1. The Work of Wind and Deserts
GEOL: CHAPTER 15
The Work of Wind and Deserts

2. The Mesquite Flat sand dunes in Death Valley, California, are a mix of dominantly transverse-type dunes with some crescent-type dunes and star-type dunes.
The Mesquite Flat sand dunes in Death Valley, California, are a mix of dominantly transverse-type dunes with some crescent-type dunes and star-type dunes.

3. Learning Outcomes
LO1: Discuss the role wind plays in transporting sediment
LO2: Explain the two processes of wind erosion
LO3: Identify the types of wind deposits
LO4: Describe the air-pressure belts and global wind patterns

4. Learning Outcomes, cont.
LO5: Describe the distribution of deserts
LO6: Identify the various characteristics of deserts
LO7: Identify the different types of desert landforms

5. More than 6,000 years ago, the Sahara was a fertile savannah supporting a diverse fauna and flora, including humans. Then the climate changed, and the area became a desert. How did this happen? Will this region change back again in the future? These are some of the questions geoscientists hope to answer by studying deserts.
By this point in the semester, you should have some theories of your own!
More than 6,000 years ago, the Sahara was a fertile savannah supporting a diverse fauna and flora, including humans. Then the climate changed, and the area became a desert. How did this happen? Will this region change back again in the future? These are some of the questions geoscientists hope to answer by studying deserts.

6. Desertification
The expansion of deserts into formerly productive lands
Human agriculture has altered natural vegetation patterns
Sahel hard hit: south of Sahara Desert
Famines
Malnutrition
Poverty

7. Wind Sediment Transport
Wind is less dense than water, so can only transport smaller sediments
Bed load: moves by saltation or by rolling or sliding
Saltation: wind lifts sand grains that dislodge other grains upon hitting the surface

8. This should look familiar. Think of stream transport.
Most sand is moved near the ground surface by saltation. Sand grains are picked up by the wind and carried a short distance before falling back to the ground, where they usually hit other grains, causing them to bounce and move in the direction of the wind.
This should look familiar. Think of stream transport.
Figure 15.1 Saltation Most sand is moved near the ground surface by saltation. Sand grains are picked up by the wind and carried a short distance before falling back to the ground, where they usually hit other grains, causing them to bounce and move in the direction of the wind.

9. Erosion Natural forces of wind and water have eroded the rocks in Monument Valley, Arizona, to create “mitten” buttes such as this one.
Figure 15.2 Erosion Natural forces of wind and water have eroded the rocks in Monument Valley, Arizona, to create “mitten” buttes such as this one.

10. Wind Sediment Transport, cont.
Suspended load:
Silt- and clay-sized particles
Will usually stay at surface unless physically disturbed
Once in the air, smaller particles can travel thousands of miles

11. Wind Erosion: Abrasion
Caused by saltating sand grains
Analogous to sandblasting
Rarely more than a meter above ground
Typically modifies existing features
Ventifacts: surfaces altered by windborne particles

12. a b
Ventifacts
a. A ventifact forms when windborne particles (1) abrade the surface of a rock, (2) forming a flat surface. If the rock is moved, (3) additional flat surfaces are formed. b. Numerous ventifacts are visible in this photo, which also shows desert pavement in Death Valley, California. Desert pavement prevents further erosion and transport of a desert's surface materials by
Figure 15.3 Ventifacts

13. Wind Erosion: Deflation
Removal of loose surface sediment by wind
Deflation hollows/blowouts from differential erosion
Desert pavement: close-fitting pebbles and cobbles caused by wind eroding away smaller particles

14. Figure 15.4 Deflation Hollow A deflation hollow, the low area, between two sand dunes in Death Valley, California. Deflation hollows result when loose surface sediment is differentially removed by wind.
A deflation hollow, the low area, between two sand dunes in Death Valley, California. Deflation hollows result when loose surface sediment is differentially removed by wind.

15. Figure 15.5 Desert Pavement

16. Figure 15.5 Desert Pavement

17. Dune Formation Mound or ridge of wind-deposited sand
Form around an obstruction that stops sand grains
Shallow windward slope
Steep leeward slope: angle of repose
Dune can migrate in direction of strong prevailing winds

18. Figure 15. 6 Sand Dunes Large sand dunes in Death Valley, California
Figure 15.6 Sand Dunes Large sand dunes in Death Valley, California. The prevailing wind direction is from left to right, as indicated by the sand dunes in which the gentle windward side is on the left and the steeper leeward slope is on the right.
Sand Dunes Large sand dunes in Death Valley, California. The prevailing wind direction is from left to right, as indicated by the sand dunes in which the gentle windward side is on the left and the steeper leeward slope is on the right.

19. Dune Migration a. Profile of a sand dune.
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.
Figure 15.7 Dune Migration
Stepped Art
Fig. 15-7, p. 308

20. Cross-Bedding Ancient cross-bedding in sandstone beds in Zion National Park, Utah, helps geologists determine the prevailing direction of the wind that formed these ancient sand dunes.
Figure 15.8 Cross-Bedding Ancient cross-bedding in sandstone beds in Zion National Park, Utah, helps geologists determine the prevailing direction of the wind that formed these ancient sand dunes.

21. Dune Types Barchan Longitudinal Transverse Parabolic Determined by:
Sand supply
Wind direction and velocity
Vegetation

22. Barchan Dunes Crescent shaped, tips point downwind
Form in areas with a flat, dry surface
Little vegetation
Nearly constant wind direction
Mobile
Up to 30 meters high

23. Barchan Dunes
Figure 15.9 Barchan Dunes

24. Longitudinal Dunes
Long parallel ridges of sand that are parallel to prevailing winds
Limited sand supply
Winds converge from slightly different directions
3-100 meters in height
Up to 100 km long

25. a b
Longitudinal Dunes
a. Longitudinal dunes form long, parallel ridges of sand aligned roughly parallel to the prevailing wind direction. They typically form where sand supplies are limited. b. Longitudinal dunes, 15 m high, in the Gibson Desert, west central Australia. The bright blue areas between the dunes are shallow pools of rainwater, and the darkest patches are areas where the Aborigines have set fires to encourage the growth of spring grasses.
Figure Longitudinal Dunes

26. Transverse Dunes
Long ridges perpendicular to prevailing wind direction
Abundant sand with little or no vegetation
“Sand seas”
Up to 200 meters high
Up to 3 km long

27. Figure Transverse Dunes Transverse dunes form long ridges of sand that are perpendicular to the prevailing wind direction in areas of little or no vegetation and abundant sand.
Transverse dunes form long ridges of sand that are perpendicular to the prevailing wind direction in areas of little or no vegetation and abundant sand.

28. Parabolic Dunes Common in coastal areas Abundant sand
Strong onshore winds
Form where vegetation cover is broken, from a deflation hollow or blowout
Tips point upwind

29. Figure 15.12 Parabolic Dunes
a. Parabolic dunes typically form in coastal areas that have a partial cover of vegetation, a strong onshore wind, and abundant sand. b. A parabolic dune developed along the Lake Michigan shoreline west of St. Ignace, Michigan.

30. Loess Wind-blown silt and clay deposits
Quartz grains, feldspar, micas, calcite
Three sources:
Deserts
Pleistocene glacial outwash deposits
River floodplains in semiarid regions

31. Loess, cont. Easily eroded Steep cliffs and rapid stream erosion
10% Earth surface
30% U.S. surface
Fertile soils

32. Global Air-Pressure Belts
Air pressure: density of air exerted on surroundings (weight)
Heated air = lower surface air pressure; much solar heating
Cooler air = higher surface air pressure; less solar heating

33. Global Air-Pressure Belts, cont.
Equatorial zone receives most solar energy
Surface air rises, cools, releases moisture
Rising air is drier as it moves poleward
At degrees latitude, it sinks, compresses, and warms to form a high-pressure area conducive to deserts

34. Global Wind Patterns
Winds: air flows from high-pressure areas to low-pressure areas
Coriolis effect: apparent deflection of moving objects because of Earth’s rotation
Winds deflected to the right in the Northern Hemisphere
To the left in the Southern Hemisphere

35. The General Circulation Pattern of Earth’s Atmosphere
Figure The General Circulation Pattern of Earth’s Atmosphere Air flows from high-pressure zones to low-pressure zones, and the resulting winds are deflected to the right of their direction of movement (clockwise) in the Northern Hemisphere and to the left of their direction of movement (counterclockwise) in the Southern Hemisphere. This deflection of air between latitudinal zones resulting from Earth’s rotation is known as the Coriolis effect.

36. Distribution of Deserts
30% of land surface
Low and middle latitudes
Potential evaporation greater than yearly precipitation
Semiarid: more precipitation than arid

37. Distribution of Deserts, cont.
Arid = desert
Deserts
Less than 25 cm precipitation per year
High evaporation rates
Poorly developed soils
Mostly devoid of vegetation

38. The Distribution of Earth’s Arid and Semiarid Regions
Semiarid regions receive more precipitation than arid regions, yet they are still moderately dry. Arid regions, generally described as deserts, are dry and receive less than 25 cm of rain per year. The majority of the world’s deserts are located in the dry climates of the low and middle latitudes.
Figure The Distribution of Earth’s Arid and Semiarid Regions Semiarid regions receive more precipitation than arid regions, yet they are still moderately dry. Arid regions, generally described as deserts, are dry and receive less than 25 cm of rain per year. The majority of the world’s deserts are located in the dry climates of the low and middle latitudes.

39. Climate Characteristics of Deserts
Hot summer days: 32ºC – 50ºC
Cooler on winter days: 10ºC - 18ºC
Precipitation variable and unpredictable
Plants are small, widely spaced, grow slowly
Stems and leaves minimize water loss

40. Desert Vegetation Desert vegetation is typically sparse, widely spaced, and characterized by slow growth rates. The vegetation shown here is in Death Valley, California.
Figure Desert Vegetation Desert vegetation is typically sparse, widely spaced, and characterized by slow growth rates. The vegetation shown here is in Death Valley, California.

41. Weathering and Soils Mechanical weathering dominates
Temperature fluctuations
Frost wedging
Some chemical weathering, though minor
Desert soils are thin, patchy and subject to erosion

42. Desert Streams
Most precipitation comes from brief and heavy cloudbursts
Rapid runoff quickly fills channels, with rapid sediment transport and much erosion
Most streams flow intermittently and don’t reach the sea (internal drainage)
Some permanent streams: Colorado River

43. Playa Lakes
Excess water from rainstorms accumulates in low-lying areas
Temporary: hours to months
Shallow and saline water
Evaporates to leave a playa (salt pan)

44. Figure 15.16 Playas and Playa Lakes
A playa lake formed after a rainstorm near Badwater, Death Valley National Park. Playa lakes last from a few hours to several months.

45. Figure 15.16 Playas and Playa Lakes
Salt deposits and salt ridges cover the floor of this playa in the Mojave Desert. Salt crystals and mud cracks are characteristics features of playas.

46. Alluvial Fans and Pediments
Sediment-laden streams leave mountains and create fan-shaped sediment deposit on flat desert areas
Common in the Basin and Range province in western North America
Pediment: an erosion surface of low relief gently sloping away from a mountain range

47. Remember the Alluvial fans from Chapter 12?
Figure Alluvial Fan A ground view of an alluvial fan, Death Valley, California. Alluvial fans form when sediment-laden streams flowing out from a mountain deposit their load on the desert floor, forming a gently sloping, fan-shaped, sedimentary deposit.

48. Mesas and Buttes
Mesa: broad and flat-topped; erosional remnant bounded by steep slopes
Buttes: mesa eroded into a pillar-like structure
Both have an erosion-resistant cap rock underlain by more easily eroded rock

49. Buttes Right Mitten Butte and Merrick Butte in Monument Valley Navajo Tribal Park on the border of Arizona and Utah.
Figure Buttes Right Mitten Butte and Merrick Butte in Monument Valley Navajo Tribal Park on the border of Arizona and Utah.

50. Virtual Field Trip The effects of wind erosion
The features of wind deposition
A desert landform