19 deserts forstudents
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This chapter discusses deserts and their formation. It begins with an outline and definition of deserts as regions receiving less than 10 inches of annual rainfall. There are five main types of deserts described - subtropical, rain shadow, coastal, continental interior, and polar. The processes of weathering, erosion, deposition and landscape development in deserts are then explained. Finally, the chapter addresses desertification and how human activities can expand desert regions.
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19 deserts forstudents
- 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 • Primary control on deserts? • Characterized by… • Extreme • Specialized • Unique
- 4. Chapter 21 What Is a Desert? • Land so arid, that… • Evaporation prevents permanent surface water • Vegetation covers <15% of surface • Annual rainfall <10’ (25 cm) • Deserts exist in both hot and cold climates
- 5. Chapter 21 Hot vs. Cold Deserts • Cold deserts: • High latitudes • High elevations • Near cold oceans 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) 2. Rain shadow (eg eastern OR) 3. Coastal (atacama) 4. Continental interiors (Gobi- southern mongolia) 5. Polar (antarctica)
- 9. Chapter 21 Subtropical Deserts • Due to patterns of atmospheric circulation. • Equator – 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 – 20o -30o N & S. • Sinking dry air wicks water • Landscape below dries • Earth’s largest deserts
- 10. Chapter 21 • At 20-30o N & S latitude across time (as plates move) • African deserts bracket equator • North: sahara and arabian • Rainforest straddling equator • South: namib and Kalahari Subtropical Deserts
- 11. Chapter 21 Rainshadow Deserts • Wet ocean winds are driven over mountains • 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 (east 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 moisture can be arid • Example: gobi desert in Mongolia
- 14. Chapter 21 Polar Deserts • Above 66º N & S latitude there is a little moisture vapor • Air circulation carries • It is so cold, air can’t
- 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 and dust and 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 – grains moved along land surface • Saltation – sand skipped, bounced by grain impact • Suspended load – sediment carried in air • High winds can carry dust across entire oceans Wind Erosion and Transport
- 22. Chapter 21 • Coarse clasts can’t be lifted by wind • Finer sediment removed; forms lag deposits (deflation) • Creates desert pavement -> Surface layer of stones • Becomes resistant 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 – 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 sandblast 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 – 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 – coarser sand sized particles 2. Suspended load – finer grained silt sized dust • 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 Desert Landscapes
- 31. Chapter 21 Desert Landscapes • Cliffs & Mesas – 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 – • Resistant – • Weak – Desert Landscapes
- 33. Chapter 21 • Cliffs & Mesas – Structure governs landforms • Non-horizontal bedding produces linear ridges • Cuesta – steep joint controlled cliff, less steep dip slope • Inselberg – 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 fan at mountain flank Desert Landscapes
- 35. Chapter 21 • Dunes – windblown sand accumulations • Sand carried by wind accumulates around obstacle • Over time, a dune grows, begins to moved downwind • Dunes generate enormous cross-beds Desert Landscapes
- 36. Chapter 21 • Dune types • Form depends on variation in.. • Wind – velocity and consistency of direction • Sediment supply – 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 – 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 – aridification of nondesert areas • Human activity expands desert areas • Examples: aral sea and mid US
- 41. Chapter 21 Desertification • Desertification caused by… • Overpopulation, overgrazing • Careless agriculture • Diversion of water supplies • climate change • Leads to hardship (i.e. famine).