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Deserts

Deserts. 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.

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Deserts

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  1. 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. World's Deserts

  8. Atmospheric Convection

  9. A Rain-Shadow Desert

  10. Ocean-Current Desert

  11. Desert Landscape (Features) Weathering and desert streams create Desert features • Weathering in Desert is mostly mechanical - 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

  12. Chemical Weathering in Deserts

  13. Desert Landforms Produced By Water

  14. Uluru (Ayers Rock) Inselberg

  15. Playas

  16. Playas (cont'd)

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

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

  19. Water-Carved Canyon

  20. 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

  21. Blowout Caused by Deflation

  22. Desert Pavements

  23. Desert Pavements (cont'd)

  24. Desert Pavements (cont'd)

  25. Desert Pavements (cont'd) – Source:Martin Miller

  26. Desert Pavements (cont'd) –

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

  28. Origin of Ventifacts

  29. Origin of Ventifacts (cont’d) Wind

  30. Yardangs , White Desert, Egypt

  31. 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

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

  33. 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

  34. Beach Sand Dunes

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

  36. Large Scale Dunes (Gobi Desert)

  37. Dune Migration Just like ripples in a stream

  38. Dune Migration (cont'd)

  39. 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

  40. Transverse Dunes

  41. Transverse Dunes

  42. Longitudinal Dunes

  43. Longitudinal Dunes

  44. Barchan Dunes

  45. Barchan Dunes in Baja California

  46. Parabolic Dunes

  47. Star Dunes

  48. Star Dunes

  49. Lithified Sand Dunes (Jurassic Navajo Sandstone)

  50. Loess • Loess are deposits of glacial outwash silt

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