Physical Geography Chapter 18 Arid Landscapes and Eolian Processes

Arid Landscapes & Eolian Processes 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

Threshold Velocity for wind to carry different sized particles 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 Landforms Deflation/Desert Pavement

Eolian Erosional Landforms Abrasion 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 Arid Landscapes and Eolian Processes Loess Deposits around the World Loess Deposits

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

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