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July 23, 2013

Physical Geography. Arid Landscapes and Eolian Processes. July 23, 2013. Arid Landscapes. 3 factors influence arid climates: Subtropical high pressure Rainshadow Distance from large bodies of water. Desert Geomorphology.

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July 23, 2013

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  1. Physical Geography Arid Landscapes and Eolian Processes July 23, 2013

  2. Arid Landscapes • 3 factors influence arid climates: • Subtropical high pressure • Rainshadow • Distance from large bodies of water

  3. Desert Geomorphology • Water important to landforms in arid regions – little vegetation to slow intermittent erosion • Arroyo – steep-sided gully cut into alluvium • In undisturbed, horiz. rock layers more resistant sandstone or limestone forms flat caprock above easily eroded shale • Result is landforms flat on top w/steep sides: • Plateau -Canyon -Butte -Mesa • Pinnacle -Playa

  4. Arroyo

  5. Desert Landforms Note: Tops of most landforms once part of same surface, since partially eroded away

  6. Playa

  7. Eolian Erosion and Transport • Wind-based processes important in deserts b/c: • Strong winds common in desert • Large supply of sand & silt to be blown • Vegetation minimal – wind free to erode

  8. Fluid Behavior of Wind • Wind acts like a fluid, like water, but less dense • Faster wind can move larger particles Threshold Velocity for wind to carry different sized particles

  9. Particle Transport • Silts and Clays carried in suspension • Sand bounces along – saltation, or • Sand rolls slowly along – creep

  10. Eolian Erosional Landforms • 2 types of wind erosion: • Deflation – wind blows loose soil away: • leaves coarser pebbles & cobbles, called Desert Pavement • when deflation causes basin to form, called Deflation Hollow

  11. Eolian Erosional Landforms Deflation/Desert Pavement

  12. Eolian Erosional Landforms • Abrasion – wind blows sand along a surface to polish & abrade it Ventifacts Yardangs

  13. Loess • Fine-grained, wind-blown silt – high in calcium – usually from alluvial deposits or glacial till • Can be transported farther than sand

  14. Loess Deposits Arid Landscapes and Eolian Processes Loess Deposits around the World

  15. Loess Terraces

  16. Human Impact/Desertification • Desertification – transforming a vegetated landscape to one that is barren & susceptible to wind erosion • Population pressure has forced more people to clear marginal, semi-arid-to-arid land for agriculture & firewood • In wind, cleared land loses topsoil and nutrients • Vegetation unlikely to reestablish

  17. Regions Prone to Desertification

  18. Desertification in African Sahel

  19. The Sahel

  20. Desertification in Great Plains • Great Plains lie east of Rocky Mts in semi-arid climate with short grass as dominant natural vegetation Dust Bowl Region

  21. Desertification in Great Plains • Early 1900s Americans moved to region to farm, plowing and clearing native grasses – unusually wet period • 1930s – terrible drought hits – topsoil blows into dust storms – called “Dust Bowl” • Many migrated to California & elsewhere • Those who stayed have employed irrigation & soil conservation, including windbreaks, and conservation tillage

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