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Arid and Semi-Arid Landscapes 1. Arid (Desert) has Precipitation (mm) < 10 x Temperature in o C 25 o C Desert < 25 cm = 250 mm rain 25 o C Desert < 25.

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Presentation on theme: "Arid and Semi-Arid Landscapes 1. Arid (Desert) has Precipitation (mm) < 10 x Temperature in o C 25 o C Desert < 25 cm = 250 mm rain 25 o C Desert < 25."— Presentation transcript:

1 Arid and Semi-Arid Landscapes 1. Arid (Desert) has Precipitation (mm) < 10 x Temperature in o C 25 o C Desert < 25 cm = 250 mm rain 25 o C Desert < 25 cm = 250 mm rain 2. UNESCO index Desert Precipitation/Potential Evaporation < 0.25 Desert Aridity Index = Potential Evaporation/ Precipitation < 4.0 Desert Aridity Index = Potential Evaporation/ Precipitation < 4.0 About 10% of a non-polar desert is dune covered Skip Lawson's Theory pp. 283-284

2 World's Deserts about 1/5 th of land Types: subtropical, rain-shadow, coastal, interior, polar

3 Semi-Arid Semi-arid (Steppe, Savannah, dry grassland) has Precipitation (mm) > 10 x Temperature in o C but 10 x Temperature in o C but < than 20x (c) 2004-2009 Charles L. Smart. All rights reserved

4 Flash Floods Role of Desert Varnish, lack of plant cover Stream Flood (c) 2004-2009 Charles L. Smart. All rights reserved

5 Desert Landforms Produced By Water playa pediment Inselberg arroyo bahada alluvial fan

6 Arroyo - Water-Carved Canyon Usually dry

7 Playa, Salar, Sabkha A Playa in Death Valley, California Evaporite Pan of dried-up Playa

8 Inselberg Uluru (Ayers Rock) Pediment

9 We have been studying the physics of wind generated landforms when we consider sand grains in wind tunnels Eolian Processes Wind storm, Lothagam Hill dunes

10 Physics of Wind Transport Air has low density, so wind is a relatively minor agent of geomorphic changeAir has low density, so wind is a relatively minor agent of geomorphic change Rock density 2000 times density same volume of airRock density 2000 times density same volume of air so rock fragments are difficult to lift with air. so rock fragments are difficult to lift with air. A one meter per second (1 m/sec) flow of water lifts the same particle as about 30 m/sec flow of airA one meter per second (1 m/sec) flow of water lifts the same particle as about 30 m/sec flow of air Sandstorm

11 Water and wind formed erosional features such as this toadstool Toadstools "Undercut, mushroom shaped pedestal rocks in desert areas are commonly attributed to wind erosion""Undercut, mushroom shaped pedestal rocks in desert areas are commonly attributed to wind erosion" Ignore Bloom's comments p291 right 1 following the line above.Ignore Bloom's comments p291 right 1 following the line above. This area is frequently hit with high sandstorms. These remove the paint from our trucks.This area is frequently hit with high sandstorms. These remove the paint from our trucks.

12 Wind-Borne Sediment causes deflation and abrasion Sahara to Caribbean Effect on Hurricanes Surface creep

13 Desert Pavements or Reg Note desert varnish, lack of sand, Iron Oxide over everything Infiltration is slow – Flash Floods, and most water leaves the area

14 Desert Pavements (cont'd) – Source: Martin Miller Makes a great runway for supply aircraft Just clear away a few big rocks We tow makeshift rakes behind our Land Rovers Hammada (Hamada) = barren rock Desert Pavement

15 Erosional Landform - Deflation Hollow Anecdote – How fast does this occur?

16 Abrasion Origin of Ventifacts

17 Erosional Landforms - Ventifacts Wind eroded (sand blasted) surfaces

18 Erosional Landform - Yardangs Sphinx? White Desert, Egypt

19 Deposition Landforms of Eolian Sands Reduced wind velocity results in sediments depositionReduced wind velocity results in sediments deposition Dunes are hills of loose wind-born sandDunes are hills of loose wind-born sand Size, shape, and orientation of dunes are determined by available sand, vegetation, and windSize, shape, and orientation of dunes are determined by available sand, vegetation, and wind Sand drift in Lee Of Mountains

20 Dunes form in a lee Dunes are accumulations in the lee of local obstructions, possibly an upwind dune High velocity zone

21 Dune Migration Just like ripples in a stream Windward slope erosion and transport Slip face (deposition)

22 Star Dunes

23 Copyright © Frank Eckardt 2002 Star Dunes Variable Wind Direction

24 Longitudinal Dunes Constant wind direction, no vegetation

25 Longitudinal Dunes Constant wind direction, no vegetation

26 Transverse Dunes Massive volumes of sand, low consistent winds

27 Transverse Dunes Massive volumes of sand, low consistent winds

28 Barchan Dunes Isolated Dunes, low sand supply, migrating across rock

29 Barchan Dunes Isolated – low sand supply; migrating across rock

30 Parabolic Dunes Horns pinned by plants Typical of coasts

31 Parabolic Dunes Stabilized “horns" point upwind

32 Dunes in Victoria Crater - Mars

33 Suspended Load Transport and Deposition In dry areas very high winds can suspend fine sand particlesIn dry areas very high winds can suspend fine sand particles

34 Suspended Load Dust

35 Loess from the Columbia River Basin Loess deposits are (usually) of glacial outwash silt. These thin downwind.

36 Global loess Origins –Desert –Glacial Layers of dust from Patagonia appear in Vostok cores just prior to interglacials

37 End Arid and semi-arid What type of dune? Wind


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