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Geologylecture19 130505151645-phpapp02
- 2. Chapter 21 Outline ⢠Deserts: extents and basics. ⢠Desert types -Subtropical, rain shadows, cont. interiors, coastal, polar. ⢠Desert Processes -Weathering, soils, desert varnish -Erosion by water and wind -Deposition and transport ⢠Desert Landscapes -Cliffs & mesas -Pediments and bajadas -Dunes, their types and controls ⢠Desertification Chapter 21
- 3. Chapter 21 Deserts ⢠Deserts cover ~25% of all land surface ⢠Primary control on deserts? Plate tectonics ⢠Characterized by⌠⢠Extreme dryness ⢠Specialized ecosystems, low human populations ⢠Unique geologic and surface processes
- 4. Chapter 21 What Is a Desert? ⢠Land so arid, that⌠⢠Evaporation prevents permanent surface water ⢠Vegetation covers <15% of surface ⢠Annual rainfall <10â (25cm) ⢠Deserts exist in both hot and cold climates
- 5. Chapter 21 Hot vs. Cold Deserts ⢠Cold deserts: ⢠High latitudes ⢠High elevations ⢠Near cold ocean currents ⢠Hot deserts: ⢠Low latitudes ⢠Low elevations ⢠Far from oceans
- 6. Chapter 21 Desert Features ⢠Desert surfaces often include⌠⢠Exposed bedrock ⢠Accumulated clasts ⢠Unweathered sediment ⢠Precipitated salt ⢠Windblown sand
- 7. Chapter 21 Outline ⢠Deserts: extents and basics. ⢠Desert types -Subtropical, rain shadows, cont. interiors, coastal, polar. ⢠Desert Processes -Weathering, soils, desert varnish -Erosion by water and wind -Deposition and transport ⢠Desert Landscapes -Cliffs & mesas -Pediments and bajadas -Dunes, their types and controls ⢠Desertification Chapter 21
- 8. Chapter 21 Types of Deserts ⢠5 Types (distinctive landscapes & biota): 1. Subtropical (Sahara, Arabian, Kalahari) 2. Rain shadow (e.g. Eastern OR?WA, Nevada) 3. Coastal (Atacama) 4. Continental interiors (Gobi- southern Mongolia) 5. Polar (Antarctica)
- 9. Chapter 21 Subtropical Deserts ⢠Due to patterns of atmospheric circulation. ⢠Equator â 0o latitude. ⢠Solar energy evaporates water, which rises as hot, moist air ⢠Rising air cools and expands, forming abundant rain ⢠This air, stripped of moisture, flows N and S ⢠Subtropics â 20o -30o N & S. ⢠Sinking dry air wicks water ⢠Landscape below dries ⢠Earthâs largest deserts
- 10. Chapter 21 ⢠At 20-30o N & S latitude across time ⢠African deserts bracket ⢠North: Sahara and Arabian ⢠Rainforest straddling equator ⢠South: Namib and Kalahari Subtropical Deserts
- 11. Chapter 21 Rainshadow Deserts ⢠Wet ocean winds are driven over ⢠Windward air is forced to rise and cool ⢠Moisture condenses and rains ⢠Leeward air, w/o moisture, sinks ⢠Sinking air warms and sucks water out of land ⢠Example: Nevada 9east side of Sierras)
- 12. Chapter 21 Coastal Deserts ⢠Cool air over cold ocean water holds little moisture ⢠Air absorbs moisture when it interacts with land ⢠Atacama desert (Peru) -> driest place on Earth
- 13. Chapter 21 Interior Deserts ⢠Air looses moisture crossing continents ⢠Land far from ocean (water source) can be arid ⢠Example: Gobi desert in Mongolia
- 14. Chapter 21 Polar Deserts ⢠Above 66Âş N & S latitude there is little moisture vapor ⢠Air circulation carries dry air to polar regions ⢠It is so cold, air canât hold water
- 15. Chapter 21 Outline ⢠Deserts: extents and basics. ⢠Desert types -Subtropical, rain shadows, cont. interiors, coastal, polar. ⢠Desert Processes -Weathering, soils, desert varnish -Erosion by water and wind -Deposition and transport ⢠Desert Landscapes -Cliffs & mesas -Pediments and bajadas -Dunes, their types and controls ⢠Desertification Chapter 21
- 16. Chapter 21 Desert Processes ⢠Deserts are characterized by: ⢠Erosion by water and wind ⢠Weathering. ⢠Soil formation. ⢠Deposition. ⢠These processes result in unique landscapes
- 17. Chapter 21 Weathering ⢠Physical weathering dominates ⢠Rare chemical weathering leaches ions ⢠Evaporation -> salts both break and cement grains
- 18. Chapter 21 Desert Soils ⢠Desert soils are thin, with poorly defined horizons ⢠Usually colored like the bedrock ⢠Trace elements/oxidation cause wide color variations/red
- 19. Chapter 21 Desert Varnish ⢠Exposed surfaces develop desert varnish. ⢠Dark coating of iron and manganese oxides ⢠Forms very slowly by bacterial activity + dust + water ⢠Native Americans left petroglyphs in desert varnish
- 20. Chapter 21 Water Erosion ⢠Though rare, water shapes desert landscapes. ⢠Lack of veg/roots magnifies sediment erosion and transport ⢠Flashy rainfall generates dangerous floods ⢠Rapid flow of thick, muddy, and viscous water sculpts land ⢠Water can quickly infiltrate dry stream beds
- 21. Chapter 21 ⢠Ground is scoured by wind ⢠Sand & silt-sized sediment is lifted and moved: ⢠Surface load â grains moved along land surface ⢠Saltation â sand skipped, bounced by grain impact ⢠Suspended load â sediment carried in air ⢠High winds can carry dust across entire oceans Wind Erosion and Transport
- 22. Chapter 21 ⢠Coarse clasts canât be lifted by wind ⢠Finer sediment removed; forms lag deposits (deflation) ⢠Creates desert pavement -> Surface layer of stones ⢠Becomes resistance to erosion⌠Wind Erosion and Transport
- 23. Chapter 21 ⢠However, deflation is now known to be followed by⌠⢠Soil accumulation below pavement (inflation!) ⢠Pavement is lifted ⢠-> Surface stones crack and fit together better ⢠Often coated with desert varnish Wind Erosion and Transport
- 24. Chapter 21 ⢠Deflation â lowering land surface via erosion⌠⢠Concentrated wind erosion creates blowouts⌠⢠Desert processes clearly operating on Mars Wind Erosion and Transport
- 25. Chapter 21 ⢠Grains in wind often âsandblastâ exposed surfaces ⢠Produces unusual products: ⢠Ventifacts -> stones with wind-faceted upper surfaces ⢠Yardangs -> wind-sculpted bedrock Wind Erosion and Transport
- 26. Chapter 21 ⢠Desert sediments accumulate in many settings ⢠Erosional debris accumulates near source areas ⢠Talus aprons, at angle of repose, collect cliff debris ⢠Rock blocks have shapes determined by jointing Desert Deposition
- 27. Chapter 21 Desert Deposition ⢠Playas â Desert (salt) lakes without outlet streams ⢠Internal drainage collects water from flash floods ⢠Dissolved solids crystallize out as water evaporates ⢠Halite, gypsum. Salar de Uyuni, Bolivia
- 28. Chapter 21 ⢠Wind carries 2 types of sediment load 1. Surface load â courser sand-sized particles 2. Suspended load â finer-grained silt-sized âdustâ ⢠Sand forms dunes in deserts; dust forms loess next to (downwind of) deserts Desert Processes
- 29. Chapter 21 Outline ⢠Deserts: extents and basics. ⢠Desert types -Subtropical, rain shadows, cont. interiors, coastal, polar. ⢠Desert Processes -Weathering, soils, desert varnish -Erosion by water and wind -Deposition and transport ⢠Desert Landscapes -Cliffs & mesas -Pediments and bajadas -Dunes, their types and controls ⢠Desertification Chapter 21
- 30. Chapter 21 ⢠Deserts exhibit diverse landscapes ⢠Reflect interplay of erosional and depositional processes over long time spans Desert Landscapes
- 31. Chapter 21 Desert Landscapes ⢠Cliffs & Mesas â bedrock landforms ⢠Bedrock exposed along cliffs- breaks away along joints ⢠Cliff retreat in flat rocks creates plateaus⌠⢠Then mesas, buttes and, finally, chimneys.
- 32. Chapter 21 ⢠Cliffs & Mesas â weathering dictates appearance ⢠Resistant â form steep cliffs as thick as rock layer ⢠Weak â weather slopes Desert Landscapes
- 33. Chapter 21 ⢠Cliffs & Mesas â Structure governs landforms ⢠Non-horizontal bedding produces linear ridges ⢠Cuesta â steep joint-controlled cliff; less-steep dip slope ⢠Inselberg â eroded remnant of almost-complete cliff retreat Desert Landscapes
- 34. Chapter 21 ⢠Pediments ⢠Ramp-like bedrock surfaces sloping up to a mountain front ⢠Bajada ⢠Sloping surface of coalesced alluvial fans at mountain flank Desert Landscapes
- 35. Chapter 21 ⢠Dunes â windblown sand accumulations ⢠Sand carried by wind accumulates around obstacle ⢠Over time, a dune grows, begins to move downwind ⢠Dunes generate enormous cross-beds Desert Landscapes
- 36. Chapter 21 ⢠Dune types ⢠Form depends on variation in⌠⢠Wind â velocity and consistency of direction ⢠Sediment supply â is sand scarce or abundant Desert Landscapes DuneDune WindWind SandSand BarchanBarchan SteadySteady 1 direction1 direction ScarceScarce StarStar SteadySteady Changing directionsChanging directions ScarceScarce TransverseTransverse Steady, moderateSteady, moderate 1 direction1 direction PlentifulPlentiful ParabolicParabolic Steady, strongSteady, strong 1 direction1 direction PlentifulPlentiful LongitudinalLongitudinal Steady, strongSteady, strong 1 direction1 direction AbundantAbundant
- 37. Chapter 21 ⢠Dunes ⢠Sand oceans (ergs) develop in some deserts ⢠Arabian peninsula, Namibia. ⢠Past ergs â large regions of cross-bedded sandstones. Desert Landscapes
- 38. Chapter 21 ⢠Desert organisms are adapted to harsh conditions ⢠Dessication. ⢠Temperature extremes. Desert Life
- 39. Chapter 21 Outline ⢠Deserts: extents and basics. ⢠Desert types -Subtropical, rain shadows, cont. interiors, coastal, polar. ⢠Desert Processes -Weathering, soils, desert varnish -Erosion by water and wind -Deposition and transport ⢠Desert Landscapes -Cliffs & mesas -Pediments and bajadas -Dunes, their types and controls ⢠Desertification Chapter 21
- 40. Chapter 21 Desertification ⢠Desertification â aridification of nondesert areas ⢠Human activity expands desert areas ⢠Examples: Aral Sea and mid US (i.e. Oklahoma 1930s)- dust bowl
- 41. Chapter 21 Desertification ⢠Desertification caused by⌠⢠Overpopulation, overgrazing ⢠Careless agriculture ⢠Diversion of water supplies ⢠Climate change ⢠Leads to hardship (i.e. famine).