Wind and DesertsChapter 14Deserts•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 lowmoisture, and high relief due to short durationcatastrophic rain with no vegetation to hold soils.• Water is the primary agent of erosion.• Wind is the secondary agent of erosion.Desert ErosionGeography of Deserts• The geographic location of deserts is controlledby descending air masses.• This occurs on the leeward side (rain shadow)of amountain range.• This also occurs at latitudes 30 degrees N and 30degrees S as a result of global air circulation.Rising and Descending Air• Rising air masses expand, cool, andprecipitate moisture.• Descending air masses compress, warm,dry, and evaporate moisture.Rising and Descending AirGeography of DesertsDesert Landforms• Desert regions with sedimentary bedrock developcharacteristic landforms.–A plateau is a broad, level, elevated regioncommonly bounded partly by cliffs.– A mesa is smaller, flat-topped hill bounded bycliffs.– A butte is a narrow hill of resistant rock bounded bycliffs.Desert Landforms:Plateau, Mesa, ButteDesert Landforms:Plateau, Mesa, ButteDesert Landforms• Desert regions with igneous or metamorphicbedrock also develop characteristic landforms with– Zones of erosion which have little or no soil orvegetation, high relief and near total outcrop ofbedrock.–Zones of deposition which have low relief, somesoil development, and sparse vegetation.• Alluvial Fans are fan-shaped deposits of material inzone of deposition.• Playa Lakes are ephemeral (temporary) lakes.Zones of Erosion and DepositionZones of Erosion and DepositionAlluvial FanAlluvial FanAlluvial FanAlluvial FanAlluvial FanAlluvial FanAlluvial FanAlluvial FanPlaya LakePlaya LakePlaya LakePlaya LakeDunes and DeflationDunes and DeflationDunes and DeflationDunes 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 forma dune field.• Pebble and cobble are left behind to form andesert pavement.• Process is called deflation.Dune fieldDune fieldDune fieldDune fieldDunes:Dunes:Dunes:Dunes:BarchanBarchanBarchanBarchanTransverseTransverseTransverseTransverseBlowoutBlowoutBlowoutBlowoutLongitudinalLongitudinalLongitudinalLongitudinal(Linear)(Linear)(Linear)(Linear)BarchanBarchanBarchanBarchan Dunes: Dunes: Dunes: Dunes:Crescent-shaped with limbsCrescent-shaped with limbsCrescent-shaped with limbsCrescent-shaped with limbsdownwind;downwind;downwind;downwind;Formed when sand movesFormed when sand movesFormed when sand movesFormed when sand movesacross outcropacross outcropacross outcropacross outcropBlowout Dunes:Blowout Dunes:Blowout Dunes:Blowout Dunes:((((Parabolic Dunes)Parabolic Dunes)Parabolic Dunes)Parabolic Dunes)Crescent-shaped with limbsCrescent-shaped with limbsCrescent-shaped with limbsCrescent-shaped with limbsupwind; formed when windsupwind; formed when windsupwind; formed when windsupwind; formed when windsbreaks through vegetationbreaks through vegetationbreaks through vegetationbreaks through vegetationTransverse Dunes:Transverse Dunes:Transverse Dunes:Transverse Dunes:Perpendicular to wind direction;Perpendicular to wind direction;Perpendicular to wind direction;Perpendicular to wind direction;formed in regions of abundantformed in regions of abundantformed in regions of abundantformed in regions of abundantsand.sand.sand.sand.Longitudinal Dunes:Longitudinal Dunes:Longitudinal Dunes:Longitudinal Dunes:Parallel to wind direction; formedParallel to wind direction; formedParallel to wind direction; formedParallel to wind direction; formedin regions of limited sand.in regions of limited sand.in regions of limited sand.in regions of limited sand.Loess• Silt and clay-sized particles are depositedby wind in wetter regions.• Loess is a well-sorted deposit of wind-deposited silt and clay sized particles.• Loess is a common peri-glacial
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