AEOLIAN PROCESSES
Erosional Landforms
Characteristics of Wind
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Just like water air behaves like a fluid and
consequently there are similarities in the
mechanics of sediment transport
Wind speed varies vertically above the ground
 Increases
rapidly close to ground but increases at
a slower rate at greater heights
 Close to the ground it is reduced due to friction
with the surface
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As wind speed and surface roughness
increase the flow becomes more turbulent
Characteristics of Wind Influence of Surface
Characteristics
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Surface Characteristics that influence the capability of the
wind to erode
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Surface roughness
Surface particle size
Surface particle cohesion (how the particles “stick” together)
Vegetation characteristics
Presence of soil moisture
Airflows rarely uniform, characterised by bursts of higher
velocity flow
RATE of aeolian transport influenced by
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Wind speed
Air density
Particle size
Surface characteristics
Erosional Landforms
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Occurs through two principal processes
 Deflation
 Removal
of loose particles by the wind
 Abrasion
 Bombardment
of rock and other surfaces by particles
carried in the wind
Lag Deposits (Deflation)
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A surface comprising particles of gravel size or
coarser
Where stone cover is continuous such surfaces
are known as desert pavements
Deflation removes silt and fine sand
This lowers the ground level and leaves a
concentrated layer of rock and coarse sand which
acts as a ‘protective blanket’
This protective layer shields the underlying
material from the wind
Cover a significant proportion of the world’s
deserts
May form from the deflation of poorly sorted
sediments such as alluvium
Formation of a Lag Deposit
Fine particles (silt & clay)
❶ Wind blowing over a surface picks up the fine
particles (such as silts and clays). Those particles
that are too large to transport get left behind.
Coarse particles (e.g. gravel)
Original Surface
❷ Over time the surface gets lowered (deflation)
with a concentration of gravel sized particles on
the surface. This serves as a ‘protective blanket’
which prevents the wind from transporting the fine
material lying underneath.
Deflation Hollows and Pans
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Deflation can scour out large or small
depressions, termed deflation hollows or
blowouts
Blowouts
 Commonest
landform produced by wind erosion
 Most common in weak unconsolidated sediments
 They are no deeper than the water table and may
attain diameters of kilometres
Deflation Hollows and Pans
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Pans
 Closed
depressions that are common in many
dryland environments
 At least partly formed by deflation
 Range in size from only a few metres wide and
centimetres deep, to kilometres across and tens
of meters deep
 They sometimes have clay or lunette dunes that
form on their leeward side
Yardangs
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Streamlined, sharp & sinuous ridges that extend
parallel to the wind and are separated by parallel
depressions
Usually less than 10m high and 100m or more in
length
Typically tapering away from the prevailing wind
Most commonly developed in soft lithologies (such
as lacustrine (lake) sediments) but can occur in
extremely resistant lithologies (such as granite &
quartzite)
Develop where wind processes dominate over
fluvial processes and are limited to extremely arid
deserts
Yardangs
Ventifacts
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Cobbles and pebbles on stony desert surfaces often
bear facets called ventifacts.
Range in size from small pebbles to large boulders
Forms through the abrasion of pebbles by dust and
silt
A facet is a relatively plane surface cut at right angles
to the wind regardless of the original shape of the
stone
They join along a sharp ridge or “keel”
The number of keels is used to describe the ventifact
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Einkanter (one-ridged)
Zweikanter (two-ridged)
Driekanter (three-ridged)
Ventifacts
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Formation influenced by
 Wind
frequency
 Wind magnitude and persistence
 Sediment supply
 Vegetation cover
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Abrasion rates determined by
 Velocity
of the wind
 Strength of the rock
 Size, shape & density of impacting grains
Formation of Ventifacts
❶ Wind blown
particles
Boulder
An exposed rock or boulder is exposed on the surface. As the wind blows, it picks
up fine particles. These particles act as an abrasive when it collides with the
boulder trimming and polishing the surface.
❷ Wind blown
particles
Boulder
Eventually the rock will be worn back to a smooth, polished sloping surface as
illustrated. The slope is due to the fact that the wind speed increases with height
up to a certain point (increasing the abrasive force) and that friction close to the
surface results inminimal abrasion occurring on the boulder.
Ventifact
3
3 Keels present on rock
2
1
Therefore this example
represents a Driekanter
keels
Wind Direction
Desert Dust
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Highly significant component of desert system
Affects:
Geomorphological processes
 Biological processes
 Atmospheric conditions
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Can also influence atmosphere, lands and oceans
far from its source
Atmospheric dust is derived mainly from aeolian
erosion with the Sahara-Sahel region and the
Gobi-Taklimakan region being the most important
source areas
Desert Dust
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Classifications
Type
Characteristics
Dust Event
Reduction of visibility to <11km
Blowing Dust
Material being entrained within sight of the observer, but not
obscuring visibility to less than 1000m
Dust Storm
Blowing dust events with visibility reductions to less than 1000m
Fugitive Dust
Dust that results from mechanical means, such as traffic on
unpaved roads
Dust Devil
Short lived and localised column of dust that does not travel for
any great distance
Canary
Islands
Sahara Desert
Namibia
Atlantic
Ocean