GEOG 140
Intro Physical Geography
Lecture Notes
Eolian Processes and Arid Landscapes
Major Concepts
-Wind is not an effective agent in eroding landscapes
-Limited ability to erode solid rock
-Can transport loose unconsolidated fragments of sand and dust
-Wind transports sand by saltation and surface creep
-Smaller particles transported in suspension
-Can remain high in the atmosphere for long periods
-Sand dunes migrate
-Sand grains blown up and over windward side
-Accumulate on the lee slope
-Internal structure of dune consists of strata inclined in the downwind direction
-Various types of dunes form based on:
-Wind velocity
-Sand supply
-Constancy of wind direction
-Surface characteristics (in the path of dune migration)
-Wind blown dust (loess) forms blanket deposits
-Can mask older landscapes
-Originated as desert dust or fine rock debris deposited by glaciers
-Eolian processes require special conditions
-An atmosphere
-Moon has no aeolian landforms
-Mars has aeolian landforms (atmosphere composed primarily of carbon dioxide)
-Earth atmosphere has low density and viscosity
-Compared to water
-Only fine particles can be carried in suspension (except at high velocities)
-Lack of complete vegetation cover
-Vegetation reduces wind speeds
-Roots bind soil particles
-Surface material that dries out periodically
-Moisture acts as binding agent for soil particles
Global Distribution of Eolian Environments

-36% of the world's land surface classified as:
-Dry savanna
-Semi-arid
-Arid (19%)
-Arid regions
-Largely devoid of vegetation
-25-33% covered with mobile sand
-Contained in "sand seas" (ergs)
-Larger than 125 sq. kilometers
-85% larger than 32,000 sq. kilometers
-Largest 560,000 sq. kilometers (Rub al Khali, Saudi Arabia)
-Desert sand sources
-Modal erg size approx. 190,000 sq. kilometers
-Present areas of sand movement generally located within 150mm isohyet
-Ephemeral stream channels and other fluvial deposits
-Coastal deposits
-Earlier dunes
-Weathering of sandstones
Eolian Erosion
-Two processes
-Deflation
-Removal of loose particles by the wind
-Sand sized particles - localized
-Silt/clay sized particles - carried great distances
-Deserts major source of atmospheric dust
-Landforms
-130 - 800 metric tons of material deflated annually
-Sahara - 60 to 200 metric tons
-Some deposited on land
-Some carried out over oceans
-Sedimentation of ocean basins
-Desert pavement
-Many deserts not covered by sand
-Lag deposits of gravel size material (and coarser)
-Form thin, continuous layer over finer material
-Result of deflation of poorly sorted deposits (alluvium)
-Mixture of gravel, sand, and silt
-Finer particles removed by wind
-Coarse material concentrated on the surface
-Deflation hollows
-Protects remaining fine material below
-Shallow depressions in deserts of low relief
-Wide range of sizes
-Form in areas with high wind velocities and bare surfaces comprised of fine-grained materials
-Occurrence (differential erosion)
-Varying soil moisture conditions
-Disturbance of vegetation cover
-Abrasion

-Airborne sand acts as sandblaster
-Effectiveness of wind borne particles matter of controversy for years
-Present consensus
-Wind action of relatively minor significance in erosional landforms
-Limited importance in near surface abrasion
-Greatest quantity of material transported near the surface
-Amount of material important to abrasion rates
-Near surface - high material/low velocity
-At height - low material/high velocity
-Studies in Mojave Desert (1952-1963)
Eolian Transportation
-Particle sizes
-110cm lucite rods
-Maximum abrasion 23cm
-Landforms
-Ventifacts
-Faceted rocks abraded by long periods of wind action
-Two or more flat faces that meet at sharp ridges
-Generally well polished
-Yardangs
-Narrow range of particle sizes transported near the ground
-0.1 - 1.0mm (fine to coarse grained sand)
-Particles larger than 1mm
-Require high velocity winds(infrequent)
-Particles smaller than 0.1mm
-Cohesive clay
-Streamlined hills
-Range in size from a meter to a kilometer
-Look like an inverted ship hull
-Aligned with prevailing wind
-Tapering away from the prevailing wind
-Usually form in soft material
-Lake sediments
-Also found in granites and quartzite (Egypt)
-Primarily eolian (evidence)
-Oriented with wind direction
-Formed by removal of material between yardangs
-Often undercut at base
-Lower surface roughness (less turbulence)
-Difficult to move
-Quartz silt
-Transported over large distances/high altitudes (loess)
-Factors affecting particle movement at a given wind speed
-Size
-Density and shape
-Packing of adjacent grains
-Cohesion between particles

-Sand moves by two processes
-Saltation (80%)
-Grains propelled into near surface moving air layer
-Aerodynamic lift (velocity increase with height)
-Higher wind speed for air flowing over upper surface than lower
-pressure difference results in lift
-Impact of other grains returning to the surface
-Ballistic impact
-Majority of transport takes place less than a meter above the surface
-Near vertical rise from the surface
-Return to surface at an angle of 6°-12°
-Cover distances .5 - 1.5 meters
-Surface creep (20%)
-Grains too large to saltate at a given velocity
-Generated by surface wind shear
-and the impact of smaller saltating grains
-Fluid threshold and impact threshold
-Fluid threshold
-Critical wind drag velocity to set particles of a given size into motion
(airborne)
-Less wind velocity required to keep particles in suspension once airborne
-Impact threshold
-Critical wind velocity at which airborne particles fall out of suspension
Eolian Depositional Landforms
-Ripples
-Wave like undulations oreinted at right angles to the wind
-Heights range from 1 to 500mm
-Wavelengths range from 0.01 to 5 meters
-Correspond to mean length of jumps by saltating grains
-Wavelengths and heights increase with increasing wind velocity
-Coarsest materials collect at crests
-Dunes
-Heights 5 to 30 meters
-Wavelengths 50 to 300 meters
-Types
Dune Characteristics
-Cresentic (barchan)
-Linear (longitudinal, seif)
-Star dunes
-Parabolic dunes
-Blowout dunes
-Dome dunes
-Reversing dunes

-Accumulation of wind blown sand forming a mound or ridge
-Gentle upwind slopes on stoss (wind-facing) side
-Steep avalanche slope on the lee slope
-Slipface
-Dunes may have more than one slipface
-Stands at angle of repose
-Maximum angle at which loose material is stable (30 to 34° for sand)
-Minimum height 30 cm (Bagnold)
-Sand particles transported up stoss slope by saltation and creep
-When load of particles reaching the brink exceeds the angle of repose a small avalanche occurs
-Reforms the slipface
-Results in dune migration
Dune Types
-Crescentic dunes (Transverse, Barchaniod, Barchan)
-Most common dune form on Earth and Mars
-Slipface on the concave side
-Fastest moving dune type
-50-100 meters per year common
-Form under dominantly unidirectional winds
-Limited/diminishing sand supply (barchan)
-Large sand supply
-Complex pattern of sinuous transverse dunes
-Barchan dunes
-Horns on the downwind ends
-No slipfaces
-Largest -Taklimakan Desert- 3.25km horn to horn
-Linear (Longitudinal, Seif) dunes
-Straight or slightly sinuous ridges with slip faces occurring alternately on both sides
-Cover more area than any other dune type
-Usually form sets of parallel ridges separated by kilometers of sand, gravel or rock interdune corridors
-Can occur as isolated ridges
-Star dunes (pyramid, radial)
-Radially symmetrical, pyramidal sand mounds
-Slipfaces on three or more arms radiating from central high point
-Accumulate in areas with multidirectional wind regime
-Grow upward rather than migrate laterally
-Occur on the margins of sand seas
-Particularly near topographic barriers
-Dominant dunes in Grand Erg Oriental
-Parabolic dunes (u-shaped, hairpin)

-U-shaped mounds of sand
-Convex noses trailed by elongated arms
-Slipfaces on their outer convex sides
-Found in coastal deserts
-Unidirectional winds
-Opposite geometry of crescentic dunes
-Horns point upwind
-Fixed by vegetation
-Bulk of sand migrates forward
-Largest near Columbia/Venezuela border
-Trailing arm 12 kilometers long
-Blowout dunes
-Small (meter size) dunes that form downwind from deflation hollows
-Blowouts scooped out by wind erosion of a sand deposit
-May evolve into parabolic dunes
-Prevailing wind advances a section of the rim
-Vegetation stabilizes the arms
-Dome dunes
-Semicircular mounds
-Lack slipfaces
-Reversing dunes
-Variation of previous types that occur when wind reverses direction
-Major and minor slipfaces
-Oriented in opposite directions
-Tend to grow in height
Loess Deposits
-Migrate slowly if at all
-(McKee) intermediate in character between star and linear dune
-(Breed) examples of reversing dunes that are modified crescentic dunes
-May cover as much as 10% of Earth's surface
-Widespread in semiarid areas along the margins of major deserts
-Equatorial tropics free of loess
-Washed away as soon as it is deposited
-Former areas of continental glaciation also loess-free
-Covered by ice a few thousand years ago
-Distinctive sedimentary deposit
-Very small particles
-Angular fragments of quartz predominate
-Deposits lack stratification and erode into vertical cliffs
-Deposits derived from
-Deserts
-Glacial deposits