Deserts and Wind

1. Deserts and Wind Chapter 14 Geology Today Barbara W. Murck Brian J. Skinner Ancient sand dunes, Utah N. Lindsley-Griffin, 1999

2. Deserts What is a desert? An area where annual precipitation is less than 250 mm (10 in) Source: U.S.G.S. N. Lindsley-Griffin, 1999

3. Desert Myths: “Deserts are barren” - Wrong, most have at least some vegetation Organ Pipe National Monument, AZ N. Lindsley-Griffin, 1999

4. Desert Myths: “It never rains” - Wrong, rains are infrequent, but over a long enough period of time, the effects are large. San Rafael Swell, UT N. Lindsley-Griffin, 1999

5. Desert Myths: “Water has little effect on deserts” - Wrong, flash floods have tremendous power to erode and transport material Tarbuck-Lutgens, 1998; N. Lindsley-Griffin, 1999

6. Desert Myths: “All erosion is by wind” - Wrong, most desert features formed by running water during wetter Ice Age climates N. Lindsley-Griffin, 1999 Source: U.S.G.S.

7. Desert Myths: “Natural arches form by wind erosion” - Wrong, mechanical weathering and mass wasting (rockfalls) are more important. Arches National Park, UT N. Lindsley-Griffin, 1999

8. Subtropical deserts are controlled by global climate zones where dry air descends to surface. (Fig. 14.2, p. 399) Desert Types N. Lindsley-Griffin, 1999

9. Desert Types Most of the western United States is subtropical arid and semiarid climate. N. Lindsley-Griffin, 1999 Source: U.S.G.S.

10. Desert Types Descending winds are dry, warm up as they compress (Mojave Desert, Death Valley) Rising winds cool, release moisture (Seattle, Oregon, Northern California) Dry winds Desert Wet winds Rainshadow Deserts form where mountains block moist air. Lee Slope Windward slope N. Lindsley-Griffin, 1999

11. Desert Types Coastal deserts are caused by global oceanic and atmospheric circulation Houghton-Mifflin, 1998; N. Lindsley-Griffin, 1999

12. Desert Types Cold coastal current chills onshore winds so they cannot carry moisture Coastal deserts form where cold coastal currents flow parallel to shore Source: U.S.G.S. N. Lindsley-Griffin, 1999

13. Polar Deserts - moisture frozen into ice sheets, not available for plant growth Air is too cold to carry moisture. Little snow falls, but what does fall does not melt.

14. Desert Weathering Mechanical weathering is more important than chemical weathering (frost wedging, root wedging) Capital Reef National Park, UT N. Lindsley-Griffin, 1999

15. Desert Weathering Salt crystallization is an important weathering process because salt expands as it crystallizes. Mudcracks and evaporites N. Lindsley-Griffin, 1999 Source: U.S.G.S.

16. Wind Erosion Eolian processes are particularly effective in arid and semi-arid regions Dust Storm Tarbuck & Lutgens, N. Lindsley-Griffin, 1999

17. Wind Erosion Sediment moves by suspension, surface creep, saltation Fig. 14.4, p. 402 N. Lindsley-Griffin, 1999

18. Wind Erosion Desert pavement forms by deflation - smaller particles blow away, leaving surface covered with tightly packed large particles. Fig. 14.6, p. 404 Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999

19. Wind Erosion Desert Pavement: pebble- to cobble-sized rock fragments covering desert surface after lighter fragments have been removed by wind. Fig. 14.6, p. 404 N. Lindsley-Griffin, 1999

20. Wind Erosion Wind-blown sand hammers at exposed rock faces to produce smooth flat surfaces Fig. 14.5, p. 403 Source: U.S.G.S. N. Lindsley-Griffin, 1999

21. Wind Erosion Ventifact - Rock faceted and polished by wind-blown sand Ventifacts have at least one smooth abraded surface facing upwind. Tarbuck & Lutgens, J.R. Griffin,N. Lindsley-Griffin, 1999

22. Blowout - a small saucer-shaped depression formed by wind erosion in dunes. Common in Nebraska sand hills where protective vegetation has been disturbed. Blowout

23. Wind Deposition Sand dunes - Mounds of wind-blown sand Loess - Thick deposits of cohesive, unstratified wind-blown dust N. Lindsley-Griffin, 1999

24. Sand Dunes Dunes form wherever a supply of sand is available,strong wind blows constantly, and a barrier causes wind to lose velocity so sand can be deposited Barchan dunes Coral Pink Sand Dunes State Park, AZ Tifernine dune field, Africa (space shuttle) N. Lindsley-Griffin, 1999

25. Wind Deposition Sand dunes dip gently on the upwind or windward side, are steep on downwind slip face. Houghton Mifflin 1998; N. Lindsley-Griffin, 1999

26. Wind Deposition Sand ripples in Monument Valley Source: U.S.G.S. N. Lindsley-Griffin, 1999

27. Sand Dunes Sand moves up windward slope by creep and saltation. It drops out as the wind loses velocity in the lee of the dune. Barchan Dunes N. Lindsley-Griffin, 1999

28. Sand Dunes Sand dune slip face: sand oversteepens at top, mass-wastes down the steep slip face of the dune Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999

29. The type of sand dune that forms depends on amount of sediment supply and wind direction Sand Dunes Transverse dunes Longitudinal dunes Barchans Parabolic dunes Star dunes Houghton Mifflin, 1998; N. Lindsley-Griffin, 1999

30. Sand Dunes Barchan dunes, crescent-shaped with horns pointing downwind, require moderate sediment supply and wind strength. (Fig. 14.9A, p. 407) Danakil Depression, Ethiopia N. Lindsley-Griffin, 1999

31. Sand Dunes Transverse dunes, continuous asymmetrical ridges perpendicular to the strongest wind, require abundant sediment. (Fig. 14.9B, p. 407) Takla Makan Desert, China N. Lindsley-Griffin, 1999

32. Sand Dunes Star dunes, isolated and stationary, form where winds blow from all directions. (Fig. 14.9C, p. 407) 300 m high star dunes in Libya; radar satellite image N. Lindsley-Griffin, 1999

33. Sand Dunes Longitudinal dunes, long narrow ridges parallel to wind, form where little sediment is available and winds are strong. Common in Africa and Australia (Fig. 14.9E, p. 407) N. Lindsley-Griffin, 1999

34. Sand Dunes Ancient sand dune deposits can be recognized by the steep sets of cross beds, separated by gently dipping sets.Steep sets = slip faceGentle sets = windward face N. Lindsley-Griffin, 1999

35. Eolian Sediment Fine-grained (sand, silt, clay) Well-sorted Distinctive cross bedding style: low angle layers separating steep layers Sand grains rounded with frosted surfaces Source: U.S.G.S.

36. Checkerboard Mesa - pattern caused by intersection of joints and eolian cross beds Source: U.S.G.S. N. Lindsley-Griffin, 1999

37. Alluvial Fans Streams deposit sediment at slope change at mouth of canyon N. Lindsley-Griffin, 1999

38. Fault-block Mountains Basins of internal drainage are common in the Basin and Range region of the U.S. Sediment-filled Playa

39. Sand Hills Nebraska’s Sand Hills - a giant sand dune field formed near the end of the Ice Ages Sand dunes held in place by vegetation - if grass is destroyed, blowouts will form Dark green areas on map are up to 75% eolian sand

40. Yellow area - Sand hills NE Conservation & Survey, J.R. Griffin , 1999

41. Wind Deposits Deposits of eolian sand and loess, Midwestern United States AGI/NAGI, J.R. Griffin, 1999

42. Desertification and Land Degradation Desertification - invasion of desert conditions into nondesert areas: Drought or climate change in areas that are already semiarid. Land degradation - desertification caused by human activities: Too many people for resource base, poor agricultural practices, overgrazing, deforestation, misuse of surface and groundwater, civilwar. Desertification in the central Plains, 1930s N. Lindsley-Griffin, 1999

43. The Aral Sea Aral Sea is drying upas water from tributary rivers is diverted to irrigate new cropland. Climate: winters colder, summers hotter, dust storms common. Economy: fishing industry gone, salinization is ruining soil. This desert was once the Aral Sea Houghton Mifflin 1998; N. Lindsley-Griffin, 1999