disturbance to vegetation without complete recovery – Overuse of regional surface and ground water resources Figure 12.3 Figure 9.17 Distribution and causes of dry lands • Dry lands are concentrated in two regions • Subtropics – Low-latitude deserts – In the vicinities of the Tropics of Cancer and Capricorn – Areas of high pressure and sinking air that is compressed and warmed • Middle-latitudes – Located in the deep interiors of continents – High mountains in the path of the prevailing winds produce a rainshadow desert Global distribution of deserts and steppe regions Rainshadow deserts are the result of major mountain ranges Figure 9.27 Geologic processes in arid climates • Weathering • Not as effective as in humid regions • Mechanical weathering produces unaltered rock and mineral fragments • Some chemical weathering in deserts does produce – Clay – Thin soils – Oxidized minerals • Role of water in arid climates • Practically all streambeds are dry most of the time • Desert streams are said to be ephemeral – Carry water only during periods of rainfall – Different names are used for desert streams in various regions » Wash and arroyo (western United States) » Wadi (Arabia and North Africa) » Donga (South America) » Nullah (India) • Desert rainfall – Rain often occurs as heavy showers • Desert rainfall – Because desert vegetative cover is sparse, runoff is largely unhindered and flash floods are common – Poorly integrated drainage systems and streams lack an extensive system of tributaries – Most of the erosional work in a desert is done by running water A dry stream channel in the desert The same stream channel following heavy rain Basin and Range Province: Evolution of a Mountainous Desert Landscape • Characterized by “interior drainage” • Landscape evolution in the Basin and Range region • Uplift of mountains – block faulting • Interior drainage into basins produces – Alluvial fans – Bajadas – Playas and Playa lakes • Ongoing erosion of the mountain mass – Produces sediment that fills the basin – Diminishes local relief – Produce isolated erosional remnants called inselbergs Stages of landscape evolution in a mountainous desert Inselbergs in southern California Copyright © 2006 Pearson Prentice Hall, Inc. • Transportation of sediment by wind • Differs from that of running water in two ways – Wind is less capable of picking up and transporting coarse materials – Wind is not confined to channels and can spread sediment over large areas • Mechanisms of transport – Bedload - sand grains » Saltation – skipping and bouncing along the surface less than 2 m high – Suspended load – silt sized » Easily carried, but not so easily picked up! Figure 12.10 Wind in the desert • Wind erosion • Wind is a relatively insignificant erosional agent with most erosion in a desert performed by intermittent running water • Mechanisms of wind erosion – Deflation » Lifting of loose material » Deflation produces blowouts (shallow depressions) and desert pavement (a surface of coarse pebbles and cobbles) – Abrasion – wearing away of solid object by impacts by wind driven particles. • Mechanisms of wind erosion – Abrasion » Produces ventifacts (stones with flat faces) and yardangs (wind sculpted ridges) – Limited in vertical extent Formation of desert pavement Figure 9.20 A and B • Wind deposits • Significant depositional landforms are created by wind in some regions • Two types of wind deposits – Dunes » Mounds or ridges of sand » Often asymmetrically shaped » Windward slope is gently inclined and the leeward slope is called the slip face » Slow migration of dunes in the direction of wind movement » Several types of sand dunes including barchan, transverse, longitudinal, parabolic and star dunes Sand dune types –Loess • Wind generally does not move sand or coarser particles very rapidly – Fine dust, or silt, can be carried off long distances by the wind and is deposited as loess – Loess can originate in either desert or glacial areas – Loess, once deposited forms a porous and open structure; holds abundant water • Loess does not make a good foundation material – hydrocompaction may cause cracks to form in foundations or structures • Structures may also settle unevenly or collapse »Extensive deposits occur in China – desert origin » and the central United States – glacial origin Figure 12.15