Deserts Aeolian Processes Distribution and causes of dry

Deserts & Aeolian Processes

Distribution and causes of dry lands • Dry regions cover 30 percent of Earth’s land surface • Two climatic types are commonly recognized • Desert or arid • Steppe or semiarid

WIND AND WAVES AOLIAN: WIND LANDFORMS COASTAL LANDFORMS

WIND EROSIAN • SALTATION: SAND GRAINS MOVED BY THE WIND IN A SERIES OF JUMPS AND BOUNCES.

WIND EROSIAN • DEFLATION: THE PROCESS BY WHICH WIND REMOVES THE TOP LAYER OF FINE VERY DRY SOIL OR ROCK PARTICLES AND LEAVE BEHIND LARGE ROCK PARTICLES. THESE PARTICLES OFTEN FORM A DENSLY PACKED SURFACE CALLED DESERT PAVEMENT

Wind Erosion • Two forms of wind erosion – Deflation • Removal of material by wind transport – Abrasion • Mechanical weathering of rock by wind blown sand

DEFLATION PROCESS

deserts: wind action blowout and pillar formation wind picks up dry fine sediment and creates blowout vegetation holds sediment in place and forms pillar

DEFLATION AND DESERT PAVEMENT

Wind Erosion • Two forms of wind erosion – Deflation • Removal of material by wind transport – Abrasion • Mechanical weathering of rock by wind blown sand

WIND EROSIAN • VENTIFACTS: ROCKS PITTED, GROOVED OR POLISHED BY WIND ABRASION

ventifacts …wind blowing in one direction abrades up wind rock faces… wind

deserts: wind action dust suspended by wind …larger sand grains move by saltation (bouncing)… ventifacts formed by sand grains moving in high speed winds “sand blasting”

Wind in the desert • Wind deposits • Two types of wind deposits – Dunes » Slow migration of dunes in the direction of wind movement » Several types of sand dunes including barchan, transverse, longitudinal, parabolic and star dunes

dunes

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

WIND DEPOSITION • DUNES: MOUNDS OF WIND DEPOSITED SAND • TYPES: BARCHAN: CRESCENT SHAPED DUNES • PARABOLIC DUNES: U SHAPED DUNES FORM WHEN PLANTS ANCHOR THE EDGES OF BARCHAN DUNES CAUSING AN INVERTED U SHAPE

types of sand dunes barchan • crescent shaped • horns point down wind • steep slip face on concave side one direction of wind and limited sand supply

types of sand dunes parabolic • deeply curved • horns anchored by vegetation • convex in down wind direction (opposite to barchans) one direction of wind and large sand supply

types of sand dunes controlled by dominant wind direction(s) and amount of sand barchan parabolic transverse longitudinal

ancient sand dunes

Sand dune migration

WIND DEPOSITION • LONGITUDINAL DUNES: LIE PARALLELL TO THE WIND • TRANSVERSE DUNES: LIE PERPENDICULAR TO THE WIND (THESE LOOK LIKE WIND RIPPLES)

types of sand dunes longitudinal • extremely long, high (100’s of meters), and straight • long axis parallel to prevailing wind direction • regularly spaced • area between dunes is swept clean of sand • cross winds may be important large sand supply -- mechanism not fully understood

types of sand dunes transverse • straight • elongate • steep slip face one direction of wind and large sand supply

• 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

• Wind deposits – Loess » Blankets of windblown silt » Two primary sources are deserts and glacial outwash deposits » Extensive deposits occur in China and the central United States

sand pillar

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

global distribution of loess deposits

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