Download PART III : AEOLIAN or WIND GEOMORPHOLOGY

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GEOMORPHOLOGY 201 READER
PART III : AEOLIAN or WIND GEOMORPHOLOGY & LANDFORMS
While water is the most important landscape-forming agent in temperate and humid climates, ice
fulfils this role in colder climates. Coastal geomorphic processes (wave and tidal action) likewise are
the major processes where fluvial systems and oceans meet. The landscaping effects of groundwater
action are limited to karst areas, which is about 10% of the earth,s surface where limestone and
dolomite formations occur. The remaining landscape creating work is done by wind in all areas where
water is in short supply. As South Africa is considered a water scarce country, aeolian, or wind-driven
processes are therefore a key player in determining the form and function of our landscapes.
Unique characteristics of arid regions include that chemical weathering occurs at a much slower rate
than where water is abundant. Physical weathering is therefore the primary weathering agent and
results in rocks and talus being deposited on the land surface. These materials are largely chemically
unchanged and are further characterised by a low content of organic material and water, which in
turn is responsible for very slow rates of soil formation. Dry regions are also characterised by sparse
vegetation, which would normally protect and anchor soil and talus; the conditions in arid regions are
therefore especially favourable for wind action to blow away dry talus and to be a major force in
landscape genesis.
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The distribution of Aeolian geomorphic action on earth
While wind occurs everywhere on the earth’s surface, it is only where surface water is scarce that
wind action becomes a major landform- creating agent. The main regulators determining if a
landscape will be prone to mainly water or wind erosion are therefore climate and more specifically
temperature and rainfall. Hot deserts and semi-deserts (BSh & BWh regions according to the Koppen
classification system) are the regions most prone to wind as landform creating agent. While wind is the
more prevalent force in shaping desert landforms, water is the more powerful. Despite water being so
scarce in these environments, it is the primary agent of erosion in the desert, with many features owing
their formation to mass wasting and running water as sheet floods. Even the driest deserts receive
occasional rainfall, which then usually occurs as downpours which cause landform creation
processes over a large scale in a short time period.
Mass action and fluvial action are therefore the main landscape-forming processes in deserts with
wind being only responsible for secondary, localised and other minor landforms. The wind action,
however, occurs over a persistent and ongoing basis and has an enduring presence and signature in
arid landscapes. Due to climate changes over time, the landscape bears evidence of former
landforms that were created when different climatic conditions controlled the prevailing landforms,
such as the sandstone deposits in the Eastern Free State and Lesotho: these date from times when
wind action and deposition by water were the chief landforming agents.
In Africa there are many hot, arid regions, such as the Namib, Sahel and Sahara deserts; and also
many semi-arid regions including Namaqualand, the Kalahari and parts of Ethiopia and Somalia.
These regions are all have many features in their landscape that were created by the wind-cascade
system. Other parts of Africa have long, dry seasons and are subject to prolonged droughts, even
though they are not classified as semi-arid regions. In these regions, as well as those where dryland
cultivation leaves large areas uncovered by vegetation or crops for part of the year, large volumes of
surface material is lost annually and wind erosion plays a major role in forming the landscape.
Furthermore, landforms created by wind can also occur in dry river valleys, along coasts and along
river floodplains where large quantities of talus are present.
Wind is one of the three dominant agents in hot deserts, after mass wasting and occasional water
action. The desert floors get heated up rapidly as they are dry and consist of mainly mineral silicas,
while being virtually devoid of organic matter and vegetation. These heated floors heat the air
directly above them, resulting in upward movements in the hot lighter air with turbulence, and any
obstructions in its path sets up eddies, whirlwinds, updrafts and downdrafts. Winds also move along
the desert floors with great speed and the obstructions in their path create turbulence, while very
destructive storm winds also occur.
Some fluvial processes and landforms found in deserts.
Deserts are defined by their lack of water and while coastal deserts may experience one or two rainfalls a
year; those further inland may get rain once or twice a decade only. Alt hough rain is scarce in deserts
(the Sahara’s average annual rainfall is only 127mm), it usually comes down torrentially and lasts mere
minutes or a few hours. This enhances rainsplash erosion on the bare, unvegetated desert soil, loosening
and spattering soil particles in all directions. The desert rock material also undergoes accelerated
mechanical and chemical weathering processes due to drastic diurnal temperature changes,
decaying fast, with the torrential rains helping to remove the weathered materials speedily. The
dislodged soil and talus is carried off with the flowing water, which picks up and transports more sediment
as it rushes along. Owing to a lack of organic-rich soil that can absorb the water, and also the absence of
vegetation, the desert landscape offers little resistance to water action and erodes rapidly. Water
therefore accomplishes within a few days what would take desert winds a year to accomplish, which
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means that the weathered debris in deserts is moved by not only wind, but also significantly by
rain/sheet wash.
Finding natural depressions, such as gullies and canyons, the sediment-laden water gains speed and
power as it is confined and flows downward. Increased velocity allows the water to pick up more and
larger sediments and rock debris, eroding them and the surface below as it rushes along. Often clogged
with so much debris, the water can resemble a mud-flow (a thick mixture of water, mud, and other surface
fragments). Arroyos, the dry streambeds created by previous rains, again fill with water. When an arroyo
finally opens onto a flat, broad plain, the rushing water flows out and drops its load of sediments, forming a
new alluvial fan. In other areas, basins or depressions in the desert floor fill with water, forming playa lakes
that soon evaporate, leaving a dry, cracked, salty lake bed that will remain until the next rain.
Alluvial fan
Precipitation that falls in higher elevations in deserts flows rapidly down to flat areas through canyons,
valleys, and other narrow, confined channels. Because most desert soil lacks plants and their root systems
to help hold the soil together, the flowing water easily picks up any loose material in its path. The faster the
water flows, the larger the pieces of material it is able to pick up and carry along.
When the rushing water finally reaches a plain or flat area, it loses power since gravity is no longer helping
it flow down a steep slope. As it slows, the water is unable to carry the sediment—gravel, clay, sand, and
silt—it picked up on its way downhill. Large rocks and other heavy material are deposited first at the base
of the canyon, followed by other material in decreasing size. No longer confined to a narrow channel, the
water spreads out the farther it moves away from the base of the canyon. The finest material it carried is
deposited at the outer edge. When the water evaporates, the sediments remain behind. Over time, as
more water flows onto the plain, more sediment is deposited, and a wide, fan-shaped pile known as an
alluvial fan forms. When two or more alluvial fans merge on a plain to create a broad, sloping surface,
they form a bajada.
Arroyo
An arroyo is a desert landform sculpted by the action of water. Sudden heavy downpours cut channels in
the desert floor, often in canyons or other low-lying areas. These fast-moving but short-lived streams create
deeper channels or gullies with steep sides and an almost flat bed or bottom. Just as quickly as the water
appears, it disappears in the normally dry desert environment. What remains is an arroyo, a dry
watercourse with a floor that is often gravel-strewn.
Stream channels in desert areas are broad, smooth and indefinite and flow for a brief time after rains.
Bajada.
When two or more alluvial fans merge on a plain to create a broad, sloping surface, they form a bajada.
THE WIND CASCADE SYSTEM
The work performed by aeolian systems is concentrated in desert regions and therefore desert is will
be used as model to describe this wind cascade system. On the basis of its morphological
characteristics, the desert landscape can be subdivided into the deflation-corrasion and the
aeolian-accumulation subsystems. The processes of deflation and corrasion dominate in the
deflation-corrasion subsystem, while the deposition of wind-transported talus is the major process in
the aeolian-accumulation subsystem.
Deflation is the process of removing talus on the earth surface by wind, while corrasion is the physical
weathering of rock by talus-carrying wind. Corrasion processes include abrasion, chipping and
polishing. The deflation-corrasion subsystem consists of the Hamada and the serir. The hamada has
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exposed rock surfaces and products of mechanical weathering , namely angular rock fragments
scattered about. The serir is a stony desert with smaller rock fragments impacted mainly by corrosion
processes. The size of the rock fragments decreases in the direction of the aeolian-accumulation
subsystem, from boulders down to sand grains. The aeolian-accumulation subsystem is therefore
characterised by fairly homogeneous sand-sized particles and landforms due to accumulation – such
as extensive sand sheets, seif dunes, transverse dunes and barchans dunes.
The following are the major morphological regulators of the two subsystems:
Deflation-corrasion subsystem
Particle size
Rock composition
Rock structure
Rock texture
Depth of soil-water table
Depth of groundwater table
Morphological characteristics
Aeolian-accumulation subsystem
Particle size
Presence of obstructions [landforms,
vegetation]
Spacing of obstructions
Slope of obstructions
Height & extent of obstructions
Deflation-corrasion subsystem
Wind velocity
Talus present or absent
Size & shape of talus particles
Regulators
Aeolian-accumulation subsystem
Wind velocity
Talus present or absent in air current
Size & shape of talus particles
Wind velocity is a threshold regulator: in practice this means that wind needs to attain a velocity of
5.3 to 7.5 m/s (19 km/h to 27 km/h) to be able to blow fine particles up into the air, or to roll grains of
sand along the surface. Velocities lower than these will not cause any movement of particles.
Therefore, if talus is available, then its particle size will act as regulator of whether the threshold
velocity will cause deflation or corrosion ( if velocity >
20 km/h) picking up or moving the talus
particle or whether it will remain stationary in the Aeolian-accumulation subsystem.
EROSIONAL LANDFORMS
Overview:
An overview of wind-driven processes and landforms in deserts
The wind moves like a fluid and has erosional ability only if it is strong enough. More often, it merely
transports material and very small particlesof less than 0.2 mm in diameter, can be picked up easily and
carried aloft for hundreds or thousands of miles by desert winds. Suspended on air currents, dust from
Africa's Sahara Desert sometimes crosses the Atlantic Ocean before landing in the west Atlantic and
Caribbean Sea, while on the other hand, sand particles of 0.2 to.6.4 mm diameter can be carried only by
extremely strong winds. Silt and other very small-sized particles fill the air during dust storms, but these and
most other wind-borne grains are too small to cause erosion or sandblasting of major landforms that stand
high above the desert floor.
Wind-blown sediment causes the most erosion at a height of no more than 250 mm above the surface,
where the wind removes fine-grained particles, which causes deflation - the lowering of the land surface
due to the removal of particles by the wind. This continued action leads to blowouts and the formation of
desert pavements. Another process is abrasion, whereby sand and silt act as effective tools to abrade
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(scour) the land surface while it is transported by wind. Any stone or part of the bedrock that has been
abraded or shaped by the wind is known as a ventifact (artefact of the wind). A yardang is one large
desert landform that is sculpted by the wind though deflation and abrasion. A third process caused by
wind action is impact, which is simply the sheer force of momentum which occurs when sand is blown
into or against a rock surface, similar to a sandblasting operation. Wind action therefore creates a
number of interesting erosional and depositional features in the deserts.
Sand storm, Tunisia
http://www.nicholas.duke.edu/eos/geo41/win002.gif
Wind storm, Tunisia
http://www.nicholas.duke.edu/eos/geo41/win003.gif
Figure 3.1: Sandstorms in deserts
The wind transports larger-grained sediments, particularly sand, through the process called saltation.
While light enough to be picked up by strong wind,
sand is too heavy to remain suspended in the air. As
a result, it is moved along the Earth's surface by the
wind through a series of short jumps and bounces.
The majority of sand transported in this way travels
within 0.6 meter of the ground. As saltating particles
crash to the ground, they can dislodge and move
slightly larger particles, such as small pebbles. The
sliding and rolling movement of these particles is
called surface creep.
Figure 3.2: Suspension, saltation and creep
LANDFORMS
Pediments and pediplains
Landscape evolution in deserts is primarily concerned with the formation and extension of pediments.
Gently inclined rocky floors close to the mountains at their foot with or without a thin cover of debris,
are called pediments. Such rocky floors form through the erosion of mountain fronts through a
combination of lateral erosion by streams and sheet flooding. Erosion starts along the steep margins
of the landmass or the steep sides of the tectonically controlled steep incision features over the
landmass. Once pediments are formed with a steep wash slope followed by a cliff or free face above
it, the steep wash slope and free face retreat backwards. This method of erosion is termed parallel
retreat of slopes through backwasting. So, through this process, the pediments extend backwards at
the expense of the mountain front, and gradually the mountain gets reduced leaving an inselberg
which is a remnant of the mountain. This explains how the high relief in desert areas is reduced to low
featureless plains called pediplains.
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Playas
Plains are by far the most prominent landforms in the deserts. In basins with mountains and hills around
and along, the drainage is towards the centre of the basin and due to gradual deposition of
sediment from basin margins, a nearly level plain forms at the centre of the basin.. When the water
falls on fairly flat areas, it may collect in a basin or other slightly depressed area, forming a small lake
that may last for a while before the water evaporates or is absorbed These shallow lakes are called
playas where water is retained only for short duration due to evaporation losses. Evaporation causes
the gradual concentration of salts in the playa, as only pure water evaporates, leaving behind the
sediments, mostly clay, silt, and various dissolved salts, which form a level, broad, cracked surface.
The playas therefore usually contain a good deposition of salt and such playa plains covered up by
salts are called alkali flats.
.When water is still present, these bodies are called playa lakes. Although they are very rare,
permanent desert lakes do exist. Two examples are the Great Salt Lake of Utah and the Dead Sea of
Israel and Jordan.
Deflation Hollows and Caves
Following water, wind is a major cause of erosion in the desert. Without plants and their anchoring roots,
loose desert soil is moved easily by near-constant blowing winds. Blowouts, also known as deflation basins
or hollows, are depressions made in sand or light soil by strong wind action and may range in size from
metres to several kilometres in diameter. As the name of the landform indicates, they are formed by the
process of deflation. These depressions may extend over several square kilometres. Blowouts can form
around desert plants with hardy roots or around rock structures, leaving them perched atop a column as
more and more sand or soil is blown away. Deflation also creates numerous small pits or cavities over
rock surfaces. The rock faces suffer impact and abrasion of wind-borne sand and first shallow
depressions called blow outs are created, and some of the blow outs become deeper and wider, to
the extent that they can be called caves.
There are some examples of deflation hollows in the Richtersveld and in Bushmanland in the northwestern extreme of the Northern Cape Province and also near Velddrift in the Western Cape
Province. Because the hollows or cavities that form trap surface water, or at least store more water
than their immediate surroundings, this promotes some chemical weathering of the underlying rock,
which in turn causes further removal of talus by wind action. The depth of the water table regulates
the depth to which a depression hollow can erode, because as soon as the water table is
intersected, an oasis, swamp or pan is formed.
This moistened landform is then not subject to further wind erosion. Pan forms are abundant in the
Kalahari (in the Northern Cape Province ) and in Botswana, with the extensive Makarikari depression
and Verneukpan as examples of large-scale landforms, while there are thousands of much smaller
pans dotting this region as well. Other examples of deflation hollows include the shallow salt lakes of
the Qattara Depression in Eqypt and the Etosha Pan in Namibia. The Etosha Pan is situated on the
floor of a deflation hollow that developed on a structural basin and is one of the largest salt pans in
the world, and an attraction for a spectacular diversity of wildlife.
Desert pavements (also: Reg, Hamada or gibber plain)
The strength of the wind determines how much and what type of material will be removed from the desert
floor. With increasing strength, wind is able to move and transport more and larger particles. Initially, very
fine particles are removed, but as more and more of these types of particles are removed, the surface of
the land is lowered. This action, known as deflation, continues until what remains on the desert floor is a
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layer of closely packed pebbles and rocks too heavy for the wind to move. Settled and wind-polished, the
entire surface is called a desert pavement or reg. The older the pavement, the smoother and flatter it
appears, like a highly worn cobblestone street.
Desert pavements are gravel covered plains such as the serirs in Libya from which the landform of the
one main component of the deflation-corrasion subsystem derives its name. These stony deserts with
small rock fragnments are referred to as ‘gibber plains’ in Australia. A similar landform is the Hamada
or reg, (e.g the Hamada el Homra in Libya and the Tanezrouft in Algeria), characterised by exposed
rock surfaces with scattered rock fragments. In the Sahara an extensive stony desert plain is known as
Reg, not to be confused with Erg, which refers to a sandy desert area. The term gibber plain is used in
Australia in parts of the Tirari-Sturt stony desert ecoregion where extensive areas are covered in
desert pavements. (Aus: pebble = gibber)
Deflation surface
The stony desert known as Reg del’Adrar in Mauritania
Desert pavement, in the Mojave Desert, California
Tunisia ; pre-Sahara
http://www.nicholas.duke.edu/eos/geo41/win008.gif
Figure 3.3: Deflation surface and desert pavement
Oasis
Beneath the Earth's surface water fills the pore spaces and openings between rocks. The upper limit of the
groundwater is called the water table, which is present even beneath desert regions. Groundwater hardly
ever reaches the desert surface, but when it does, it can transform the stark landscape into a fertile haven
thriving with many species of plants and animals that otherwise would not exist in such a hot, dry
environment. This green area, existing like an island in a sand sea, is an oasis. Many oases are artificial
ecosystems, created by people living in the desert using large pipes to tap into the groundwater to bring it
to the surface. A few are the result of natural forces and are centred on springs that have been exposed
because of blowouts and other erosive actions by the wind, resulting in a lowering of the land surface.
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Abrasion acts on free-standing rock outcrops, modifying them into interesting and unique landforms.
Yardangs are the first of these landforms primary formed by abrasion. The others are zeugen and
then collectively landfoms such as mushroom rocks, rock pedestals and ventifacts.
Yardangs and zeugen
Yardangs are extensively grooved, fluted, pitted and irregular rock ridges or reliefs of one to about
ten metres high running parallel to the prevailing winds. They are caused by differential erosion, when
the sand-laden wind corrades zones of softer or weaker rock between harder vertical ridges from old
lake sediment where soft, poorly consolidated rock and bedrock surfaces are eroded into alternating
ridges and furrows. Large- scale yardangs in Africa are to be found in Egypt (near Kom Ombo, north
of Lake Aswan), Chad, Libya and Algeria. Local, but smaller scale yardangs are to be seen near the
Umtamvuma River mouth in Kwazulu-Natal; these are 4m high and were formed within a 60-year
period.
Yardangs and allied "ridge and furrow" features are probably the only landforms in deserts that can
be ascribed to wind abrasion alone.
Yardang, Egypt © H Kehl
Top:
Yardang, Egypt © Urs Möckli
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Yardangs up to 180 m tall in the Lut Desert or Dasht-e Lut
("Desert of Emptiness") of Iran's southeastern province of
Kerman. This great sand and stone desert is among the
driest places on the planet’
The Lut desert is extremely barren, having the only region
devoid any life, not even bacteria, on Earth with annual
rainfall ranging from 0 to only 30 mm.
Figure 3.4 : Yardangs around the world.
Zeugen
Hoodoos - Mushroom Rocks - Pedestal Rocks
When differential erosion of horizontal rock layers take place, we refer to the resulting landform as
zeugen. Typically zeugen are mushroom-shaped rock that has been eroded by the abrasive action
of windblown sand. The undercutting effect is concentrated near ground level, where sand
movement is greatest, and is enhanced in areas of near-horizontal strata when the lowest bed is
relatively weak and shows distinct signs of basal abrasion on the upwind side only. A smoothly
rounded top (as opposed to a top with rough edges) is created by spheroidal weathering into a
form resembling a mushroom.
The supporting column of a mushroom or pedestal rock is caused by etching, differential weathering
and natural sandblasting of sedimentary deposits. Often an erosion resistant layer will be deposited
above softer sediment layers and after burial and lithification, joints and fractures form in the upper
layer, allowing faster erosion of the soft strata below. Abrasion near the ground by windblown sand
causes the lower portion of a rock mass to be undercut leaving the upper portion relatively
unaffected.
Mushroom, Table and Pedestal Rocks
More localised and on a smaller scale than the previous landforms, many rock-outcrops in the deserts
are easily susceptible to wind deflation and abrasion. Softer layers in these outcrops are worn out
easily, leaving remnants of more resistant rocks in the shape of mushroom with a slender stalk and
capped by a broad and rounded pear shape above. Mushroom rocks and similar landforms are
actually the products of mainly chemical weathering, but wind abrasion also contributes in that the
chemically weakened (weathered) softer layers are easily abraded by wind. The resulting landform is
the pedestal-shaped toadstool capped by a more resistant upper layer. As wind-borne talus has
bigger-sized particles nearer the land surface compared to higher up, free standing rock outcrops
are subject to more active or more concentrated abrasion at their bases relative to higher sections,
resulting in the familiar stem or pedestal shape.
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The Mushroom Rock near Clarens, Free State.
Mukorob, the “Finger of God” near Mariental , Namibia, before
its collapse in 1988
Figure 3.5: Examples of Zeugen, Hoodoos, Mushroom Rocks or Pedestal Rocks
The Finger of God, or Mukorob, near Asab and Mariental in Namibia (which toppled in December
1988) was an excellent example of this landform (bottom rigt in Figure . ). It consisted mostly of
sandstone and stood 12 m high and 4.5 m wide, weighing some 450 tons, but had a base of just 3 m
long and 1.5 m wide. The Mushroom Rock near Clarens in the eastern Free State Province (top left) is
another local, but much smaller, example.
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Ventifacts
On a much smaller scale than the mega-landforms of yardangs and zeugen, rock fragments and
stones are shaped into highly-polished conical shapes by wind-driven sand. Typically larger rocks
too big to be pushed or moved along by the wind, once they are polished, display smooth faces or
facets and are then termed ‘ventifacts’, analogous to artefacts.
The German term ‘einkanters’ is used for rocks with one face only polished, but when such a onesided polished stone falls over, it presents another face to the prevailing wind, and after a second
toppling over, three facets are smoothed and polished. Such three-sided ventifacts are known as
‘dreikanters’. The ergs of the Sahara and the stony northern Namib Desert have extensive sheets of
ventifacts.
Ventifacts from all over
http://www.nicholas.duke.edu/eos/geo41/win011.gi
Ventifact showing facets
http://www.nicholas.duke.edu/eos/geo41/win012.gif
Figure 3.6: Ventifacts
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DEPOSITIONAL LANDFORMS
Wind is a good sorting agent. Depending upon the velocity of wind, different sizes of grains are
moved along the floors by rolling or saltation and carried in suspension and in this process of
transportation itself, the materials get sorted. When the wind slows or begins to die down, depending
upon sizes of grains and their critical velocities, the grains will begin to settle. Therefore good sorting of
grains can be found in depositional landforms made by wind. More formally, wind will deposit its talus
load when its supply of kinetic energy is too low to carry the talus mass. Since a desert has an
abundant supply of sand and with nearly constant wind directions prevailing, depositional features in
arid regions can develop anywhere.
The three commonly distinguished landforms formed by wind deposition of talus are draas, dunes and
ripples.
Draas and ergs
Draas are huge sand accumulations and where these converge, a landform termed erg is formed –
such as the star-shaped Great Continental Erg in Algeria. There are approximately twenty ergs in the
Sahara, which collectively cover about 15% of the Sahara’s surface area.
The Great Sand sea near Luderitz in the
Namib Desert
Ar Rub al Khali Sand Sea, Arabian Peninsula
Figure 3.7: Sand seas, or draas (ergs)
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These ‘sand seas’ are extensive accumulations of sand up to 300 m thick and are essentially water
deposits, while most of them occupy large depressions which are former (paleo) lakes or shallow
oceans, which were subsequently filled with water-borne (i.e. fluvial) talus. The surface of the draas
are shaped by wind – with dunes typically developing on the wind side. On a smaller scale, individual
ripples are formed on the surface of a draa or dune. All three of these wind-shaped landforms, draas,
dunes and ripples, are in essence waves – and can therefore be described in terms of their geometric
characteristics like wavelength and amplitude.
The photograph on the top right (Figure 3.7) shows a part of The Ar Rub al Khali sand sea or erg - also
known as the "Empty Quarter" – in Oman. In the western portion there is a large region of SW-NE oriented
transverse sand dunes (reddish brown) alternating with the blue-grey interdune salt flats, known as sabkha.
The dominant winds are Shamal winds, which are northwesterly trade winds originating in Iraq. The
formation of secondary barchan and star dunes on the parallel transverse dunes is due to southwesterly
Kharif winds that blow during the monsoon season. The long l transverse dunes begin to break up into
isolated star dunes to the northeast and east (right), probably as a result of both wind pattern interactions
and changes in the sand supply to the dunes.
The Great Sand Sea of the Namib extends northward for 400km from Lüderitz Bay to Walvis Bay and as far
as 140km inland from the coast. The sand consists of quartz grains mixed in a ratio of 9:1 with heavy
minerals like garnet, ilmenite and magnetite as well as a little mica. The colours darken from pale buff in
the west to deep red in the east as the iron oxide content of the sand increases.
Sand Dunes
Dry hot deserts are good places for sand dune formation. Obstacles to initiate dune formation are
equally important. Dune form, i.e. their shape and size, is controlled or determined by three factors,
namely the strength and direction of wind, the amount of sand available and the amount (if any) of
vegetation present. All dunes are mobile to some extent, and can be classified into live dunes and
fixed dunes on the basis of their mobility.
Figure 3.8: Sand dune form controls: sand supply, vegetation & wind strength
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Live dunes or free dunes have no fixed position, but migrate downwind by erosion on the gently
inclined windward side and deposition on the leeward side (slip face) in the same way as described for
fixed dunes.
The smallest free dunes are common
wind ripples that measure only a few
centimetres in height. Large dunes are
found in extensive dune areas in
deserts, in sand seas known as `ergs'.
Free dunes move frequently and their
shapes vary. They are dependent on
the speed the wind is depositing or
removing sands and the size of the
materials being transported
Coastal dunes occur along beaches
or sand-flats that form part of a nonerosional sandy or deltaic coast. The
source areas of the sand will eventually
lose all sand, silt and clay particles;
some become wet (groundwater)
depressions whereas others acquire a
rocky or boulder-strewn surface known
as a `desert pavement'.
Figure 3.9: Four of the most common dune forms: barchan, transverse,
longitudinal (or linear), and blowout (or parabolic).
Fixed dunes, by contrast, tend not to move and their shapes are relatively stable and static and they
are usually secured down by vegetation, rocks and opposing winds. They are formed when transported
sand settles in the lee of an obstacle such as a bush or a rock, causing the obstruction to grow in size,
capturing more sand. The transport capacity of the wind decreases as it drives the sand grains to the top
of the dune, causing an increasing part of the transported sand to settle before reaching the dune crest.
This steepens the angle of the slope, particularly near the crest. Once the slope angle exceeds the angle
of repose of the deposited sand (typically 34˚ for dry sand), shearing sets in along a slightly less steep
plane. Thus, a slip face is formed on the leeward side of the dune. Vegetation growing on especially the
lower part of dunes may eventually keep most of the sand in place. Dunes along coasts are often fixed by
vegetation, where shrub-coppice dunes and also parabolic dunes may develop by landward migration of
beach sand.
During transport, selection of particles (sorting and winnowing) occurs; the momentary wind speed and
the size, shape and density of minerals determine how far a particular grain will be transported. Fine gravel
travels by creep and sand-sized particles by saltation. Silt-sized particles can be carried over great
distances (Saharan `dust' settles regularly in central Europe and, in the past, loess formations have formed
extensive blankets far from the source areas). Fine, plate-shaped clay minerals and micas are blown out
and travel even farther (which explains why wind-borne sediments are normally poor in micas). This sorting
of grains results in deposits that consist of pure sand with a uniform particle size. Many aeolian sand
deposits show characteristic large-scale cross bedding, indicative of sand deposition on the slip faces of
dunes. See Figure 3.10.
Common to all dunes is the contrast between the gentle slope of the windward side (the side facing into
the wind) and the steep slope of the leeward side (the side facing away from the wind). The leeward side
is known as the slip face of the dune.
15
Figure 3.10 : Schematic dune structure
Longitudinal dunes, also known as linear dunes, form when the supply of sand is poor and the wind
direction is constant, or where sand is more abundant and cross winds converge - often along coasts
where the winds from the sea and those from the land meet and push the sand into long lines. They
appear as long ridges of considerable length but low in height. The wind channels between existing
dunes and forms a vortex flow, which then shapes and maintains the dune form. In the central and
greater part of the Namib sand sea, linear or longitudinal dunes lie parallel to one another and to the
southerly winds, formations which in places are over 100km long and 120m high. They are arranged in
nearly straight rows with valleys or so-called "dune streets" between them. Their slipfaces change
position with the seasons. Whereas they face north-east for much of the year when southerly winds
prevail, they are turned around to face south-west in winter when the easterlies blow.
Longitudinal dune [USGS]
http://www.nicholas.duke.edu/eos/geo41/win028.gif
Longitudinal dune Australia
http://www.nicholas.duke.edu/eos/geo41/win031.gif
[NASA]
Figure 3.11 : Longitudinal (Linear) dunes
Longitudinal dunes, Arabian peninsula
http://www.nicholas.duke.edu/eos/geo41/win029.gif
[NASA]
16
Seif dunes, a sinuous variety of longitudinal sand dunes, or are usually found in very dry deserts. They are
large compound forms where smaller, secondary, linear ridges coalesce and are superposed on the main
ridges of the underlying longitudinal dunes. The secondary forms on seif dunes originate from barchan-
like forms, but have only one wing or point due to shifting wind conditions disturbing the one point.
The remaining lone wings of seifs can grow very long and high. Seifs are the dominant dune form in
the Sahara and some of them are up to 100 m long and have a local relief of up to 100 m as well. The
seifs in the central Namib Desert south of Walvis Bay reach heights between 50 m and 250 m, reputed
to be of the highest dunes in the world. These dunes have their origin as longitudinal dunes formed in
the prevailing south-westerly winds, which have been changed into seifs as a result of sporadic
easterly winds.
Figure 3.12 : Seif dunes of the Namib desert www.nationalgeographic.com
Barchan dunes are crescent shaped and have their points or wings directed downwind, or away from
the wind direction.
They form where the original surface over
which sand is moving is almost uniform
and where the wind direction is constant
and moderate. Barchan dunes are a very
mobile dune type and are popularly called
"wandering dunes".
The rate of advancement of the sand is
roughly inversely proportional to the
height of the crest. This causes the flanks
of a shifting dune to advance more
rapidly than the central part until the
flanks become sheltered by the main
mass of the dune. Coalescing barchans
produce `transverse dunes'.
In the Namib Desert they grow to a height
of about 30m where strong south-westerly
winds blow mainly from coastal regions
with relatively little sand. The barchans of
the Skeleton Coast and those near Walvis
Bay point to the north-east and also travel
in that direction – due to the southwesterly winds shaping and driving them.
Figure 3.13 : The formation of barchan dunes. Source: Bagnold, 1965
17
Barchan, White sands New Mexico
http://www.nicholas.duke.edu/eos/geo41/win026.gif
Small barchans, Tunisia
http://www.nicholas.duke.edu/eos/geo41/win025.gif
Figure 3.14 : Barchan dunes
Transverse dunes
When the supply of sand is plentiful, regular shaped dunes like barchans can coalesce and lose their
individual characteristics, forming crescent-shaped (or barchanoid) ridges. If the ridges become
fairly straight, they are called transverse dunes. The ideal conditions for their formation is when the
wind direction is constant and the source of sand is an elongated feature at right angles to the wind
direction. In the Namib Desert transverse dunes are found a short distance inland from the barchans,
reaching heights of two and a half times that of the barchans, with their long axes similarly across the path
of the wind. They may be very long and low in height.
Transverse dunes {USGS]
http://www.nicholas.duke.edu/eos/geo41/win027.gif
Figure 3.15 : Transverse dunes
18
Star dunes
Mountainous piles of sand known as star dunes dominate the inland margin of the sand sea where high
winds blow from all directions. They are reputed to be among the highest dunes in the world, as high as
220 m or even higher when they rest on a raised surface. Star dunes are named for their shape as seen
from above, a lot of sharp ridges winding outwards and downwards from a central crest.
Figure 3.16 : Star dunes
Shrub-coppice dunes
Shrub-coppice dunes, (also known as Nebkha dunes) usually only a metre or two high, are common at the
sea-shore, where clumps of grass or shrubs such as mesquite form obstructions to windblown sand. The
sand starts to collect around the vegetation, gradually forming a small mound. As the mound grows, the
roots lengthen to keep the plant above the surface, thereby anchoring the dune, while it also grows taller.
These dunes are frequently found on sand sheets and on and around larger parabolic dunes.
19
Figure 3.17: Coppice dune
Parabolic dunes (blowout dunes)
Parabolic dunes form when sandy surfaces are partially covered with vegetation. They are also
crescent shaped, but unlike the barchan dunes, their points or wings (horns) point into the wind, while
the arch is downwind. They typically grow from the horns or trailing arms that are anchored around
an obstruction like vegetation or rock blocking the wind, while its central part migrates with the wind
after a blowout or depression hollow has formed. Parabolic dunes reach heights of up to 20 or 30 m
except at their crescent, where more sand piles up as it is halted or slowed by surrounding
vegetation.
20
Parabolic dunes, California © 2012 Nature Education : K. Adelman.
Figure 3.18 : Parabolic dunes
These dunes are usually less regularly shaped than barchans and in plan view can be either U-shaped
or V-shaped and are more elongated. They consist of mounds of well-sorted, very fine to medium
sand and usually occur on beaches, downwind from blowouts in partially vegetated sand sheets.
They also occur in semiarid areas, i.e. where some precipitation is common and near-surface moisture
is available in the lower parts of the dune and underlying soils. Parabolic dunes can also extend
inland into vegetated areas in coastal zones and on shores of large lakes.
Most parabolic dunes do not grow to heights greater than a few tens of meters except at their
forward portions, where sand piles up as its advance is halted or slowed by surrounding vegetation.
There are two principal sub-types of parabolic dunes: long walled and elliptical types.
Loess
This is the lightest material carried by the winds which form a so-called blanket covering the existing
land. This blanket is easily eroded and rain penetrates through them rapidly. A large portion of the
world’s loess has its origin from deserts.
Loess is a predominantly silt-sized sediment originating from broken-down rock fragments, which is
formed by the accumulation of wind-blown dust. Loess is fairly even- sized sediment, pale yellow or
buff in colour, typically non-stratified and often calcerous – i.e. of calcium carbonate origin. Loess
grains are angular with little polishing or rounding and composed of crystals of quartz, feldspar , mica
and other minerals. Loess can be described as a rich, very fertile, dust-like soil, which erodes very
easily.
The word loess, with connotations of origin by wind-deposited accumulation, can probably be traced
back to Alemannic dialects of German.
21
Loess profile in Vicksburg, Mississippi.
USA.
Loess near Hunyuan, in China’s Shanxi
province.
A loess ioutcrop in Patagonia,
Southern Argentina.
Figure 3.19 : Loess as found in different continents
Loess deposits may become very thick; more than a hundred meters in areas of China and the
Midwestern United States. It generally occurs as blanket deposit that covers areas of hundreds of
square kilometres and tens of metres thick. Loess often stands in either steep or vertical faces.
Because the grains are angular, loess will often stand in banks for many years without slumping. This
soil is easily excavated to form cave dwellings, a popular method of making human habitations in
some parts of China.
In several areas of the world, loess ridges have formed that are aligned with the prevailing winbds
during the last glacial maximum. These are called paha ridges in America and greda ridges in
Europe. The form of these loess dunes has been explained by a combination of wind and tundra
conditions.
Glacial or peri-glacial loess is derived from the floodplains of glacial braided rivers that carried large
volumes of glacial meltwater and sediments from the annual melting of continental ice sheets and
mountain icecaps during the summer, while non-glacial loess can originate from deserts, dune fields,
playa lakes and volcanic ash. The thick Chinese loess deposits are non-glacial loess having been
blown in from deserts in northern China. The loess covering the Great Plains of Nebraska, Kansas,
and Colorado is non-glacial desert loess. Non-glacial desert loess is also found in Africa and Australia.
Sandy parent materials are also abundant in areas where sand accumulates after selective transportation
of weathering material by wind or water. Aeolian (wind-borne) sands will be discussed in this paragraph.
Dune and other desert features: Words to Know
Abrasion:
The erosion or wearing away of bedrock by continuous friction caused by sand or rock fragments in
water, wind, and ice.
Aeolian:
Formed or deposited by the action of the wind.
Bajada:
Several alluvial fans that have joined together.
Basin:
A hollow or depression in Earth's surface with no outlet for water.
Crescent:
The sickle-moon shape of a barchan dune and also the shape of a parabolic dune.
Crest:
The highest point or level; summit.
Deflation:
The lowering of the land surface due to the removal of fine-grained particles by the wind.
22
Erg:
A vast area deeply covered with sand and topped with dunes.
Erosion:
The gradual wearing away of Earth surfaces through the action of wind and water.
Gully:
A channel cut into the Earth's surface by running water, especially after a heavy rain.
Leeward:
On or toward the side facing away from the wind.
Saltation:
The jumping movement of sand caused by the wind.
Silt:
Fine earthy particles smaller than sand carried by moving water and deposited as a sediment.
Slip face:
The steeply sloped side of a dune that faces away from the wind.
Surface creep:
The rolling and pushing of sand and slightly larger particles by the wind.
Ventifact:
A stone or bedrock surface that has been shaped or eroded by the wind.
Windward:
On or toward the side facing into the wind.
Bibliography
Books
Gallant, Roy A. Sand on the Move: The Story of Dunes. New York: Franklin Watts, 1997.
Lancaster, Nicholas. The Geomorphology of Desert Dunes. New York: Routledge, 1995.
Websites
http://www2.agc.army.mil/research/products/desert_guide/lsmsheet/lspara.htm dunes
http://www.nps.gov/grsa/naturescience/dune-types.htm
http://geog-leics.blogspot.com/2012/08/desert-depositional-features.html 13 april 2013
http://www.indiana.edu/~geol116/Week11/wk11.htm .
http://hrsbstaff.ednet.ns.ca/mcallip/Deserts/desert_deposit_features.htm
http://www.ux1.eiu.edu/~cfjps/1300/deserts.html
http://www.google.co.za/url?sa=i&source=images&cd=&docid=1Gk1J6g1KgticM&tbnid=5T83zLScaKezM:&ved=0CAgQjRwwAA&url=http%3A%2F%2Fwww.handsontheland.org%2Fgrsa%2Fresources%2Fc
urriculum%2Fmid%2Fdunes%2Fphoto_files%2Flongitudinal.htm&ei=eQppUc_OBJSThgeE9IHYCQ&psig=AF
QjCNF6A1YoY3Cy3nbL8vAnK7FrILGpxw&ust=1365924857233144
http://www.scienceclarified.com/landforms/Basins-to-Dunes/Dune-and-Other-DesertFeatures.html#b#ixzz2QKPl4to5
http://www.desertusa.com/mag99/sep/papr/desfeatures.html
http://pubs.usgs.gov/gip/deserts/
http://www.eos.ubc.ca/courses/eosc110/fletcher/slideshow/deserts/deserts.html
http://alic.arid.arizona.edu/sonoran/Physical/geomorphology.html
http://www.aqd.nps.gov/grd/tour/sanddune.html
http://www.desertusa.com/geofacts/sanddune.html
http://www.fao.org/docrep/003/Y1899E/y1899e06.html
http://www.scienceclarified.com/landforms/Basins-to-Dunes/Dune-and-Other-Desert-Features.html#b
23
APPLIED GEOMORPHOLOGY
Was the Sphinx Originally a Landform?
In 2001 Farouk El-Baz, Boston University professor and director of the university's Center for Remote Sensing,
published a paper in which he suggested that the pyramids and the Great Sphinx located on Egypt's Giza
Plateau were based on natural landforms found in the eastern Sahara Desert. El-Baz pointed out that the
landscape of the Nile River valley features coneshaped hills that have lasted many years because their
shape forces the strong winds in the area upward, preventing the wind from eroding them or wearing
them down. The pyramid builders, El-Baz believes, would have looked to these landforms in their quest to
build lasting structures.
Extending his theory even further, El-Baz asserted that the Great Sphinx, the enormous sculpture with the
head of a man and the body of a reclining lion, might even have been carved by ancient Egyptians in
2500 B.C.E. from an existing desert landform. He cited the works of early twentieth-century explorers and
geologists that described wind-eroded yardangs in northwestern China and southwestern Egypt as "sphinxlike" or "lionlike." El-Baz believes the head of the sphinx was an existing yardang (a wind-sculpted ridge) the
Egyptians reshaped. They then formed the body, which sits in a hollow or depression, by digging out the
naturally occurring limestone on the plateau around it. due to compression caused by atmospheric
pressure (atmospheric pressure increases closer to the planet's surface).
Great Sphinx, Egypt. Photograph reproduced by permission of the Corbis Corporation
.
During its descent, the warming air pushes the air below it back toward the equator, since air flows always
move toward areas of low pressure. Passing over land on its way back to the equator, the now heated, dry
air evaporates any moisture in the air, creating dry regions or deserts.
These wind patterns explain the formation of many of the world's deserts in the area between 15° and 35°
latitude north and south of the equator. Although much weaker, similar atmospheric circulation (with
heated, dry air evaporating moisture as it moves over Earth's surface) occurs over both poles, creating
polar deserts. While heated because of compression by the atmosphere, the air over the polar regions is
not as warm as in equatorial regions simply because the Sun is farther away from Earth in these areas. Polar
deserts, however, are similar to hot deserts because they have very low humidity and precipitation levels.
The topography (physical features) of Earth in combination with atmospheric pressure creates other
deserts around the planet. Rain-shadow deserts are those that lie on the leeward side of mountain ranges
located near coasts. As moisture-laden air flows inland from an ocean and encounters a mountain range,
it is pushed upward. Cooling as it rises, the air begins to lose its moisture on the windward side of the
mountain range through rainfall. Once on the leeward side, in the "shadow" of the mountain range, the air
has little moisture left. Heated by compression as it descends, the warm, dry air forms deserts in the slope of
the range. All deserts in North America are formed by this action.