Physical Geography Chapter 18 Arid Landscapes and Eolian Processes.

Physical Geography

Chapter 18

Arid Landscapes and Eolian Processes

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

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 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

Eolian Erosional LandformsDeflation/Desert Pavement

Eolian Erosional LandformsAbrasion

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

• 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

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