1. Deserts

2. Definition
Desert:
A region so arid that it contains no permanent streams except for those that bring water in from elsewhere, and has very sparse vegetation cover.
NOT related to temperature!
Deserts can be
Hot (>35 °C)
Cold (< 20 °C)

3. Location Dry regions cover 30 percent of Earth’s land surface
Two climatic types are commonly recognized
Desert or arid
Steppe or semiarid

4. Location of Deserts

5. Earth’s dry regions coincide with the subtropical high pressure belts & solar heating

6. Location Types of deserts Classified by location Subtropics
Low-latitude deserts
In the vicinities of the Tropics of Cancer and Capricorn
Areas of high pressure and sinking air that is compressed and warmed

7. Location Types of deserts Classified by location Middle-latitudes
Located in the deep interiors of continents
High mountains in the path of the prevailing winds produce a rainshadow desert

8. Location Types of deserts
Classified by environment in which they are formed
subtropical: in the hot dry latitudes between 20 and 30°, both north and south
rain shadow: on the landward side of coastal mountain ranges
coastal: along coasts bordering cold ocean currents
continental interior: deep within continents, far from major water sources
polar: in the cold dry polar regions, both north and south

9. Formation of Coastal Deserts

10. Weathering & Erosion Not as effective as in humid regions
Mechanical weathering forms unaltered rock and mineral fragments
Some chemical weathering does occur
caliche-rich soils
desert varnish

11. Weathering & Erosion Water Erosion Desert rainfall
Rain often occurs as heavy showers
Causes flash floods
Poorly integrated drainage
Most erosional work in a desert is done by running water

13. Weathering & Erosion Water Erosion Streams are dry most of the time
Desert streams are said to be ephemeral
Flow only during periods of rainfall
Different names are used for desert streams including wash, arroyo, wadi, donga, and nullah

14. A dry stream channel in the desert

15. The same stream channel following heavy rainfall

16. Weathering & Erosion Wind erosion
Differs from that of running water in two ways
Wind is less capable of picking up and transporting coarse materials
Wind is not confined to channels and can spread sediment over large areas

17. Weathering & Erosion Wind erosion Mechanisms of transport Bedload
Saltation – skipping and bouncing along the surface
Suspended load
In the air as duststorms

18. Weathering & Erosion Wind erosion Mechanisms of wind erosion Deflation
Lifting of loose material
Produces
Blowouts
Desert pavement

19. Weathering & Erosion Wind erosion Mechanisms of wind erosion Abrasion
Produces ventifacts (stones with flat faces) and yardangs (wind sculpted ridges)
Limited in vertical extent

20. Depositional Environments
Where does all of the eroded sediment go?
Water Deposits
Talus Aprons
Alluvial Fans
Bajada
Playas and Salt Lakes

21. Depositional Environments
Where does all of the eroded sediment go?
Wind deposits
Dunes
Mounds or ridges of sand
Often asymmetrically shaped
Characteristic features
Slip face
Cross beds

22. Depositional Environments

23. Depositional Environments
Where does all of the eroded sediment go?
Wind deposits
Dunes
Types of sand dunes

24. Depositional Environments
Where does all of the eroded sediment go?
Wind deposits
Loess
Deposits of windblown silt
Extensive blanket deposits
Primary sources are deserts and glacial stratified drift

25. Desert Landforms Butte Chimney cap rock Plateau Mesa Alluvial fan
sandstone
Alluvial fan
Pedestal
Rocky desert pavement
Playa lake
shale
Dunes

26. Desert Landforms
Rocky desert areas change over time

27. Desert Landforms
Rocky desert areas change over time

28. Desert Landforms

29. Basin and Range: the evolution of a desert landscape
Uplifted crustal blocks
Interior drainage into basins produces:
Alluvial fans and bajadas
Playas and playa lakes
Erosion of mountain mass causes local relief to continually diminish
Eventually mountains are reduced to a few large bedrock knobs called inselbergs projecting above a sediment filled basin

30. Basin and Range landscape

31. Landscape evolution in a mountainous desert – early stage

32. Landscape evolution in a mountainous desert – middle stage

33. Landscape evolution in a mountainous desert – late stage

34. Inselbergs in Southern California

35. ~ End ~