About 10% of a
non-polar desert is
dune covered
Arid and Semi-Arid
Landscapes
1. Arid (Desert) has Precipitation (mm) < 10 x Temperature in oC
25o C Desert < 25 cm = 250 mm rain
2. UNESCO index
Desert Precipitation/Potential Evaporation < 0.25
Desert Aridity Index = Potential Evaporation/ Precipitation < 4.0
Skip Lawson's Theory pp. 283-284
World's Deserts
about 1/5th of land
Types: subtropical, rain-shadow,
coastal, interior, polar
Semi-Arid
Semi-arid (Steppe, Savannah, dry grassland) has
Precipitation (mm) > 10 x Temperature in oC but < than 20x
(c) 2004-2009 Charles L. Smart. All rights reserved
Flash Floods
Role of Desert Varnish, lack of plant cover
Stream Flood
(c) 2004-2009 Charles L. Smart. All rights reserved
Desert Landforms Produced By
Water
alluvial fan
Arroyo - Water-Carved Canyon
Usually dry
Playa, Salar, Sabkha
A Playa in Death Valley, California
Evaporite Pan of dried-up Playa
Inselberg
Uluru (Ayers Rock)
Pediment
Eolian Processes
We have been studying the physics of
wind generated landforms when we
consider sand grains in wind tunnels
Wind storm, Lothagam Hill dunes
Physics of Wind Transport
Sandstorm
• Air has low density, so wind is a relatively minor
agent of geomorphic change
• Rock density 2000 times density same volume of 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 air
Toadstools
• "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.
• This area is frequently hit with high
sandstorms. These remove the paint from
our trucks.
Water and wind formed erosional features such as this toadstool
Wind-Borne Sediment causes
deflation and abrasion
Sahara to Caribbean
Effect on Hurricanes
Surface creep
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
Desert Pavements (cont'd) –
Hammada (Hamada) = barren rock
Makes a great runway for supply aircraft
Just clear away a few big rocks
We tow makeshift rakes behind our Land Rovers
Desert Pavement
Source: Martin Miller
Erosional Landform - Deflation Hollow
Anecdote – How fast does this occur?
Abrasion Origin of Ventifacts
Erosional Landforms - Ventifacts
Wind eroded (sand blasted) surfaces
Erosional Landform - Yardangs
Sphinx?
White Desert, Egypt
Deposition Landforms of Eolian
Sands
• Reduced wind velocity results in sediments
deposition
• Dunes are hills of loose wind-born sand
• Size, shape, and orientation of dunes are
determined by available sand, vegetation, and
wind
Sand drift in Lee Of Mountains
Dunes form in a lee
High velocity zone
Dunes are accumulations in the lee of local obstructions, possibly an upwind dune
Dune Migration
Windward slope
erosion and transport
Just like ripples in a stream
Slip face (deposition)
Star Dunes
Star Dunes
Variable Wind Direction
Copyright © Frank Eckardt 2002
Longitudinal Dunes
Constant wind direction, no vegetation
Longitudinal Dunes
Constant wind direction, no vegetation
Transverse Dunes
Massive volumes of sand, low consistent winds
Transverse Dunes
Massive volumes of sand, low consistent winds
Barchan Dunes
Isolated Dunes, low sand supply, migrating across rock
Barchan Dunes
• Isolated – low sand supply; migrating across rock
Parabolic Dunes
Horns pinned by plants
Typical of coasts
Parabolic Dunes
• Stabilized “horns" point upwind
Dunes in Victoria Crater Mars
Suspended Load Transport and Deposition
• In dry areas very high winds can suspend fine sand particles
Suspended Load Dust
Loess from the Columbia River Basin
•Loess deposits are (usually) of glacial outwash silt. These thin downwind.
Global loess
• Origins
– Desert
– Glacial
Layers of dust from Patagonia appear in Vostok cores just prior to interglacials
End Arid and semi-arid
What type of dune?