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Winds and Deserts

Winds and Deserts. Stanley Breeden/DRK. 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

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Winds and Deserts

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  1. Winds and Deserts Stanley Breeden/DRK

  2. 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.

  3. Location of Deserts • Tropic of Capricorn, Tropic of Cancer • High pressure  subsiding air heats  loses moisture • Center of continent • Rain shadow (Orographic desert) • 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.

  4. Major Deserts of the World Fig. 14.20

  5. Fig. 14.2

  6. Fig. 14.3

  7. Atmospheric Circulation Patterns Fig. 14.1

  8. Rain Shadow (Orographic) Deserts Fig. 12.3

  9. 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.

  10. Erosion and deserts (2) 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.

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

  12. The Day After Fig. 14.22b Peter Kresan

  13. Wind Blow Sedimentary Deposits • Sand dunes (sand sized material – cross bedded) • Loess (silt and clay sized material – massive)

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

  15. Dune Migration Fig. 14.14

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

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

  18. Types of Dunes Fig. 14.17

  19. Barchan Dunes

  20. Linear (Longitudinal) Dunes

  21. “Star Dunes”

  22. Coastal Dunes in Peru Fig. 14.12 Loren McIntyre

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

  24. Desert Varnish • Surface coating of Fe and Mn oxides • Can be used to date exposure of rock surfaces.

  25. Petroglyphs in Desert Varnish Fig. 14.21 Peter Kresan

  26. 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

  27. 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.

  28. 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).

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

  30. Wind (2) 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.

  31. Wind • Wind-borne material can become extremely concentrated in air:in 1 km3, there may be up to 1000 tons of dust. • Sand grains carried by wind get a frosted exterior (diagnostic of eolian transport).

  32. Dust Storm, 1937 Library of Congress

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

  34. 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”.

  35. Deflation Hollow Fig. 14.7 Breck P. Kent

  36. Formation of Desert Pavement Fig. 14.9b

  37. Desert Pavement Fig. 14.8a David Muench

  38. Ventifact Fig. 14.9 E.R.Degginger

  39. Yardangs in Iran Fig. 14.10 Comstock

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

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

  42. 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.

  43. Playa Lake Fig. 14.23 David Muench

  44. Faulting Fig. 14.25a

  45. Deposition of Alluvial Fans Fig. 14.25b

  46. Erosional Retreat Forms Pediment Fig. 14.25c

  47. Pediment Expands with Continued Erosion Fig. 14.25d

  48. Evolution of a MesaRivers Breach Resistant Cap Fig. 14.26a

  49. Evolution of a MesaContinued Erosion Fig. 14.26b

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