1. Understanding Earth Chapter 19: WINDS AND DESERTS Grotzinger • Jordan
Seventh Edition
Chapter 19:
WINDS AND DESERTS
© 2014 by W. H. Freeman and Company

2. Chapter 19
Winds and
Deserts

3. About Winds and Deserts
Wind is a major agent of erosion and deposition that moves vast quantities of sand, silt, and dust over large areas.
Eolian processes shape the land surface in areas, such as deserts, where few plants live.
Deserts expand and contract, depending upon climatic changes.

4. Lecture Outline 1. Global wind patterns 2. Wind as a transport agent
3. Wind as an agent of erosion
4. Wind as a depositional agent
5. The desert environment

5. ● wind is unconfined, except in narrow valleys
Global Wind Patterns
● Wind (the flow of air)
● wind is unconfined, except in
narrow valleys
● wind obeys all the laws of flow
● wind is parallel to surface, and
may go upward too

6. ● Characteristics of wind ● turbulence ● wind belts ● trade winds
Global Wind Patterns
● Characteristics of wind
● turbulence
● wind belts
● trade winds
● westerlies

7. ● Complications to simple circulation ● Earth’s rotation
Global Wind Patterns
● Complications to simple circulation
● Earth’s rotation
● Coriolis effect
● equatorial upward movement
● dry, sinking air over deserts

8. Wind as a Flow of Air: Earth’s Belts North Pole Westerlies
30°N latitude
NE trade winds
Equator
SE trade winds
30°S latitude

9. Wind as a Flow of Air: Earth’s Belts North Pole Westerlies
At the equator,
the Sun’s rays are perpendicular to
the surface, concentrating heat.
30°N latitude
NE trade winds
Equator
SE trade winds
30°S latitude

10. Wind as a Flow of Air: Earth’s Belts
At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures.
North Pole
Westerlies
At the equator,
the Sun’s rays are perpendicular to
the surface, concentrating heat.
30°N latitude
NE trade winds
Equator
SE trade winds
30°S latitude

11. At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures.
North Pole
There is little surface wind at the equator, and the air rises, forming clouds and rain as it cools.
Westerlies
At the equator,
the Sun’s rays are perpendicular to
the surface, concentrating heat.
30°N latitude
NE trade winds
Equator
SE trade winds
30°S latitude

12. At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures.
North Pole
There is little surface wind at the equator, and the air rises, forming clouds and rain as it cools.
Westerlies
At the equator,
the Sun’s rays are perpendicular to
the surface, concentrating heat.
30°N latitude
NE trade winds
At 30°N and 30°S latitudes, the cooled air sinks, warms up, absorbs moisture, and yields clear skies.
Equator
SE trade winds
30°S latitude

13. At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures.
North Pole
There is little surface wind at the equator, and the air rises, forming clouds and rain as it cools.
Westerlies
At the equator,
the Sun’s rays are perpendicular to
the surface, concentrating heat.
30°N latitude
NE trade winds
At 30°N and 30°S latitudes, the cooled air sinks, warms up, absorbs moisture, and yields clear skies.
Equator
SE trade winds
30°S latitude
These two motions set up the horizontal circulation between the equator and the North and South Poles.

14. At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures.
North Pole
There is little surface wind at the equator, and the air rises, forming clouds and rain as it cools.
Westerlies
At the equator,
the Sun’s rays are perpendicular to
the surface, concentrating heat.
30°N latitude
NE trade winds
At 30°N and 30°S latitudes, the cooled air sinks, warms up, absorbs moisture, and yields clear skies.
Equator
SE trade winds
30°S latitude
These two motions set up the horizontal circulation between the equator and the North and South Poles.
In the temperate zones,
the prevailing wind belts
come from the west.

15. At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures.
North Pole
There is little surface wind at the equator, and the air rises, forming clouds and rain as it cools.
Westerlies
At the equator,
the Sun’s rays are perpendicular to
the surface, concentrating heat.
30°N latitude
NE trade winds
At 30°N and 30°S latitudes, the cooled air sinks, warms up, absorbs moisture, and yields clear skies.
Equator
SE trade winds
30°S latitude
These two motions set up the horizontal circulation between the equator and the North and South Poles.
In the temperate zones,
the prevailing wind belts
come from the west.
In the tropics, the prevailing wind belts blow from the east. The Coriolis effect is responsible for the deflection of air eastward or westward as illustrated by the curved arrows.

16. ● Factors in how wind carries things ● wind strength ● particle size
2. Wind as a Transport Agent
● Factors in how wind carries things
● wind strength
● particle size
● surface material

17. Wind
as a
Transport
Agent:
Rate of
Sand
Movement
versus
Wind Speed

18. Wind
as a
Transport
Agent:
Sand Being
Blown from
the Desert
in Namibia
toward the
South Atlantic

19. ● Materials carried by the wind ● windblown dust
2. Wind as a Transport Agent
● Materials carried by the wind
● windblown dust
● volcanic, organic, soil
● aerosols
● windblown sand

20. Wind
as an
Agent
of Erosion:
Ventifacts

21. Wind
as an
Agent
of Erosion:
Rounded
and
Frosted
Grains
of Sand

22. Wind
as an
Agent
of Erosion:
Desert
Deflation
Surface

23. Thought questions for this chapter
What evidence might you find in an ancient sandstone that would point to its eolian origin?

24. ● What the wind does to erode surficial materials
3. Wind as an Agent of Erosion
● What the wind does to erode
surficial materials
● sand blasting (ventifacts;
frosting on sand grains)
● deflation (desert pavements)

25. Thought questions for this chapter
You have just driven a truck through a sandstorm and discover that the paint has been stripped from the lower parts of the truck, but the upper parts are barely scratched. What process is responsible, and why is it restricted to the lower parts of the truck?
Compare the heights to which sand and dust are carried in the atmosphere and explain the differences and similarities.

26. ● deserts, beaches, lake shores, floodplains, etc.
4. Wind as a Depositional Agent
● Sand dunes
● deserts, beaches, lake shores,
floodplains, etc.
● need a ready supply of loose
sand
● vegetation stabilizes dunes

27. Wind
as a
Depositional
Agent:
Sand dunes

28. Wind
as a
Depositional
Agent:
Ripples at
Stovepipe
Wells,
California

29. Wind
as a
Depositional
Agent:
Sand Dunes
in Gusev
Crater, Mars

30. ● How sand dunes form and move ● saltation of sand
4. Wind as a Depositional Agent
● How sand dunes form and move
● saltation of sand
● transverse piling of sand or
sand piles up behind an
obstruction
● piling and avalanche of sand

31. Wind
as a
Depositional
Agent:
Sand Piling
Downwind
of an
Obstruction

32. 4. Wind as a Depositional Agent: Formation of Wind Shadow Sands

33. 4. Wind as a Depositional Agent: Formation of Sand Dunes

34. 4. Wind as a Depositional Agent: Formation of Sand Dunes

35. 4. Wind as a Depositional Agent: Formation of Sand Dunes

36. 4. Wind as a Depositional Agent: Formation of Sand Dunes

37. 4. Wind as a Depositional Agent: Formation of Sand Dunes

38. ● Types of sand dunes ● barchans ● blowout dunes ● transverse dunes
4. Wind as a Depositional Agent
● Types of sand dunes
● barchans
● blowout dunes
● transverse dunes
● linear dunes

39. 4. Wind as a Depositional Agent:
Barchans
Wind

40. Barchans are crescent-shaped dunes, always the products of limited sand supply and unidirectional winds.
Wind

41. 4. Wind as a Depositional Agent:
Blowout Dunes

42. Blowout dunes are almost the reverse of barchans (the horns point “backward”).

43. 4. Wind as a Depositional Agent:
Transverse Dunes

44. Transverse dunes form in arid regions where abundant sand is available.

45. 4. Wind as a Depositional Agent:
Linear Dunes

46. Linear dunes occur in areas that have a moderate sand supply, a rough pavement, and winds in the same general direction.

48. ● fallen atmospheric dust creates a deposit of loess (loess blanket)
4. Wind as a Depositional Agent
● Dust falls and loess
● fallen atmospheric dust creates a
deposit of loess (loess blanket)
● loess covers 10% of Earth’s
surface

49. Wind
as a
Depositional
Agent

50. Wind
as a
Depositional
Agent:
Pleistocene
Loess
in Central
China

51. Thought questions for this chapter
What factors determine whether sand dunes will form on a stream floodplain?
Trucks continually have to haul away sand covering a coastal highway. What do you think might be the source of the sand? Could its encroachment be stopped?
Which of the following would be a more reliable indication of the direction of the wind that formed a barchan: cross-bedding or the orientation of the dune’s shape on a map? Why?

52. Thought questions for this chapter
Mars has large areas of sand dunes. From this fact alone, what can you infer about conditions on the Martian surface?
What aspects of an ancient sandstone would you study to show that it was originally a desert sand dune?
What evidence would cause you to infer that dust storms and strong winds were common in glacial times?

53. 5. The Desert Environment:
Where the Deserts Are

54. ● Where deserts are found ● areas of low rainfall ● rain shadows
5. The Desert Environment
● Where deserts are found
● areas of low rainfall
● rain shadows
● far from oceans
● in polar regions

55. ● Various factors play a role in the formation of deserts
5. The Desert Environment
● Various factors play a role
in the formation of deserts
● plate tectonics
● climate change
● human actions
● in polar regions

56. ● Desert weathering phenomena ● desert colors (rusty, orange brown)
5. The Desert Environment
● Desert weathering phenomena
● desert colors (rusty, orange
brown)
● desert varnish
● stream erosion (when water
is present)

57. The
Desert
Environment:
Rusty Colors
and Desert
Varnish
(carved by
aboriginal
people),
Canyonlands,
Utah

58. The
Desert
Environment:
Stream
Erosion
(when water
is present),
Saguaro
National
Monument, Arizona

59. The
Desert
Environment:
Stream
Erosion
(when water
is absent),
Saguaro
National
Monument, Arizona

60. The
Desert
Environment:
Desert Playa
Lake, Death
Valley,
California

61. ● Desert sediments; sedimentation ● alluvial sediments
5. The Desert Environment
● Desert sediments; sedimentation
● alluvial sediments
● eolian sediments
● evaporite sediments (playas
and playa lakes)

62. ● Desert landscapes ● playas ● desert pavements ● dune fields
5. The Desert Environment
● Desert landscapes
● playas
● desert pavements
● dune fields
● dry washes (wadis)
● pediments

63. The
Desert
Environment:
Pediment,
Cima Dome,
Mojave
Desert,
California

64. 5. The Desert Environment: Formation of a Pediment
Elevated
mountains
Downfaulted
lowlands
Fault
Time 1
The lowlands are downfaulted, and the mountains are elevated.

65. 5. The Desert Environment: Formation of a Pediment
Early erosion of
mountains
Alluvial fans
Stream
floodplain
Time 2
Erosional debris is deposited as alluvial fan and stream floodplain sediments.

66. 5. The Desert Environment: Formation of a Pediment
Alluvial deposits
Pediment
Time 3
Erosion produces a pediment with thin covering of alluvial deposits.

67. 5. The Desert Environment: Formation of a Pediment
Mountain
remnants
Pediment
Time 4
Continued erosion produces a more extensive pediment.

68. Thought questions for this chapter
What features of a desert landscape would lead you to believe it was formed mainly by streams, with secondary contributions from eolian processes?
How does desert weathering differ from or resemble weathering in more humid climates?
What kinds of landscape features would you ascribe to the work of the wind, to the works of streams, or to both?

69. Key terms and concepts Desert pavement Desert varnish Desertification
Deflation
Desert pavement
Desert varnish
Desertification
Dry wash
Dust
Eolian
Loess
Pediment
Playa
Playa lake
Sandblasting
Slip face
Ventifact
Wadi