19 deserts forstudents
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19 deserts forstudents

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19 deserts forstudents 19 deserts forstudents Presentation Transcript

  • Chapter 21Deserts
  • Chapter 21Outline• 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• DesertificationChapter 21
  • Chapter 21Deserts• Deserts cover• Primary control on deserts?• Characterized by…• Extreme• Specialized• Unique
  • Chapter 21What 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
  • Chapter 21Hot vs. Cold Deserts• Cold deserts:• High latitudes• High elevations• Near cold oceans currents• Hot deserts:• Low latitudes• Low elevations• Far from oceans
  • Chapter 21Desert Features• Desert surfaces often include…• Exposed bedrock• Accumulated clasts• Unweathered sediment• Precipitated salt• Windblown sand
  • Chapter 21Outline• 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• DesertificationChapter 21
  • Chapter 21Types 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)
  • Chapter 21Subtropical Deserts• Due to patterns of atmospheric circulation.• Equator – 0olatitude.• 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-30oN & S.• Sinking dry air wicks water• Landscape below dries• Earth’s largest deserts
  • Chapter 21• At 20-30oN & S latitude across time (as plates move)• African deserts bracket equator• North: sahara and arabian• Rainforest straddling equator• South: namib and KalahariSubtropical Deserts
  • Chapter 21Rainshadow 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)
  • Chapter 21Coastal 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
  • Chapter 21Interior Deserts• Air looses moisture crossing continents• Land far from ocean moisture can be arid• Example: gobi desert in Mongolia
  • Chapter 21Polar Deserts• Above 66º N & S latitude there is a little moisture vapor• Air circulation carries• It is so cold, air can’t
  • Chapter 21Outline• 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• DesertificationChapter 21
  • Chapter 21Desert Processes• Deserts are characterized by:• Erosion by water and wind• Weathering.• Soil formation.• Deposition.• These processes result in unique landscapes
  • Chapter 21Weathering• Physical weathering dominates• Rare chemical weathering leaches ions• Evaporation -> salts both break and cement grains
  • Chapter 21Desert Soils• Desert soils are thin, with poorly defined horizons• Usually colored like the bedrock• Trace elements/oxidation cause wide color variations/red
  • Chapter 21Desert 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
  • Chapter 21Water 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
  • 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 oceansWind Erosion and Transport
  • 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 erosionWind Erosion and Transport
  • 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 varnishWind Erosion and Transport
  • Chapter 21• Deflation – lowering land surface via erosion• Concentrated wind erosion creates blowouts• Desert processes clearly operating on marsWind Erosion and Transport
  • Chapter 21• Grains in wind often sandblast exposed surfaces• Produces unusual products:• Ventifacts -> stones with wind-faceted upper surfaces• Yardangs -> wind sculpted bedrockWind Erosion and Transport
  • 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 jointingDesert Deposition
  • Chapter 21Desert 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
  • Chapter 21• Wind carries 2 types of sediment load1. Surface load – coarser sand sized particles2. Suspended load – finer grained silt sized dust• Sand forms dunes in deserts: dust forms loess next to (downwindof ) desertsDesert Processes
  • Chapter 21Outline• 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• DesertificationChapter 21
  • Chapter 21• Deserts exhibit diverse landscapes• Reflect interplay of erosional and depositionalDesert Landscapes
  • Chapter 21Desert 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.
  • Chapter 21• Cliffs & Mesas –• Resistant –• Weak –Desert Landscapes
  • 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 retreatDesert Landscapes
  • Chapter 21• Pediments• Ramp-like bedrock surfaces sloping up to a mountain front• Bajada• Sloping surface of coalesced alluvial fan at mountain flankDesert Landscapes
  • 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-bedsDesert Landscapes
  • Chapter 21• Dune types• Form depends on variation in..• Wind – velocity and consistency of direction• Sediment supply – is sand scarce or abundantDesert LandscapesDuneDune WindWind SandSandBarchanBarchanSteadySteady1 direction1 directionScarceScarceStarStarSteadySteadyChanging directionsChanging directionsScarceScarceTransverseTransverseSteady, moderateSteady, moderate1 direction1 directionPlentifulPlentifulParabolicParabolicSteady, strongSteady, strong1 direction1 directionPlentifulPlentifulLongitudinalLongitudinalSteady, strongSteady, strong1 direction1 directionAbundantAbundant
  • Chapter 21• Dunes• Sand oceans (ergs) develop in some deserts• Arabian peninsula, Namibia.• Past ergs – large regions of cross-bedded sandstones.Desert Landscapes
  • Chapter 21• Desert organisms are adapted to harsh conditions• Dessication.• Temperature extremes.Desert Life
  • Chapter 21Outline• 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• DesertificationChapter 21
  • Chapter 21Desertification• Desertification – aridification of nondesert areas• Human activity expands desert areas• Examples: aral sea and mid US
  • Chapter 21Desertification• Desertification caused by…• Overpopulation, overgrazing• Careless agriculture• Diversion of water supplies• climate change• Leads to hardship (i.e. famine).