Deserts

1. Deserts

2. PART 1 Introduction to deserts (not desserts!!!)

3. What is a desert… really?

4. The common definition of “desert” is a region that receives less than 25cm of rain a year on average Compare thisto Vancouver: approximately 110 cm a year Total yearly evaporation > precipitation

5. Tropical desert characteristics Sparse vegetation Strong daily winds Extreme heat Flash floods Hard rocky surfaces

6. Did you know? Only 15% of the world' s desert surface is pure sand. (Parts of the Sahara and Arabia desert)

7. How do deserts come form?

8. Rainfall cycle Evaporation lifts pure water up from the sea.. Onshore winds from the oceans carry air that is rich in moisture over the land. Air rises and cools. Water vapour condense and falls as rain. It provides vital sustenance before returning to the sea through ground water, streams and rivers.

9. 1) Latitude The equator is an area of constant low pressure Hot air at the equator rises and spreads (up and away) north and south before cooling, condensing and releasing its moisture over the tropical zones (high pressure belts)

11. Deserts occurs in 2 broad belts: at 20-30 degrees north and south of the Equator, along the Tropics of Cancer and Capricorn. Desert areas develop under the influence of the quite permanent high-pressure areas.

12. 2) Mountain barriers Winds carry moisture-laden air into the mountains where it is forced to rise over mountain barriers Rising air cools, water vapours condense, and heavy rain falls Moving past the range, the air descends, heats up and driesout  No clouds and sunny skies on the other side!

15. 3) Cold ocean currents Warm air over cold water  air cools, condenses and falls as rain Air drops moisture over the water As it travels to warm land, air temperature rises Warm air can hold a lot of moisture  no cloud or rain over desert

16. Cold ocean currents responsible for creating: Namib Desert Atacama Desert

17. 4) Ocean influence Lands far away from oceanic influence The longer an air mass is travelling, the greater its chances of losing moisture An example is the Gobi Desert

18. How does wind affect deserts?

19. Wind erosion 1 Wind picks up light surface materials and carry them in suspension Saltation: a method of transport where particles roll and bounce along the ground

20. Wind erosion 2 Sparse vegetation = strong winds Lighter materials: silt, clay - carried higher up by wind Heavier: sand - carried close to the ground by stronger wind - Grinds and scours

21. What is a toadstool?

22. No, silly…

23. Even telephone poles are affected!

24. What is deflation?

25. Deflation Wind blows away sand, silt and clay so rocks underneath are exposed Creates blowouts in Badlands Blowouts can form oases if it is deep enough to reach an underground water source (aquifers)

26. Badlands can have spectacular coloured rock layers

27. Did you know? Desert occupies about one fifth to one third of the earth' s surface.

28. How does water affect deserts?

29. Rain is rare but when it happens it usually does in the form of sudden, intense thundershowers Flash floods Alluvial fan (did you get this right?)

30. Alluvial fans

31. Alluvial fans (in colour!)

32. PART 2 Desert sand dunes and landforms

33. What is a dune? A dune is any accumulation of sand-size, windblown materials.

34. Where are dunes found? Dunes occur where there is a large supply of sand, wind to move it and a place where it can accumulate. Dunes are found on coasts, near rivers and in desert basins

35. Desert sand dunes Individual sand granules (grains of sand) accumulate through both water and wind (eolian) transportation Individual granules form perpendicularly to the wind's direction, forming small ripples. As more granules collect, dunes form.

36. Dunes are the most common depositional landform of the desert Windward slope: long, gentle (saltation) Leeward slope (SLIP FACE): steep (rolling and sliding rocks)

37. Did you know? Quick sand is wet sand held up by the water within it Usually not more than thigh deep

38. There are four types of domes…

39. But first… please note that… Dunes are influenced by the strength and direction of the wind The vegetation The landforms in the area

40. 1) Transverse dune Abundant sand is available A continuous sand ridge Right angles to the wind

41. 2) Barchan dune Crescent-shaped Over a small obstruction Horns point downward Migrate slowly in direction of wind May be a part of transverse dunes if they align together

42. 3) Parabolic dune Crescent-shaped Forms around a blowout Horns point up wind Vegetation anchors horns

43. 4) Longitudinal dune Thin layer of sand Winds blow continuously one-way Long, straight and parallel to the wind

45. Try not to “meanwhile” in class…

46. Physical weathering Because of the limited precipitation, physical weathering is relatively more important Heating-cooling, freeze-thaw and exfoliation are common Produces angular rock materials

47. Paradoxically,… Water is the chief agent of erosion in deserts Heavy rains occur during intense thunderstorms Hard, bare ground is easily eroded by run-off Few plants= cannot break the fall of raindrops, slow down water’s movement, and hold on to the soil

48. A flash flood in Southern Utah http://www.youtube.com/watch?v=kV4aF4AZtY0

49. A reminder

50. Unique desert landforms Running water is responsible for creating many of these landscapes