Physical GeographyArid 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• 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

Arroyo

Desert Landforms

Note: Tops of most landforms once part of same surface, since partially eroded away

Playa

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 LandformsDeflation/Desert Pavement

Eolian Erosional Landforms• Abrasion – wind blows sand along a surface to polish

& abrade it

Ventifacts Yardangs

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

Loess Terraces

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

Regions Prone to Desertification

Desertification in African Sahel

The Sahel

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