1. Classroom presentations to accompany Understanding Earth, 3rd edition prepared by Peter Copeland and William Dupré University of Houston Chapter 14 Wind and Deserts

2. Stanley Breeden/DRK

3. Deserts Deserts are usually thought of as hot and dry, but there are different ways to define a desert: Annual rainfall (<25 cm) Less precipitation than the potential for evaporation Deserts can be cold if there is an extremely small amount of precipitation.

4. Fig. 14.1 Atmospheric Circulation Patterns

5. Erosion and deserts Wind is often thought to be the most important agent of erosion in deserts. However, even in deserts, most of the work of erosion is done by water. Because there is so little water in deserts, erosion is very intermittent.

6. Erosion and deserts Typically, when storms take place in desert regions, dry stream courses fill quickly with water. With little vegetation to hold water, flash floods can be brief, but violent.

7. Erosion and deserts When rainfall is unusually heavy, desert soil may become saturated with water and begin to flow. This is known as a debris flow.

8. Fig. 14.2

9. Fig. 14.3

10. Fig. 14.4 Tom Bean Wind Direction

11. Fig. 14.5 Rate of Sand Movement as a Function of Wind Velocity

12. Wind Transportation of material: Because wind is much less dense than water, it can transport only small particles, mainly fine sand and silt (clay is usually too cohesive). Particles move by either saltation (sand) or suspension (dust).

13. Wind Dust can be transported over great distances. Skiers in the Alps commonly encounter a silty surface on the snow. The silt comes from the Sahara desert in Africa, over 1500 km away.

14. Wind Wind-borne material can become extremely concentrated in air: in 1 km 3, there may be up to 1000 tons of dust. Sand grains carried by wind get a frosted exterior (diagnostic of eolian transport).

15. Dust Storm, 1937 Library of Congress

16. Fig. 14.6 Frosted and Rounded Wind-blown Sand Walter N. Mack

17. Deflation The process of removing all of the small (easily moved) particles. As this process proceeds, only larger rocks are left. This is known as “desert pavement”.

18. Fig. 14.7 Breck P. Kent Deflation Hollow

19. Formation of Desert Pavement Fig. 14.9b

20. Fig. 14.8a David Muench Desert Pavement

21. Ventifact Fig. 14.9 E.R.Degginger

22. Yardangs in Iran Fig. 14.10 Comstock

23. Fig. 14.11 Linear Dunes in Saudi Arabia Prevailing Winds ERIM

24. Fig. 14.12 Coastal Dunes in Peru Loren McIntyre

25. Formation of a Wind- shadow Dune Fig. 14.13

26. Dune Migration Fig. 14.14

27. Fig. 14.15 Dune Migration and the Formation of Cross Bedding

28. Fig. 14.16 Compression of Streamlines over Dune Increases Velocity

29. Types of Dunes Fig. 14.17

30. Fig. 14.18 Pleistocene Loess E.R.Degginger

31. Loess in China Fig. 14.19 Stephen C. Porter

32. Where deserts are Tropic of Capricorn, Tropic of Cancer High pressure  subsiding air heats  loses moisture Center of continent Rain shadow Interaction with ocean currents: e.g., Atacama Desert (Peru and Chile). Air moves from above cold ocean waters to warm land and expands, absorbing moisture.

33. Major Deserts of the World Fig. 14.20

34. Desert varnish Surface coating of Fe and Mn oxides Can be used to date exposure intervals.

35. Fig. 14.21 Petroglyphs in Desert Varnish Peter Kresan

36. Streams and lakes in deserts Often streams in the desert dry up before they reach the sea. Those that don’t dry up are usually fed from a wetter area (e.g., Colorado River). Interior drainages are common in deserts — the two are linked. Examples: Nevada, Tibetan plateau

37. Fig. 14.22a “Dry wash” in Flood Peter Kresan

38. Fig. 14.22b The Day After Peter Kresan

39. Fig. 14.23 Playa Lake David Muench

40. Typical Landscape Formed by Desert Weathering Fig. 14.24 Peter Kresan

41. Playa lakes Formed in a closed basin. Water accumulates after rain; may last days to months before complete evaporation, leaving a playa, a flat lake bed of clay, silt, and evaporites.

42. Faulting Fig. 14.25a

43. Deposition of Alluvial Fans Fig. 14.25b

44. Erosional Retreat Forms Pediment Fig. 14.25c

45. Pediment Expands with Continued Erosion Fig. 14.25d

46. Evolution of a Mesa Rivers Breach Resistant Cap Fig. 14.26a

47. Evolution of a Mesa Continued Erosion Fig. 14.26b

48. Evolution of a Mesa Long-continued Erosion Fig. 14.26c