Arid and Semi-Arid Arid and Semi-Arid LandscapesLandscapes

1. Arid (Desert) has Precipitation (mm) < 10 x Temperature in 1. Arid (Desert) has Precipitation (mm) < 10 x Temperature in ooCC 2525oo C Desert < 25 cm = 250 mm rain C Desert < 25 cm = 250 mm rain

2. UNESCO index 2. UNESCO index Desert Precipitation/Potential Evaporation < 0.25Desert Precipitation/Potential Evaporation < 0.25 Aridity Index = Potential Evaporation/ Precipitation > 4.0Aridity Index = Potential Evaporation/ Precipitation > 4.0

About 10% of a About 10% of a non-polar desert is non-polar desert is dune covereddune covered

Skip Lawson's Theory pp. 283-284

World's DesertsWorld's Deserts about 1/5th of land

Types: subtropical, rain-shadow,coastal, interior, polar

Semi-AridSemi-AridSemi-arid (Steppe, Savannah, dry grassland) has Semi-arid (Steppe, Savannah, dry grassland) has Precipitation (mm) > 10 x Temperature in Precipitation (mm) > 10 x Temperature in ooC but < than 20xC but < than 20x

(c) 2004-2013 Charles L. Smart. All rights reserved

Flash FloodsFlash Floods

Role of Desert Varnish, lack of plant cover

Stream Flood

(c) 2004-2013 Charles L. Smart. All rights reserved

Desert Landforms Produced By Desert Landforms Produced By WaterWater

playa

playa

pediment

Inselberg

arroyo

bajadaalluvial fan

Arroyo - Water-Carved CanyonArroyo - Water-Carved Canyon

Usually dry

Playa, Salar, SabkhaPlaya, Salar, SabkhaA Playa in Death Valley, CaliforniaA Playa in Death Valley, California

Evaporite Pan of dried-up PlayaEvaporite Pan of dried-up Playa

Inselberg, Monadnock, KopjeInselberg, Monadnock, Kopje

UluruUluru (Ayers Rock) (Ayers Rock)

Pediment

The physics of wind generated The physics of wind generated landforms is studied using sand landforms is studied using sand

grains in wind tunnelsgrains in wind tunnels

Eolian Processes Eolian Processes

Wind storm, Lothagam Hill dunesWind storm, Lothagam Hill dunes

Physics of Wind TransportPhysics of Wind Transport

• Air has low density, so wind is a relatively minor Air has low density, so wind is a relatively minor agent of geomorphic changeagent 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 A one meter per second (1 m/sec) flow of water lifts

the same particle as about 30 m/sec flow of airthe same particle as about 30 m/sec flow of air

Sandstorm

Water and wind formed erosional features such as this toadstool

ToadstoolsToadstools• "Undercut, mushroom shaped pedestal rocks in "Undercut, mushroom shaped pedestal rocks in

desert areas are commonly attributed to wind desert areas are commonly attributed to wind erosion"erosion"

• Ignore Bloom's comments p291 right 1 Ignore Bloom's comments p291 right 1 following the line above. following the line above.

• This area is frequently hit with high sandstorms. This area is frequently hit with high sandstorms. These remove the paint from our trucks.These remove the paint from our trucks.

Wind-Borne Sediment causes Wind-Borne Sediment causes deflation and abrasiondeflation and abrasion

Sahara to CaribbeanEffect on Hurricanes

Surface creep

Desert Pavements or RegDesert Pavements or Reg

Note desert varnish, lack of sand, Iron and Note desert varnish, lack of sand, Iron and Manganese oxides over everythingManganese oxides over everything

Infiltration is slow – Flash Floods, and most water Infiltration is slow – Flash Floods, and most water leaves the arealeaves the area

Desert Pavements (cont'd) Desert Pavements (cont'd) – –

Source: Martin Miller

Makes a great runway for supply aircraftMakes a great runway for supply aircraftJust clear away a few big rocksJust clear away a few big rocks

We tow makeshift rakes behind our Land RoversWe tow makeshift rakes behind our Land Rovers

Hammada (Hamada) = barren rockHammada (Hamada) = barren rock

Desert PavementDesert Pavement

Erosional Landform - Deflation HollowErosional Landform - Deflation Hollow

Anecdote – How fast does this occur?

Abrasion Origin of VentifactsAbrasion Origin of Ventifacts

Erosional Landforms - Erosional Landforms - VentifactsVentifactsWind eroded (sand blasted) surfaces

Erosional Landform - Erosional Landform - Yardangs Yardangs

Sphinx?

White Desert, EgyptWhite Desert, Egypt

Deposition Landforms of Eolian Deposition Landforms of Eolian SandsSands

• Reduced wind velocity results in sediments Reduced wind velocity results in sediments depositiondeposition

• Dunes are hills of loose wind-born sandDunes are hills of loose wind-born sand

• Size, shape, and orientation of dunes are Size, shape, and orientation of dunes are determined by available sand, vegetation, and determined by available sand, vegetation, and windwind

Sand drift in Lee Of Mountains

Only 10% of desert surface

Dunes form in a leeDunes form in a lee

Dunes are accumulations in the lee of local obstructions, possibly an upwind dune

High velocity zone

Dune MigrationDune Migration

Just like ripples in a stream

Windward slopeerosion and transport

Slip face (deposition)

Star DunesStar Dunes

Copyright © Frank Eckardt 2002

Star DunesStar Dunes

Variable Wind DirectionVariable Wind Direction

Longitudinal DunesLongitudinal Dunes

Constant wind direction, no vegetation

Longitudinal DunesLongitudinal Dunes

Constant wind direction, no vegetation

Transverse Dunes Transverse Dunes

Massive volumes of sand, slow but consistent winds

Transverse Dunes Transverse Dunes

Massive volumes of sand, low consistent windsMassive volumes of sand, low consistent winds

Barchan DunesBarchan Dunes

Isolated Dunes, low sand supply, migrating across rock

Barchan DunesBarchan Dunes• Isolated – low sand supply; migrating across rock

Parabolic DunesParabolic Dunes

Horns pinned by plantsTypical of coasts

Parabolic DunesParabolic Dunes

• Stabilized “horns" point upwind

Dunes in Victoria Crater - Dunes in Victoria Crater - MarsMars

Suspended Load Transport and DepositionSuspended 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

Suspended Load DustSuspended Load Dust

Loess from the Columbia River BasinLoess from the Columbia River Basin

•Loess deposits are (usually) of glacial outwash silt. These thin downwind.

Global loessGlobal loess

• Origins– Desert– Glacial

Layers of dust from Patagonia appear in Vostok cores just prior to interglacials

End Arid and semi-aridEnd Arid and semi-arid

What type of dune?

Wind