Lecture 16 - Deserts

1. Lecture 16 - Deserts

2. Definition of Desert • A desert is an area with less than 25 cm (10 inches) of annual precipitation • aridity index = potential evaporation/precipitation greater than 4.0 • Deserts may be cold, temperate or hot. All major continents have one type of desert or the other.

3. Wind-Swept Sand Dunes

4. Desert Life Forms (Roadrunner)

5. Desert Life Forms (Lizard)

6. Types of Deserts • Desert Types - Subtropical Desert – 30o Latitude - Deserts on Leeward side of major Mountain ranges - Interior Deserts- center of continents far from ocean - Coastal desert- prevailing onshore wind cooled by cold ocean current - Polar deserts- extremely cold and dry

7. Coriolis “turns” them The major wind cells

8. Subtropical Deserts +/- 30o latitude

9. A Rain-Shadow Desert

10. Ocean-Current Desert

11. Also Interior and Polar Deserts

12. Desert Landscape (Features) Weathering and desert streams create Desert features • Weathering in Desert is mostly mechanical - A little chemical weathering produces manganese and iron-oxide stains, called desert vanish • Stream Erosion - Arroyo- channel with water during periods of high discharge but dry most part of the year - Pediments- large-scale gently inclined surfaces - Inselberg- steep-sided knob of durable rock - Playa- dry lake bed

13. Chemical Weathering in Deserts

14. Desert Landforms Produced By Water

15. Uluru (Ayers Rock) Inselberg

16. Playas

17. Playas (cont'd)

18. Playas (cont'd) –A Playa in Death Valley, California

19. Swimmers in hypersaline Dead Sea Evaporite deposits indicate a dry climate in the geologic record

20. Water-Carved Canyon

21. Work of Winds • Erosion by Wind - Deflation- wind removes finer particles from the surface - Desert pavement- layer of pebbles left behind after deflation - Abrasion- sand blasting - Ventifacts- wind-shaped stones with sharp-edge faces - Yardangs- streamlined desert ridges

22. Blowout Caused by Deflation

23. Desert Pavements

24. Desert Pavements (cont'd)

25. Desert Pavements (cont'd)

26. Desert Pavements (cont'd) – These make good landing strips Source:Martin Miller

27. Desert Varnish slows infiltration Causes Flash Floods

28. Remnants of Wind Abrasionin addition to occasional flash flood erosion(surface grains are frosted)

29. Origin of Ventifacts

30. Origin of Ventifacts (cont’d) Wind

31. Yardangs , White Desert, Egypt

32. Transport By Wind • No dissolved load • Suspended Load- most consist of dust (silt, clay, pollen, bacteria, salt crystals, etc.) • Bed Load- sediments moved along or near the ground • Rolling or saltation- bed loads lifted off the ground momentarily due to force of collision with other grains

33. Transport of Wind-Borne Sediment Suspended Load Transports Sahara sediment to Caribbean and Amazon Rain Forest

34. Deposition of Dunes • Reduced wind velocity results in sediments deposition • Dunes are hills of loose wind-born sand - Size, shape, and orientation of dune are determined byavailable sand, vegetation, and wind

35. Beach Sand Dunes

36. Deposition of Wind’s Bed Load Rain – Shadow Desert in Lee Of Mountains

37. Large Scale Dunes (Gobi Desert)

38. Dune Migration Just like ripples in a stream

39. Dune Migration (cont'd)

40. Deposition and Dune Types • Dune Types - Transverse- ridges that are perpendicular to prevailing wind direction - Longitudinal- ridges that are parallel to prevailing wind direction - Barchans- crescent-shaped with horns pointing downwind - Horseshoe (Parabolic)- crescent-shaped with horns pointing upwind - Star- winds from three or more directions

41. Transverse Dunes

42. Transverse Dunes

43. Longitudinal Dunes

44. Longitudinal Dunes

45. Barchan Dunes

46. Barchan Dunes in Baja California

47. Parabolic Dunes

48. Star Dunes

49. Star Dunes

50. Lithified Sand Dunes (Jurassic Navajo Sandstone)