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