1. Deserts & Wind
Arid Landscapes

2. Deserts Land of extremes Little life So harsh - named: Extreme heat
Extreme dryness
Little life
Sparse vegetation
Little animal life
So harsh - named:
Death Valley
Badwater
Devil’s playground

3. What is a Desert? An area where evaporation exceeds precipitation
< 10” of rain per year

4. Where Do Deserts Occur? Approximately 30° N&S
Covers about 20% of Earth’s surface
Saharan Desert – largest (3,500,000 mi2)
Atacama – driest < 0.5”/year)

5. Types of Deserts Low Latitude Deserts Mid-latitude Deserts Subtropical
Rainshadow deserts

6. Low Latitude Deserts A band circulating Earth around 30° N&S Examples:
Sahara, Arabian Deserts, Africa
Mojave Desert, North America
Controlled by global air pressure and wind circulation

7. Low Latitude Deserts - Causes
Depends on angle sunlight hits surface
Sunlight at polar latitudes covers wider area; therefore, less heat
At equator, sunlight covers less area; more heat

8. Low Latitude Deserts - Causes
Excess heat is transferred from equator to poles
Air Circulation
Ocean currents

9. Low Latitude Deserts - Causes
At equator, warm air rises
Zone of low pressure
Clouds and precipitation
Reaches troposphere and moves poleward
As it spreads, it cools
30° N&S, cool air sinks
Area of high pressure
Dry conditions
Location of world deserts

10. Low Latitude Deserts - Causes
At equator, warm air rises, expands, and cools
Warm air holds more H20 than cool air
condenses and precipitates
At 30°, cool air sinks, compresses and warms
Dry and hot

11. Mid-latitude Deserts Deep interiors of large landmasses Examples:
Gobi Deserts, Central Asia
Great Basin & Sonoran Deserts, North America
Controlled by location
Far inland away from ocean
Behind large mountain belts

12. Mid-latitude Desert - Cause
Rainshadow Effect
Windward  high precipitation
Air rises, expands and cools
 H20-holding capacity, condenses, precipitates
Leeward  much drier
Air descends, compresses and warms
H20 has been removed by precipitation  dry

13. Geologic Processes in Arid Climates
The Role of Water
Does most of the erosional work
The Role of Wind
Main role is transportation and deposition

14. Water - Ephemeral Streams
Flow intermittently
Only lasts a few hours to days
Produce flash floods
However infrequent, main shaper of topography

15. Water – Alluvial Fan Cone-shaped deposit
Rivers flow out of canyon, soak into dry ground and sediments deposited at mouth
Bajada – coalescing alluvial fan

16. Water - Playas Shallow, dry lake Closed basin - H20 cannot escape
H20 evaporates leaving its salts behind

17. Owens Lake

18. Racetrack Playa & Devil’s Golf Course

19. Basin and Range
Alluvial fans and playas are typical of Basin & Range province
Fault block mountains formed by extension
Form horsts (ranges) – uplifted blocks and grabens (basin) – down-dropped blocks

20. Basin & Range Stages Early Stage Advanced Middle Stage Late Stage
Recent uplift; early erosion
Relief is greatest
Advanced Middle Stage
Continued erosion
Relief decreases
Late Stage
Mountains are gone
Few knobs - inselbergs

21. The Role of Wind Erosion Transportation Deposition Deflation Abrasion
Suspended load
Bed load
Deposition
Sand Dunes

22. Erosion - Deflation Blowouts – shallow depressions
Strong winds pick up and transport sand creating hollow depressions

23. Erosion - Deflation Desert Pavement – Layer of pebbles and gravel
Wind carries smaller sand-sized particle away leaving larger particles behind
Forms protective layer – no more erosion

24. Erosion - Abrasion
Wind-blown sand cuts and polishes exposed rock surfaces (sandblasts)
Produces ventifacts – polished stone

25. Transportation – Suspended Load
Fine dust particles stay suspended
May travel 1000’s of miles
Suspended for days to weeks
Marine maintenance technicians are wearing gas masks so that they can work on aircraft in conditions that can only be described as “extreme.” The dust storm that blew in early this evening has made the air outside appear to be foggy – like it does along the Atlantic or Pacific coasts when there is a large storm offshore. It has the strange effect of turning daylight into dusk, blotting out the sun and changing the hue of every man and machine. Visibility is reduced to less than 20 yards. The wind, blowing steadily at 25 to 30 knots, howls like a banshee through the antenna guy wires. But the “fog” in the air isn’t water vapor – it’s particles of sand that the Marines inhale with every breath, that they swallow with every mouthful of food. It whips in beneath the sidewalls and portals of the tents. It jams weapons. It seeps into every crevice and clogs the intakes of jet engines and the filters of the gas masks everyone carries everywhere, all the time.

26. Transportation – Bed load
Wind-blown sand can skip, bounce, roll or scoot - saltation

27. Deposition – Dune Formation
Sand dunes migrate
Windward side  low slope face
Faces wind direction
Sediment eroded by saltation
Leeward side  steep slip face
Points downwind
Wind slows and deposits load

28. Deposition – Sand Dunes
Barchan – crescent-shaped dunes that concave downwind
limited sand and constant wind

29. Deposition - Sand Dunes
As sand supply increases, barchan dunes begin to connect – Barchanoid dunes
When ridges straighten, they are called transverse – linear ridges that are perpendicular to wind direction
form with abundant sand and no vegetation

30. Deposition - Sand Dunes
Longitudinal – linear dune that is parallel to wind direction
Sand supply is good and wind is not constant

31. Deposition - Sand Dunes
Parabolic – crescent-shaped dune that is concave upwind
Some vegetation and good sand supply

32. Deposition - Sand Dunes
Star Dunes – complex, pyramidal-shaped, with 3+ slip faces
Grow upward
Several wind directions
Namib Desert