Landform Geography Lecture 9

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Landform GeographyLecture 9
Arid Landscapes and Eolian Processes
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Arid Landscapes

• 3 factors influence arid climates
• Subtropical high pressure
• Rainshadow
• Distance from large bodies of water

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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

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Arroyo
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Desert Landforms
Note Tops of most landforms once part of same
surface, since partially eroded away
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Playa
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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

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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
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Particle Transport

• Silts and Clays carried in suspension
• Sand bounces along saltation, or
• Sand rolls slowly along creep

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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

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Eolian Erosional Landforms

• Abrasion wind blows sand along a surface to
  polish abrade it
• Ventifacts rocks shaped by abrasion pitted,
  grooved, polished
• Yardangs elongated, wind-sculpted ridges caused
  by abrasion

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Eolian Erosional Landforms
Deflation/Desert Pavement
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Eolian Erosional Landforms
Abrasion
Ventifacts
Yardangs
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Loess

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

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Loess Deposits around the World
Arid Landscapes and Eolian Processes
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Loess Terraces
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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

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Regions Prone to Desertification
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Desertification in African Sahel

• Semi-arid region in transition region from Sahara
  Desert in north to rainforest in south
• Traditionally nomadic herders small, sedentary
  farmers north-south migrations to follow rain
• Into 20th century, European borders resource
  exploitation made people more sedentary
  over-cultivation of soil, overgrazing, and tree
  removal
• Add in extended drought since late 1960s, you
  have desertification

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The Sahel
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Atmospheric Dust Storm
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Eolian Dust Storm
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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
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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