Wind Erosion and Deserts

1. Wind Erosion and Deserts

2. Wind as a Geologic Agent • Arid regions have little to no soil moisture and mineral fragments. • Wind has the ability to transport, erode, and deposit sediment. • Transportation occurs by: • Saltation: short jumps of grains dislodged from the surface • Suspension: smaller particles carried by the wind

3. Wind Erosion • Wind can erode the surface through deflation and abrasion. • Deflation: lowering of the land surface due to removal of fine-grained particle by the wind. • Creates desert pavement • Abrasion (sandblasting): winds blow sand or silt against rocks and other objects.

4. Wind Deposits • Wind deposits sediment when its velocity decreases to the point where the sediments can no longer be carried. • Velocity of wind is slowed on downwind side of barriers • Rocks, vegetation, and human made structures

5. Sand Dunes • Sand dunes are asymmetrical mounds with a gentle slope in the upwind direction and steep slope called a slip face on the downwind side. • Sands form when • A ready supply of sand • A steady wind • An obstacle to trap some sand (rock, vegetation, or human structure)

6. Types of Dunes Star Dune Linear Dune Barchan Dune Parabolic Dune Transverse Dune

7. Wind Blown Dust • Dust consists of clay sized particle that are often packed together with smooth surfaces • Large deposits of dust are called loess. • Dust typically originates from glacial erosion • Can also consist of volcanic ash

8. Origins of Deserts • Different mechanisms create different types of deserts. • Subtropical – mid-latitudes (30N, 30S) • Continental – continental interiors far from moisture • Rainshadow – mountainous regions • Coastal - cold upwelling seawater • Polar – cold polar regions dominated by dry air

9. Polar Rainshadow Mid-latitudes Continental Interior Coastal

10. Surface Processes • Weathering and mass wasting • Mechanical weathering dominates • rockfalls, slides, accumulation of coarse grained material, steep slopes

11. Streams and Fluvial Landforms • Surface waters are rare in deserts • Originate at higher elevations • Intermittent streams (flow only when it rains) • Flash floods and braided streams are common • Alluvial Fans – mountain stream enters a broad flat valley • Pediments – broad bedrock surface with a gentle slope • Playa Lakes – form during rainfall but quickly evaporate away