deserts!every continent has an extensive dry region!"…deserts are as much as 1/3 of Earth’s surface!Hollywood portrayal of vast stretches of sand dune!"…Sahara has only 10% covered by sand!"…Arabian (sandiest) has only 30% covered by sand!deserts!any region that receives < 25 cm (~10 inches) rain per year!despite this, running water is predominant force in landscape!deserts!flash floods produce most desert erosion!deserts are not “deserted”!vegetation that grows where rain < 25 cm per year!if index is 1.0, precipitation = potential for evaporation!"--area is humid!if index is > 4.0, evaporation is 4 times > precipitation!"--area is a desert!2 driest spots on planet: !Atacama Desert, Peru !and eastern Sahara!aridity index !~200 (hyperarid)!Atacama desert!deserts!aridity index!potential annual evaporation !recorded average annual precipitation!distribution of deserts!deserts can be found anywhere air is dry!global wind patterns!deserts associated!with descending air!at ~30° N or S latitude!(Hadley Cell)!hot, moist air rises in tropics!--loses moistures in atmosphere!--descends as dry air at 30°!image source: http://www.newmediastudio.org!global wind zones!controls are:! • solar heating,! • atmospheric circulation,! • “Coriolis effect” (rotation)!global distribution of rainfall (deserts)!deserts: rain shadows!deserts also form downwind of where moist air rises!"over mountains!characteristics of deserts!streams flow intermittently!• desert streambeds are dry most of the time!• most deserts lack through-flowing streams!• desert regions tend to have internal drainage, such that!"streams drain into land-locked basins!• rain comes from occasional, often violent, thunderstorms!"producing flash flooding!characteristics of deserts!desert washes (arroyos)!• are steep-sided!• have flat sandy floors!• result from rare, but highly erosive flash flooding events!desert landforms of SW United States!2 distinct landscapes!• Colorado Plateau!• Basin and Range!plateaus!• broad, flat-topped areas bounded by cliffs!Colorado plateau!flat-lying sediments that are heavily eroded!…erosion produces characteristic landforms…!mesas!• flat-topped hills bounded by cliffs, capped by resistant rock!buttes!• small, flat-topped hills, capped by resistant rock!Monument Valley, UT (Colorado plateau)!Book Cliffs, CO and UT (Colorado plateau)!Basin and Range!linear mountains separated by flat-floored valleys!…mountains are bounded by faults…!narrow canyons carry sediment to valley floor during rains!Basin and Range!• overlapping and coalescing alluvial fans!• ephemeral, saline lakes that evaporate and leave minerals !plateaus!• broad, flat-topped areas bounded by cliffs!bajadas!alluvial fans!playas!landforms in the Basin and Range!• cone shaped sites of sediment accumulation at mouth of canyon!3 km of relief!Death Valley (Basin and Range)!bajada!playa!alluvial fans!bajada (coalesced alluvial fans)!satellite image!downstream view to alluvial fan and playa!playas!Racetrack Playa, Death Valley!how did the rocks get here?!Devil’s Golf Course!Death Valley!deserts: wind action!important agent for transportation and erosion!…caused by large daily temperature and pressure changes…!dust storms!occur if fine grained sediments are available!dust storm!deserts: wind action!dust suspended by wind!…larger sand grains move by saltation (bouncing)…!formed by sand grains moving in high speed winds!ventifacts!“sand blasting”!…wind blowing in one direction abrades up wind rock faces…!wind!ventifacts!blowout and pillar formation!deserts: wind action!wind picks!up dry fine!sediment!and creates!blowout!vegetation holds!sediment!in place!and forms!pillar!from: http://www.exploroz.com/Journal/DTrip/Simpson/SD_WAABlowouts.jpg!blowout!sand pillar!desert pavement!closely packed stones!left behind by!deflation!deserts: wind action!deflation: loss of fine grained material by wind transport!" and concentraton of coarse-grained particles!deserts: wind action!loess: wind deposited clay and silt sized particles!"…sources are glacial outwash and playas!forms fertile soils that erode easily!Dust Bowl of 1930’s produced by!erosion of loess during drought!Iowa!global distribution of loess deposits!deserts: wind action!sand dunes: mounds of loose sand piled up by wind!• large sand supply!• winds that blow in same direction!sand erodes from and!blows up the!slope facing the wind!sand falls down the!down wind slope!(slipface)!and deposits!(calm air)!dune migrates through time!dunes!types of sand dunes!controlled by dominant wind direction(s) and amount of sand!barchan! parabolic!transverse! longitudinal!types of sand dunes!• crescent shaped!barchan!• horns point down wind!• steep slip face on concave side!one direction of wind and limited sand supply!types of sand dunes!• straight!transverse!• elongate!• steep slip face!one direction of wind and large sand supply!types of sand dunes!• deeply curved!parabolic!• horns anchored by vegetation!• convex in down wind direction (opposite to barchans)!one direction of wind and large sand supply!types of sand dunes!• extremely long, high (100’s of meters), and straight!longitudinal!• regularly spaced!• area between dunes is swept clean of sand!large sand supply -- mechanism not fully understood!• cross winds may be important!• long axis parallel to prevailing wind direction!sand dunes in the rock record!dunes migrate in wind direction, pile up, and lithify!each white line corresponds to dune surface at some point!ancient sand dunes!ancient sand dunes!sand dunes on Mars (barchans)!invasion of desert conditions into non-desert!• significant lowering of water table!• reduction in surface-water supply!• increased salinity in waters and soils!• destruction of native vegetation!• accelerated rate of erosion!rapid in northern and western Africa for last 2000 years!…8000 to 2000 years ago, Namid Desert was fertile grassland!"that supported advanced Stone Age societies…!2 causes:!"1) natural: drought!"2) human-induced: overpopulation and mismanagement!Sahel--region south of Sahara--is hardest hit!desertification!1973! 2000!Dust Bowl of 1930’s!drought and removal of prairie grass for agriculture!…decrease in rainfall to < 20 inches per year!…high winds swept away topsoil (no roots to hold it)!ended as rainfall increased and farm practices