Chapter 15 Wind and Deserts Desert Erosion Water is the primary agent of erosion Wind is a secondary agent of erosion Rising and Descending Air Deserts A desert is any region that receives less than 25cm 10in of rain per year Evaporation equals or exceeds rainfall Boulder gets about 62 cm 24 in of rain per year Deserts tend to have sparse vegetation due to low moisture and high relief due to short duration catastrophic rain with no vegetation to hold soils Geography of Deserts The geographic location of deserts is controlled by descending air masses This occurs on the leeward side rain shadow of a mountain range This also occurs at latitudes 30 degrees N and 30 degrees S as a result of global air circulation Rising and Descending Air Rising air masses expand cool and precipitate moisture Descending air masses compress warm dry and evaporate moisture 1 Rising and Descending Air Geography of Deserts Desert Landforms Desert Landforms Desert regions with sedimentary bedrock develop characteristic landforms Plateau Mesa Butte plateau is a broad level elevated region commonly bounded partly by cliffs A mesa is smaller flat topped hill bounded by cliffs A butte is a narrow hill of resistant rock bounded by cliffs A Desert Landforms Plateau Mesa Butte Desert Landforms Desert regions with igneous or metamorphic bedrock also develop characteristic landforms with Zones of erosion which have little or no soil or vegetation high relief and near total outcrop of bedrock Zones of deposition which have low relief some soil development and sparse vegetation Alluvial Fans are fan shaped deposits of material in zone of deposition Playa Lakes are ephemeral temporary lakes 2 Zones of Erosion and Deposition Zones of Erosion and Deposition Alluvial Fan Alluvial Fan Playa Lake Dunes and Deflation Wind moves sand and silt in zone of deposition Silt and Clay are removed Sand is moved to leeward end of valley to form a dune field Pebbles and cobbles are left behind to form an desert pavement Process is called deflation 3 Dune field Great Sand Dunes Dunes Barchan Transverse Blowout Longitudinal Linear Barchan Dunes Crescent shaped with limbs downwind Formed when sand moves across outcrop Transverse Dunes Perpendicular to wind direction formed in regions of abundant sand Blowout Dunes Parabolic Dunes Crescent shaped with limbs upwind formed when winds breaks through vegetation Longitudinal Dunes Parallel to wind direction formed in regions of limited sand 4 Yardangs Iran Result from wind erosion Desert Pavement Desert Varnish Ventifact is a windfeceted cobble or pebble Loess Silt and clay sized particles are deposited by wind in wetter regions Loess Loess is a well sorted deposit of wind deposited silt and clay sized particles Loess is a common peri glacial deposit 5 Desert Terms Desert Descending air Leeward Plateau Mesa Butte Zone of erosion Zone of deposition Alluvial fan Playa Desert pavement Deflation Yardang Ventifact Dune Barchan Transverse dune Longitudinal dune Blowout Loess 6
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