aeolian process and landforms desert landscapes aeolus, greek god of the winds
TRANSCRIPT
Aeolian Process and LandformsDesert Landscapes
Aeolus, Greek god of the winds
Wind as a geomorphic agent
requires fine grained sediment and little vegetation
10 -20 in. precip
< 10 in. precip12 consecutive months with no precip
Arid and Semi-arid
EgyptLibya
Tunisia
Algeria
Morocco
Wind Erosion
• Deflation– removal of loose particles by wind– max effectiveness when grain size = 100m– silt and clay size move most easily and carried in
suspension by turbulence– takes a long time to move sand
• Abrasion– bombarding of rock by airborne particles– natural “sandblasting”– sand-sized most effective– uncommon > 2m. above ground
Making the Sahara Desert
• http://www.sciencefriday.com/videos/watch/10070
by Salt Weathering
Arches NP: Arch Formation
Arches N.P.• Greatest concentration of
natural rock arches in the world
• Arches form by– (Salt) weathering of vertical
joints, in Entrada sandstone, produced by folding
– Salt-cored anticlines undergoing dissolution
– Exfoliation weathering• Sand grains are cemented
by CaCO3• Rain forms carbonic acid
dissolving bond
– Wind and water erosion exploit joints
– Groundwater sapping
Arch formation
Wind Erosion
• Deflation– Variations in
lithology or cementation enable preferential wind scour
– Desert pavement
• Abrasion– Ventifacts
Wind Transport• Saltation
– bounce off the ground and other grains
– effective < 2m. above ground
– powers the remaining 3 processes
• Reptation– impacting grain releases
shower of particles• Suspension
– silt and clay-sized– travel around the world
• Creep or dry ravel– individual particles (sand
and pebble size) roll and slide by momentum imparted by an impacting particle
Wind deposition
• Where does the “dust” go?– 90% on land; 10% in oceans
• Sedimentation– Grains fall to ground– Air velocity decreases– Air currents bring silt/clay sizes near ground
• Accretion– Where saltating grains come to rest
• Encroachment– Creep is stopped by surface roughness
Aeolian Forms: Transport by Suspension
• Loess– Grain size: 20-40 microns– Accumulation rates: fractions of mm’s/yr
• Mixed grain sizes are needed to entrain dust
• Large saltating grains disrupt the laminar sublayer to entrail dust
• Dust sources: alluvial fans/rivers, glacial outwash plains
• Thickness declines with distance from source
Chinese Loess• 247,000 mi2 loess plateau
• Up to 300m thick
• Source: deserts of interior China and Tibet http://www.clw.csiro.au/ReVegIH/Maps.htm
Loess Strat
• Soils separated by unweathered silt
• Used to correlate with deep sea records of paleoclimate
• Pleistocene– windier and drier
glacial Interglacial, odd numbers