Wind and DesertsDesertsDesert ErosionGeography of DesertsRising and Descending AirRising and Descending AirRising and Descending AirGeography of DesertsDesert LandformsDesert LandformsZones of Erosion and DepositionZones of Erosion and DepositionAlluvial FanBarchan Dunes:Crescent-shaped with limbs downwind; Formed when sand moves across outcropLoessDesert TermsWind and DesertsChapter 15Deserts•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.Desert Erosion•Water is the primary agent of erosion.•Wind is a secondary agent of erosion.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.Rising and Descending AirRising and Descending AirGeography of DesertsDesert Landforms•Desert regions with sedimentarybedrock develop characteristic landforms.–A 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.Desert Landforms:Plateau, Mesa, ButteDesert Landforms:Plateau, Mesa, ButteDesert 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.Zones of Erosion and DepositionZones of Erosion and DepositionAlluvial FanAlluvial FanPlaya LakeDunes 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.Dune field, Great Sand DunesDunes:BarchanTransverseBlowoutLongitudinal(Linear)Barchan Dunes:Crescent-shaped with limbs downwind; Formed when sand moves across outcropBlowout Dunes:(Parabolic Dunes)Crescent-shaped with limbs upwind; formed when winds breaks through vegetationTransverse Dunes:Perpendicular to wind direction; formed in regions of abundant sand.Longitudinal Dunes:Parallel to wind direction; formed in regions of limited sand.Yardangs, IranResult from wind erosionDesert VarnishDesert PavementVentifactis a wind-fecetedcobble or pebbleLoess•Silt and clay-sized particles are deposited by wind in wetter regions.•Loess is a well-sorted deposit of wind-deposited silt and clay sized particles.•Loess is a common peri-glacial deposit.LoessDesert 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
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