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?