Deserts and Wind Processes

1. Deserts and Wind Processes • Definition : an area that receives < 10” rain annually and is sparsely populated • Desert - arid • Steppe – semiarid and usually surrounding desert • Aridity index : ratio of potential annual evaporation/potential annual precipitation • Temperature or elevation don’t factor into equation • Cover ~30% of Earth’s land surface

2. Types of Deserts • Subtropical : between 20 and 30 degrees N & S of equator, convection cells cause warming and cooling cycle that drops moisture first and then allows high rate of evaporation • Northern : Sahara, Arabian, Mexico/US southwest • Southern : Kalihari, Namib, Atacama, Austrailia • Rain-Shadow (Orographic) : form on leeward side of mountain ranges so adiabatic currents drop moisture as they rise over high topography • Sierra Nevada : California, Nevada, Arizona • Cascades : Eastern Washington state

3. Types of Deserts (cont’d) • Continental Interiors : caused by great distance from moist, ocean air and orographic effects – Gobi, African Interior • Coastal deserts : Warm subtropical areas with cold ocean currents offshore • Namib, Kalihari, Atacama • Polar deserts : global circulation brings in only cold, dry air • Greenland, Antarctica (Dry Valleys)

4. Deserts(World Distribution)

5. Weathering in Deserts • Little water = few chemical reactions • Steep topography, angular grains/rocks, thin, nutrient poor soils • Mechanical processes dominate : thermal expansion/contraction, crystal wedging, oxidation, wind-driven saltation, episodic/chaotic water processes because of lack of vegetation

6. Water Erosion • Streams : head-ward expansion of arroyos, move large amounts of debris quickly • Pediments : large low-angle erosional/depositional surfaces sloping away from mountains, probably formed during Pleistocene time when more water was available • Alluvial fans : deposition of material at slope change • Playas : evaporite deposits in short lived, shallow, closed run-off systems

7. Saltation

8. Wind Deposition • Loess : wind blown silt deposit • Dunes : wind deposited mounds and/or ridges • very well-sorted • frosted grains • migrate downwind • Slip face on leeward side • Types • Transverse : source material is plentiful, single dominant wind direction, scarce vegetation, perpendicular to wind direction • Longitudinal : moderate source of sand, wind varies in narrow range, parallel to wind direction • Barchan : crescent shaped, little sand source, constant wind, horns migrate downwind, start in vegetation anchors • Parabolic : horseshoe shaped, reactivated transverse dunes, horns stay anchored by vegetation, deflation pushes center ridge downwind • Star : complex, wind shifts from three or more directions

9. Barchan Dunes(Peru)

10. Linear Dunes(Egypt)

11. Star Dunes(Egypt)

12. Wind Erosion • Abrasion : erosive effect of wind driven particles colliding with other materials • Ventifacts : orange peel surface look • Rock pedestals : erosion of softer, underlaying material • Yardangs : single directional winds form “over-turned canoe” features • Deflation : removal of sand and silt sized particles • Desert pavement : surface layer of stones left behind, usually covered by desert varnish (manganese and iron oxide)

13. Desertification • Definition : the invasion of desert conditions into non-desert areas • Significant lowering of water table • Increased salinity in ground water and soils • Destruction of vegetation • Accelerated rate of erosion • Sahara : imaging radar tracks older, buried evidence of more wet environment • Box 19.2 The Disappearing Aral Sea • Box 19.4 The Expanding Deserts

14. Desertification(Sand encroaching on a rural town in Africa)