Physical Geography Chapter 18

1. Physical GeographyChapter 18 Arid Landscapes and Eolian Processes

2. Arid Landscapes & Eolian Processes • Arid Landscapes • Eolian Erosion & Transportation • Eolian Deposition & Landforms • Human Interactions with Eolian Processes

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

4. 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

5. Arroyo

6. Desert Landforms Note: Tops of most landforms once part of same surface, since partially eroded away

7. Playa

8. 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

9. 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

10. Particle Transport • Silts and Clays carried in suspension • Sand bounces along – saltation, or • Sand rolls slowly along – creep

11. 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

12. 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

13. Eolian Erosional Landforms Deflation/Desert Pavement

14. Eolian Erosional Landforms Abrasion Ventifacts Yardangs

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

16. Loess Deposits Arid Landscapes and Eolian Processes Loess Deposits around the World

17. Loess Terraces

18. 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

19. Regions Prone to Desertification

20. 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

21. The Sahel

22. 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

23. 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