1. The Work of Wind and Deserts
Chapter 15
The Work of Wind and Deserts

2. Introduction What is desertification?
Desertification is the expansion of deserts into formerly productive lands.
These expansions destroy croplands and rangelands.
Can cause massive starvation and even force hundreds of thousands of people from their homelands

3. Introduction
By understanding how desert processes operate people can take steps to reduce the spread of desertification and its effects
Overgrazing and improper cultivation destroys vegetation and soils, and can result in desertification.
Desertification is also closely tied to global warming.
We will also be able to better understand and deal with present-day environmental changes.

4. Sediment Transport by Wind
Sediment may be transported by winds as:
bed load (by the process of saltation) or
suspended load
Fig. 15.1, p. 367

5. Sediment Transport by Wind
Bed load is the material that is too large or heavy to be carried in suspension by water or wind. Particles move along the surface by saltation, rolling, or sliding.
Fig. 15.1, p. 367

6. Sediment Transport by Wind
Suspended load - material that can be carried in suspension by water or wind.
Suspended load is composed of silt- and clay-sized particles
They can be carried thousands of kilometers

7. Wind Erosion
Material is eroded by wind either through abrasion or deflation.
Abrasion is the impact of saltating sand grains on an object. Its effect is similar to sandblasting.
Fig p. 367

8. Wind Erosion
Ventifacts are rocks whose surfaces have been polished, pitted, grooved, or faceted by the wind abrasion.
Fig p. 368

9. Wind Erosion
Deflation is the removal of loose surface material by wind.
Desert pavement and deflation hollows are common features of deserts resulting from differential erosion by deflation.
Fig. 15.5, p.369

10. Wind Erosion Deflation
The formation of desert pavement prevents further deflation.
Fig. 15.3b, p. 368
Fig. 15.6, p. 369

11. Wind Deposits
The wind is responsible for two important desert deposits, sand dunes and loess.
The Formation and Migration of Dunes
Fig. 15.7, p. 370

12. Wind Deposits The Formation and Migration of Dunes
Dunes are mounds or ridges of wind-deposited sand.
They form when wind flows over and around an obstruction, resulting in the deposition of sand grains, which accumulate and build up a deposit of sand.
Fig. 15.7, p. 370

13. Wind Deposits The Formation and Migration of Dunes
Most dunes have an asymmetric profile, with a:
gentle windward slope
steeper downwind, or leeward, slope that is inclined in the direction of the prevailing wind
Fig. 15.7, p. 370

14. Wind Deposits The Formation and Migration of Dunes
Dunes migrate by sand moving up and over the gentle windward slope by saltation and accumulating and sliding down the windward side.
Fig. 15.8, p. 370

15. Wind
Direction of
dune migration
b. Dunes migrate when sand moves up the windward side and slides down the leeward slope. Such movement of the sand grains produces a series of crossbeds that slope in the direction of wind movement.
Sand moves by saltation
Windward
side
Leeward
slope
Wind
a. Profile of a sand dune.
Stepped Art
Fig. 15-8, p. 370

16. Wind Deposits Dune Types The five major dune types are: Barchan
Longitudinal
Transverse
Parabolic
Star

17. Wind Deposits Dune Types
Barchan Dunes - Crescent- shaped dunes whose tips point downwind
Fig , p. 371

18. Wind Deposits Dune Types
Barchan Dunes form in the areas that have a generally flat, dry surface with:
little vegetation
a limited supply of sand
a nearly constant wind direction
Fig b, p. 371

19. Wind Deposits Dune Types
Longitudinal Dunes - Long, parallel ridges of sand aligned generally parallel to the direction of the prevailing winds
They form where the sand supply is somewhat limited.
Fig , p. 372

20. Wind Deposits Dune Types
Transverse Dunes - Form long ridges perpendicular to the prevailing wind direction
Fig , p. 372

21. Wind Deposits Dune Types Transverse Dunes - Found in areas that have:
abundant sand
little or no vegetation
Fig , p. 372

22. Wind Deposits Dune Types Parabolic Dunes - Common in coastal areas
Characteristics:
abundant sand
strong onshore winds
partial cover of vegetation
Fig , p. 373

23. Wind Deposits Dune Types Star Dunes - huge pyramidal hills of sand
Characteristics:
variable wind direction
do not migrate, form desert landmarks used by the nomadic people for centuries
Fig , p. 373

24. Wind Deposits
Loess - is wind-blown silt and clay deposits composed of angular quartz grains, feldspar, micas, and calcite
Loess is derived from deserts, Pleistocene glacial outwash deposits, and river floodplains in semiarid regions.
Loess covers approximately 10% of Earth’s land surface and weathers to a rich, productive soil.
Fig , p. 373

25. Air-Pressure Belts and Global Wind Patterns
The global wind patterns are determined by air-pressure belts and the Coriolis effect.
Fig , p. 374

26. Air-Pressure Belts and Global Wind Patterns
Air flows from high-pressure to low-pressure zones.
Moisture air rises at the equator, in the doldrums, producing equatorial rains as it rises and condenses.
Fig , p. 374

27. Air-Pressure Belts and Global Wind Patterns
The doldrums and the horse latitudes are areas of very little to no wind because air is moving vertically.
Fig , p. 374

28. Air-Pressure Belts and Global Wind Patterns
Dry air falls at the horse latitudes and returns to the equator. As the air returns to the equator, it moves horizontally, producing wind, the tradewinds.
Fig , p. 374

29. Air-Pressure Belts and Global Wind Patterns
The winds are deflected as the earth rotates due to the Coriolis effect.
Fig , p. 374

30. Air-Pressure Belts and Global Wind Patterns
The winds are deflected clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Fig , p. 374

31. The Distribution of Deserts
Dry climates occur in the low-middle latitudes.
They occur where the loss of water by evaporation is greater than the annual precipitation.
Dry climates cover 30% of the Earth’s land surface.
Dry climates are subdivided into semiarid and arid regions.
Semiarid regions receive more precipitation, yet they are moderately dry and support grasslands.
Arid regions, generally described as deserts, are dry and receive less than 25 cm of rain per year.

32. The Distribution of Deserts
The majority of the world’s deserts are located in the between 20 and 30 degrees north and south latitudes. Their dry climate results from a high-pressure belt of descending dry air.
Examples: Southwest U.S. and Mexico, Sahara, Arabian Peninsula, Australia, Atacama (Chile and Peru)
Fig , p. 375

33. The Distribution of Deserts
The remaining deserts are in the middle latitudes, where their distribution is related to the rain- shadow effect, and in the dry polar regions.
Examples: Gobi and Central Eurasian, Great Basin (U.S.)
Fig , p. 375

34. The Distribution of Deserts
Rain shadow effect
Many middle and high latitude deserts are located far from the coasts. Often, mountain ranges block the flow of moist marine air creating a “rain- shadow”.
The Himalayas block the Gobi desert, and the Sierra Nevadas block the Great Basin.
Fig , p. 375

35. Moist marine air Warm dry air Rain-shadow desert Stepped Art
Fig , p. 375

36. Characteristics of Deserts
Temperature, Precipitation, and Vegetation
Most deserts are characterized by:
high temperatures (except Polar)
little precipitation and
sparse plant cover
Fig , p. 376

37. Characteristics of Deserts
Weathering and Soils
Mechanical weathering is the dominant form of weathering and, coupled with slow rates of chemical weathering, results in poorly developed soils.
Fig , p. 377

38. Characteristics of Deserts
Mass Wasting, Streams, and Groundwater
Running water is the major agent of erosion in deserts, with most streams being poorly integrated and flowing intermittently.

39. Characteristics of Deserts
Wind
Wind, though secondary to water as an erosional agent in deserts, is still capable of producing a variety of distinctive erosional and depositional features.
Wind forms sand deposits
Important alternate energy source
Fig. 15.4, p. 368

40. Desert Landforms Major landforms of deserts
Important desert landforms include
Playa lakes and playas
Alluvial fans and bajadas
Pediments
Inselbergs
Buttes and mesas

41. Desert Landforms
Playas are dry lakebeds characterized by mud cracks and precipitated salt crystals.
When temporarily filled with water following a rainstorm, they are known as playa lakes.
Fig , p. 380

42. Desert Landforms Alluvial fans are fan-shaped sedimentary deposits.
They form when sediment-laden streams flow out from mountain fronts and deposit their load on the relatively flat desert floor.
Coalescing alluvial fans form bajadas
Fig , p. 380

43. Desert Landforms Pediments are erosional bedrock surfaces Low relief
Slope gently away from mountain bases
Fig , p. 381

44. Desert Landforms Inselbergs
Isolated, steep-sided erosional remnants that rise above desert plains
Uluru (Ayers Rock) of Australia
Fig , p. 381

45. Desert Landforms
Buttes and mesas - flat-topped erosional remnants with steep slopes
Buttes – pillar-like
Mesas – table-like
Add Fig b, p. 403 on right
Fig , p. 382

46. End of Chapter 15