1. Chapter 14 Geology Today Barbara W. Murck Brian J. Skinner
Deserts and Wind
Chapter 14
Geology Today
Barbara W. Murck
Brian J. Skinner
Ancient sand dunes, Utah
N. Lindsley-Griffin, 1999

2. What is a desert? Deserts
An area where annual precipitation is less than 250 mm (10 in)
Deserts
Source: U.S.G.S. N. Lindsley-Griffin, 1999

3. “Deserts are barren” - Wrong, most have at least some vegetation
Desert Myths:
“Deserts are barren” - Wrong, most have at least some vegetation
Organ Pipe National Monument, AZ
N. Lindsley-Griffin, 1999

4. “It never rains” - Wrong, rains are infrequent, but over a long enough period of time, the effects are large.
Desert Myths:
San Rafael Swell, UT
N. Lindsley-Griffin, 1999

5. “Water has little effect on deserts” - Wrong, flash floods have tremendous power to erode and transport material
Desert Myths:
Tarbuck-Lutgens, 1998; N. Lindsley-Griffin, 1999

6. Desert Myths:
“All erosion is by wind” - Wrong, most desert features formed by running water during wetter Ice Age climates
N. Lindsley-Griffin, 1999
Source: U.S.G.S.

7. “Natural arches form by wind erosion” - Wrong, mechanical weathering and mass wasting (rockfalls) are more important.
Desert Myths:
N. Lindsley-Griffin, 1999
Arches National Park, UT

8. Desert Types
Subtropical deserts are controlled by global climate zones where dry air descends to surface (Fig. 14.2, p. 399)
N. Lindsley-Griffin, 1999

9. Desert Types
Most of the western United States is subtropical arid and semiarid climate.
N. Lindsley-Griffin, 1999
Source: U.S.G.S.

10. Rainshadow Deserts form where mountains block moist air.
Desert Types
Rainshadow Deserts form where mountains block moist air.
Descending winds are dry,
warm up as they compress
(Mojave Desert, Death Valley)
Rising winds cool, release moisture
(Seattle, Oregon, Northern California)
Dry winds
Desert
Wet winds
Lee Slope
Windward slope
N. Lindsley-Griffin, 1999

11. Desert Types
Coastal deserts are caused by global oceanic and atmospheric circulation
Houghton-Mifflin, 1998; N. Lindsley-Griffin, 1999

12. Coastal deserts form where cold coastal currents flow parallel to shore
Desert Types
Cold coastal current chills onshore winds so they cannot carry moisture
Source: U.S.G.S. N. Lindsley-Griffin, 1999

13. Air is too cold to carry moisture. but what does fall does not melt.
Polar Deserts - moisture frozen into ice sheets, not available for plant growth
Air is too cold to carry moisture.
Little snow falls,
but what does fall does not melt.

14. Desert Weathering
Mechanical weathering is more important than chemical weathering (frost wedging, root wedging)
Capital Reef National Park, UT
N. Lindsley-Griffin, 1999

15. Salt crystallization is an important weathering process because salt expands as it crystallizes.
Desert Weathering
Mudcracks and evaporites
N. Lindsley-Griffin, 1999
Source: U.S.G.S.

16. Eolian processes are particularly effective in arid and semi-arid regions
Wind Erosion
Dust Storm
Tarbuck & Lutgens, N. Lindsley-Griffin, 1999

17. Sediment moves by suspension, surface creep, saltation
Wind Erosion
Sediment moves by suspension, surface creep, saltation
Fig. 14.4, p. 402
N. Lindsley-Griffin, 1999

18. Wind Erosion
Desert pavement forms by deflation - smaller particles blow away, leaving surface covered with tightly packed large particles.
Fig. 14.6, p. 404
Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999

19. Desert Pavement: pebble- to cobble-sized rock fragments covering desert surface after lighter fragments have been removed by wind.
Wind Erosion
Fig. 14.6, p. 404
N. Lindsley-Griffin, 1999

20. Wind Erosion
Wind-blown sand hammers at exposed rock faces to produce smooth flat surfaces
Fig. 14.5, p. 403
Source: U.S.G.S. N. Lindsley-Griffin, 1999

21. Ventifacts have at least one smooth abraded surface facing upwind.
Wind Erosion
Ventifacts have at least one smooth abraded surface facing upwind.
Ventifact -
Rock faceted and polished by wind-blown sand
Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999

22. Blowout - a small saucer-shaped depression formed by wind erosion in dunes. Common in Nebraska sand hills where protective vegetation has been disturbed.
Blowout

23. Wind Deposition Sand dunes - Mounds of wind-blown sand
Loess - Thick deposits of cohesive,
unstratified wind-blown dust
N. Lindsley-Griffin, 1999

24. Sand Dunes
Dunes form wherever a supply of sand is available, strong wind blows constantly, and a barrier causes wind to lose velocity so sand can be deposited
Barchan dunes
Coral Pink Sand Dunes State Park, AZ
Tifernine dune field, Africa (space shuttle)
N. Lindsley-Griffin, 1999

25. Wind Deposition
Sand dunes dip gently on the upwind or windward side, are steep on downwind slip face.
Houghton Mifflin 1998; N. Lindsley-Griffin, 1999

26. Sand ripples in Monument Valley
Wind Deposition
Source: U.S.G.S.
N. Lindsley-Griffin, 1999

27. Sand moves up windward slope by creep and saltation
Sand moves up windward slope by creep and saltation. It drops out as the wind loses velocity in the lee of the dune.
Sand Dunes
Barchan Dunes
N. Lindsley-Griffin, 1999

28. Sand Dunes
Sand dune slip face: sand oversteepens at top, mass-wastes down the steep slip face of the dune
Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999

29. The type of sand dune that forms depends on amount of sediment supply and wind direction
Sand Dunes
Transverse dunes
Longitudinal dunes
Barchans
Parabolic dunes
Star dunes
Houghton Mifflin, 1998; N. Lindsley-Griffin, 1999

30. Sand Dunes
Barchan dunes, crescent-shaped with horns pointing downwind, require moderate sediment supply and wind strength. (Fig. 14.9A, p. 407)
Danakil Depression, Ethiopia
N. Lindsley-Griffin, 1999

31. Sand Dunes
Transverse dunes, continuous asymmetrical ridges perpendicular to the strongest wind, require abundant sediment. (Fig. 14.9B, p. 407)
Takla Makan Desert, China
N. Lindsley-Griffin, 1999

32. Sand Dunes
Star dunes, isolated and stationary, form where winds blow from all directions. (Fig. 14.9C, p. 407)
300 m high star dunes in Libya; radar satellite image
N. Lindsley-Griffin, 1999

33. Longitudinal dunes, long narrow ridges parallel to wind, form where little sediment is available and winds are strong.
Sand Dunes
Common in Africa and Australia
(Fig. 14.9E, p. 407)
N. Lindsley-Griffin, 1999

34. Sand Dunes
Ancient sand dune deposits can be recognized by the steep sets of cross beds, separated by gently dipping sets. Steep sets = slip face Gentle sets = windward face
N. Lindsley-Griffin, 1999

35. Sand grains rounded with frosted surfaces
Eolian Sediment
Fine-grained
(sand, silt, clay)
Well-sorted
Distinctive cross bedding style: low angle layers separating steep layers
Sand grains rounded with frosted surfaces
Source: U.S.G.S.

36. Checkerboard Mesa - pattern caused by intersection of joints and eolian cross beds
Source: U.S.G.S.
N. Lindsley-Griffin, 1999

37. Streams deposit sediment at slope change at mouth of canyon
Alluvial Fans
Streams deposit sediment at slope change at mouth of canyon
N. Lindsley-Griffin, 1999

38. Fault-block Mountains
Sediment-filled Playa
Basins of internal drainage are common in the Basin and Range region of the U.S.

39. Dark green areas on map are up to 75% eolian sand
Nebraska’s Sand Hills - a giant sand dune field formed near the end of the Ice Ages
Sand Hills
Sand dunes held in place by vegetation - if grass is destroyed, blowouts will form
Dark green areas on map are up to 75% eolian sand

40. Yellow area - Sand hills
NE Conservation & Survey, J.R. Griffin , 1999

41. Wind Deposits
Deposits of eolian sand and loess, Midwestern United States
AGI/NAGI, J.R. Griffin, 1999

42. Desertification and Land Degradation
Desertification - invasion of desert conditions into nondesert areas:
Drought or climate change in areas that are already semiarid.
Land degradation - desertification caused by human activities:
Too many people for resource base, poor agricultural practices, overgrazing, deforestation, misuse of surface and groundwater, civil war.
Desertification in the central Plains, 1930s
N. Lindsley-Griffin, 1999

43. The Aral Sea
Aral Sea is drying up as water from tributary rivers is diverted
to irrigate new cropland.
Climate: winters colder, summers
hotter, dust storms common.
Economy: fishing industry gone,
salinization is ruining soil.
This desert was once the Aral Sea
Houghton Mifflin 1998; N. Lindsley-Griffin, 1999