1. Deserts and Winds Chapter 19

2. A typical desert landscape, Sonoran Desert east of Phoenix, Arizona

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

4. Arid and Semiarid Climates Cover @ 30% of Earth’s Land Surface
Arid and Semiarid Climates 30% of Earth’s Land Surface. No Other Climate Group Covers So Much Area

5. Distribution and causes of dry lands
Dry regions are characterized by a combination of two important factors
Minimal rainfall (typically 5-10 inches annually)
High rates of evaporation (evaporation > precipitation)

6. Deserts are Defined by a Combination of Average Annual Temperature, Average Annual Rainfall, & How the Rainfall is Distributed

7. Distribution and causes of dry lands
Because evaporation is greatly reduced in a cold climate, areas near the earth’s poles can experience relatively little rainfall and still not exhibit desert conditions.
Climatologists consider the average annual temperature, the total annual precipitation & when the rain falls in determining desert conditions

8. Distribution and causes of dry lands
Dry lands are concentrated in two regions
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, resulting in little rainfall

9. Earth’s Wind Circulation Patterns Produce Sinking Air & Little Rain Near the Subtropical High Pressure Belts

10. Distribution and causes of dry lands
Dry lands are concentrated in two regions
Middle-latitudes
Located in the deep interiors of continents
High mountains in the path of the prevailing winds produce a rainshadow desert

11. Rainshadow deserts form in earth’s mid-latitude regions when high mountains cause air to rise and rain on their windward sides, then sink without raining on the leeward sides

12. Note the Distribution of Desert & Steppe Climates
Note the Distribution of Desert & Steppe Climates. They are Concentrated along the Subtropical Highs and in the Rainshadows of High Mountain Ranges. There are NO Deserts in the Polar Regions

13. Distribution and causes of dry lands
Human activity can hasten the “desertification” of steppe regions, usually as the result of excess water comsumption for grazing or irrigation
Areas at particular risk today include the southern Sahara steppe and the Aral Sea

14. The Aral Sea was replenished primarily by water from the Amu & Syr Rivers. Since 1960, most of their waters have been diverted for irrigation

15. Blooming, But This Region is Being Destroyed.
The Aral Sea is Shrinking Rapidly. It’s Fishing Economy is Gone & Entire Towns Are Disappearing. Other Areas Are
Blooming, But This Region is Being Destroyed.

16. Geologic processes in arid climates
Weathering
Not as effective as in humid regions
Mechanical weathering produces unaltered rock and mineral fragments
Some chemical weathering in deserts does produce
Clay
Thin soils
Oxidized minerals

17. Geologic processes in arid climates
Role of water in arid climates
Practically all streambeds are dry most of the time
Desert streams are classified as ephemeral
Carry water only during periods of rainfall
Different names are used for desert streams in various region
Wash and arroyo (western United States)
Wadi (Arabia and North Africa)
Despite its scarcity, water is still the major erosive agent in deserts

18. Desert Streams are Ephemeral – They Rarely Contain Water – But Erosion is Dramatic When “The Rains” Come

19. Geologic processes in arid climates
Role of water in arid climates
Desert rainfall
Rain often occurs as heavy, localized showers
Because desert vegetative cover is sparse, runoff is largely unhindered and flash floods are common
Poorly integrated drainage systems and streams lack an extensive system of tributaries
Most of the erosion work in a desert is done by running water

20. Tucson, Arizona, a desert
Rains in the Desert Can Be Spectacular and Quite Heavy, But They Cover Small Geographic Areas
Tucson, Arizona, a desert
Van Horn, Texas, a steppe

21. Basin and Range: Evolution of a desert landscape
Characterized by interior drainage
Landscape evolution in the Basin and Range region
Uplift of mountains – block faulting
Interior drainage into basins produces
Alluvial fans
Bajadas
Playas and Playa lakes

22. Typical “Block Faulting” (Normal Faulting) Creates Basin and Range Topography in Parts of the Western United States

23. Most Block Faulted Mountains in the Basin & Range Are Actually Horsts and Grabens. The Mountains Are Faulted on Both Sides and Moved “Up” Relative to the Intervening Valleys

24. Alluvial Fans Develop Along Horst Mountain Blocks in the Basin and Range Province Where Stream Gradients Change Abruptly from Steep to Flat

25. Coalescing Alluvial Fans Form Bajadas Along the Horst Block Mountain Fronts in Death Valley, Basin & Range Province

26. Playas Are Dry Lake Beds that are Common in the Graben Valleys of the Basin & Range. They Often Display Evaporative Salt Crusts
Bajada
Playa

27. When Localized Rains Fills the Playa with Water, it Becomes a Playa Lake

28. Basin and Range: Evolution of a desert landscape
Landscape evolution in the Basin and Range region
Ongoing erosion of the mountain mass
Produces sediment that fills the basin
Diminishes local relief
Produce isolated erosional remnants called inselbergs

29. Without Continued Uplift, the Horst Block Mountains are Gradually Eroded. The Resulting Sediment Fills the Graben Valleys, Leaving Only Isolated Inselbergs as Erosional Remnants

30. An Inselberg (Erosional Mountain Remnant) in the Basin & Range of Southern California

31. Mount Uluru (Formerly Ayers Rock) is an Inselberg in the Center of the Australian Outback

32. Wind in the desert Transportation of sediment by wind
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

33. Wind in the desert Transportation of sediment by wind
Mechanisms of transport
Bedload
Saltation – skipping and bouncing along the surface
About 20 to 25 percent of the sand transported in a sandstorm is moved this way
Suspended load
Silt and clay can be carried by wind for long distances

34. A Cloud of Saltating Sand Grains (Bed Load) Moves up the Gentle Slope of a Dune During a Time of High Desert Wind Velocity

35. Silt and Clay (Suspended Load) Transported by Wind During the “Dust Bowl” Days of the 1930’s

36. Dust from the Sahara Desert is transported by wind toward the Canary Islands. The Sahara is the world’s largest dust source today.

37. Wind in the desert Wind erosion
Wind is a relatively insignificant erosional agent with most erosion in a desert performed by intermittent running water
Mechanisms of wind erosion
Deflation
Lifting of loose material
Deflation produces blowouts (shallow depressions) and desert pavement (a surface of coarse pebbles and cobbles)

38. Formation of a Blowout
(A) Area Prior to Deflation (B) Shallow Depression After Deflation Has Removed Fine-Grained, Loose Material

39. Soil Mound Below The Bush Shows the Land Surface Prior to Blowout Formation

40. Formation of Desert Pavement
Formation of Desert Pavement! If Undisturbed, the Concentrated Layer of Coarse Pebbles Will Prevent Further Erosion, But The Layer is Thin & Easily Disrupted

41. Undistrubed Desert Pavement

42. Wind in the desert Wind erosion Mechanisms of wind erosion Abrasion
Produces ventifacts (stones with flat faces) and yardangs (wind sculpted ridges)
Limited in vertical extent

43. Ventifacts – Rocks That Are Polished and Shaped by Sandblasting

44. Wind in the desert Wind deposits
Significant depositional landforms are created by wind in some regions
Two types of wind deposits
Dunes
Mounds or ridges of sand
Often asymmetrically shaped
Windward slope is gently inclined and the leeward slope is called the slip face

45. Dunes Have an Asymmetrical Shape, with a Gently Sloping Windward Side and a Steeply Dipping Leeward Side (the Slip Face)

46. The Slip Face is Aptly Named
The Slip Face is Aptly Named. Because Grains are at or near the Angle of Repose, they readily Slide Down the Slip Face

47. Dune Deposits Are Characterized by their Dramatic Cross-Bedding, Which is Produced When Wind Directions Shift

48. Wind in the desert Wind deposits Two types of wind deposits Dunes
Slow migration of dunes occurs in the direction of wind movement
Several types of sand dunes are formed, including barchan, transverse, longitudinal, parabolic and star dunes

49. Dunes Display Various Shapes & Sizes, Controlled by a Combination of Wind Direction & Velocity, Abundance of Sand & Amount of Vegetation

50. Barchan Dunes – Crescent-Shaped with Tips Pointing Downwind
Barchan Dunes – Crescent-Shaped with Tips Pointing Downwind. Form When Sand Supply is Limited & Ground is Flat, Hard & Largely Free of Vegetation
Wind Direction

51. Barchanoid Dunes – Transverse Dunes with Scallops
Barchanoid Dunes – Transverse Dunes with Scallops. White Sands National Monument
Wind Direction
Transverse Dunes – Long Ridges Separated by Troughs. Form When Sand Supply is Abundant, Winds Are Steady & Vegetation is Sparse

52. Longitudinal Dunes – Long Sand Ridges Somewhat Parallel to the Prevailing Wind. Form When Sand Supply is Moderate & Wind Direction Variable, But Out of the Same Quadrant
Parabolic Dunes – Crescent Shaped with Tips Pointing Upwind. Form along Coastlines Where There is a Strong Onshore Wind, Abundant Sand & Vegetation to Stabilize the Dune

53. Star Dunes – Dunes that Create Isolated Hills of Sand that Display Complex Shapes. Form When Wind Directions Are Variable. Largely Confined to the Sahara & Arabian Deserts

54. Wind in the desert Wind deposits Two types of wind deposits Loess
Blankets of windblown silt
Two primary silt sources are deserts and glacial outwash deposits
Extensive deposits occur in China and the central United States

55. Loess Bluff @ 9 feet high along the Mississippi River
Loess 9 feet high along the Mississippi River. The Source of the Silt is Glacial Outwash