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Arid Landscapes & Eolian Processes
• Arid Landscapes
• Eolian Erosion & Transportation
• Eolian Deposition & Landforms
• Human Interactions with Eolian Processes
Arid Landscapes• 3 factors influence arid climates:
– Subtropical high pressure– Rainshadow– Distance from large bodies of water
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
Desert Landforms
Note: Tops of most landforms once part of same surface, since partially eroded away
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
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
Particle Transport• Silts and Clays carried in suspension
• Sand bounces along – saltation, or
• Sand rolls slowly along – creep
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
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
Loess
• Fine-grained, wind-blown silt – high in calcium – usually from alluvial deposits or glacial till
• Can be transported farther than sand
Loess Deposits around the World
Loess Deposits
Arid Landscapes and Eolian Processes
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
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
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
• 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
Desertification in Great Plains