Deserts & AeolianProcesses
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
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
blowout and pillar formationdeserts: wind action
wind picksup dry finesediment
and createsblowout
vegetation holdssedimentin place
and formspillar
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
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…wind blowing in one direction abrades up wind rock faces…
wind
ventifacts
deserts: wind actiondust suspended by wind…larger sand grains move by saltation (bouncing)…
formed by sand grains moving in high speed winds
ventifacts
“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 actionsand dunes: mounds of loose sand piled up by wind
• large sand supply• winds that blow in same direction
sand erodes from andblows up the
slope facing the wind
sand falls down thedown 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
• crescent shapedbarchan
• horns point down wind• steep slip face on concave side
one direction of wind and limited sand supply
types of sand dunes
• deeply curvedparabolic
• horns anchored by vegetation• convex in down wind direction (opposite to barchans)
one direction of wind and large sand supply
types of sand dunescontrolled 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
• extremely long, high (100’s of meters), and straightlongitudinal
• 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
types of sand dunes
• straighttransverse
• 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 actionloess: 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 byerosion of loess during drought
global distribution of loess deposits
deserts: wind actionimportant agent for transportation and erosion
…caused by large daily temperature and pressure changes…
dust stormsoccur if fine grained sediments are available
dust storm