Aeolian Processes and Landforms: Erosional & Depositional

1 year ago

Atiqur Rahman

19 minutes

Aeolian Environment

All land features formed by wind are also called ‘Aeolian features’ after the name of the Greek God of wind, ‘Aeolus’.

Although present everywhere, the wind is a powerful agent of landform creation only in dry regions and is mostly free of vegetation cover. Wind erodes, transports and deposits smaller particles like sand but may move even larger particles under special conditions.

These processes lead to a variety of features. The scale of features ranges from very small to a vast expanse of several thousand square kilometres. The main features are hollows and grooves on rocky surfaces and mounds of sand. They have different shapes and sizes and are named and classified as such.

Arid, semi-arid and sub-humid climatic regions are best suited for aeolian features (Fig.). These drier parts occupy central and western regions of the continents that are on the leeward side of easterly rain-bearing winds coming from the oceans.

Distribution of aeolian environments in the world

Aeolian Processes

The three main processes that play a vital role in the development of aeolian landforms are:

Erosion by wind,
Transportation by wind, and
Deposition by the wind.

Erosion by Wind

Wind erosion follows two different processes: a) Abrasion and b) Deflation

a) Abrasion

When strong winds blow and carry hard sand particles with them, they attack rocky surfaces in their path of flow. This is known as ‘sandblasting’.

According to Bagnold, wind can lift sand particles only up to a maximum height of 45 centimetres or 2 metres (Thornbury 1958 p. 300).

As such, abrasion by sandblasting is limited to a zone close to the surface. Blackwelder has classified three different impacts of abrasion – i) polishing and pitting, ii) grooving, and iii) shaping and cutting smooth surfaces on the windward side of the rock, also called faceting.

b) Deflation

The process of lifting dry and loose particles and carrying them away is called deflation. Both the size of the particles and wind velocity plays an important role in deflation.

Richard Huggett states particles with a 100-micrometre diameter are most suited for deflation. Particles larger than this cannot be lifted and carried unless the wind is especially strong. Deflation transports sand only to a limited distance.

On the other hand, fine particles show a tendency to stick to each other and are not easily carried away by wind deflation. But once the finer particles are picked up, they can be lifted up vertically higher up, kept afloat and transported to great distances. The process of deflation leaves hollowed features in its wake. Dust storms are another evidence of wind deflation.

Transportation by Wind

Once loose particles are picked up by wind, they are transported away by four different processes: a) creep, b) suspension, c) saltation and d) reptation. Each has a different role in landform creation.

a) Creep

This mode of transportation involves the movement of particles along the surface. Particles are pushed ahead as they roll and slip across the surface. This process influences larger grain sizes, like sand and even pebbles, that cannot be lifted but can be pushed by strong winds (Fig.).

b) Suspension

Smaller particles with a diameter of fewer than 100 micrometres are light in weight and can be lifted above the ground by wind eddies. Once these start floating, they keep moving at a height until the force of the wind totally dies out. Suspension may carry dust particles thousands of kilometres away from their point of origin.

c) Saltation

Large grains of sand and gravel move ahead in intermittent jumps. They are lifted and dropped repeatedly by gusts of wind. The maximum height for saltation is 2 metres. As they land and hit the ground, they push other particles ahead.

According to W. B. Sparks- in saltation, “the impact derived from saltation is able to move grains six times the size of those forming the saltation.”(Sparks 1983 p. 325). As a result, large particles are pushed by this process which cannot be moved otherwise by wind (Fig.).

d) Reptation

This process of wind transportation is closely related to saltation. As particles fall on the ground after jumping up, they create a splash, and smaller particles are displaced in different directions.

Deposition by Wind

When the velocity of wind is reduced, it loses its transporting capacity and drops the suspended fine particles carried by it or stops creep and saltation started by it. The result of this inaction is deposition.

Based on particle size, there are two categories – fine particle deposition takes place far away from the point of origin, while sand is deposited closer to the source region.

Deposition may take place within the aeolian environments mentioned above or may be outside such areas. Four such areas have been mentioned by W. D. Thornbury – shoreline, semi-arid river banks, extensive sandstone weathering areas and glacial out-wash zone.

The deposited material forms various types of land features. Dunes of various types are the most conspicuous of all.

Types of Aeolian Landforms

The aeolian processes mentioned above are responsible for creating a variety of typical landforms, each indicative of the process responsible for its formation. Based on the two major actions of wind, aeolian landforms are classified into two broad categories- erosional and depositional aeolian features.

Aeolian Erosional Landforms

Wind erodes in two ways; first, it picks up loose particles and removes them to create depressions. Secondly, wind attacks rock with sand particles and destroy weak rock beds. The following are the features formed by these actions.

Lag Deposit

While blowing over a surface, wind removes all unconsolidated fine particles. Those with less than 100 micrometres diameter are suspended and are taken to long distances. Those particles that are 100 micrometres in diameter, like sand, are removed gradually to short distances. The larger ones are left at their place of origin and keep rolling and shifting their place till they are tightly packed by this random jostling.

These surfaces are called ‘lag deposits’ because the surface is made of particles that could not keep pace with the rest of the smaller ones moving out and ‘lagged behind’. They are also known as ‘desert pavement’ as the grains are fitted tightly, just like any man-made tiled pavement surface.

The top of these desert pavements are polished by wind abrasion and have a thin shiny layer of oxides of iron and manganese, called desert varnish. These lag deposits have different names, e.g., desert armour in North America, serir and hammada in the Arab world and gibber in Australia (Richard Huggett 2011p. 319).

Deflation Hollow

As the name suggests, these low-lying surfaces have been cleared of all loose particles and converted into hollows. The size of these depressions may range from a few metres in diameter and depth, to several kilometres.

The underground water table controls the dimension, especially the depth. As deepening reaches humid layers close to the water table, wind fails to move the moist particles, and no further hollowing is allowed. These are also known as blowouts.

Some examples of these deflation hollows are:

P’ang Kiang Hollows – Several hollows are found in Mongolia that are 8 kilometres wide and 60 to 120 metres deep. These have been described by C. P. Berkey and F. K. Morris as a work of wind deflation (Thornbury 302).

Laramie Basin in Wyoming is 14.5 kilometres long, 4.8 kilometres wide and about 46 metres deep.

Quattara Depression in North Egypt has its deepest part 134 metres below sea level.

The process involved in their formation essentially shows an alternation of wet and dry periods. During the wet period, moisture helps in the destruction of rocks due to agents of weathering. Unconsolidated rock grains created during this period are later transported by wind during the dry phase of the cycle. Repetition of the two phases gradually enlarges the depression.

Yardang and Zuegen

Yardangs are elongated grooves, first described by Hedin in Turkestan (Thornbury p.299). Eliot Blackwelder used the term ‘Yardang’ for these grooves in 1934. These are parallel ridges separated by parallel U-shaped grooves, both developed in the direction of dominating wind flow in the region. They are called Mega-yardang when they are large in scale. In central Sahara and Egypt, yardangs are 100 metres long and 1000 metres wide (Richard Huggett p.123).

It is believed that the formation of yardangs is initiated by some conditions favouring differential wind erosion. Some believe that initial depressions are made by the erosive action of water, and are later enlarged by wind deflation and abrasion.

Arthur Bloom has given the example of such narrow parallel gullies in the cold desert of the central Andes. Here the gullies are cut along the joints that run parallel to the strong winds (Bloom 2003 pp. 291-92).

Such initial grooving is mostly held responsible for the evolution of yardangs everywhere. The feature itself is considered ‘dynamic’; that is, yardangs are both destroyed and made continuously.

Zuegen (singular Zuege)- These are similar to yardang, except they are smaller in scale, and grooving is related to softer material alternated with more resistant rock beds.

Ventifacts and Dreikanter

Ventifacts are rock pieces with smooth, sand-blasted facets pointing to the direction of dominant wind flow.

Sometimes several such facets may be developed on a rock, indicating varying wind directions. All facets in this type of case intersect along sharp and angular edges. If the rock piece has a pyramidal three-faceted shape, it is called a dreikanter.

Besides the above main landforms, mesa, butte, and mushroom rocks are other features resulting from near-surface abrasion by sand-laden winds. These are formed when the base of a rock projection is eroded while the top is untouched by wind action or is protected by some harder rock. Weak rock beds are eroded, while harder beds stand out.

Aeolian Depositional Landforms

All particles transported by wind are ultimately dropped under two conditions – first, if the velocity of wind drops, or second if the wind meets an obstacle in its path.

Different circumstances lead to the formation of different landforms. The most important of these are sand dunes. Besides these, sand ripples and sand ridges cover vast areas. Bagnold has classified all depositional features into two broad classes on the basis of scale.

Bagnold's Classification of Aeolian Depositional Landforms — Bagnold’s Classification of Aeolian Depositional Landforms

Sand Dunes

Sand dunes are defined as hills and mounds of sand. They have a large variety and are classified on different bases.

Bagnold defines dunes as “mobile heap of sand whose existence is independent of either ground form or fixed wind obstruction”. His classification mentioned only two types – barchans (transverse dune) and seif (longitudinal dune).

Dune formation- All dunes have more or less similar morphology and require similar ideal conditions. When wind meets an obstacle, it slows down and leaves some of the transported sand on the windward side of the obstacle.

Gradually this deposition adds height to the evolving dune. When the crest of the deposit gains the maximum height possible under the available supply of sand and wind velocity, and the front becomes too steep, particles begin to slip forward and the leeward slip-face slope slumps (Fig.).

At this stage, two simultaneous processes shape the dune. First, on the windward side, sand particles arrive and move up the slope by creeping action; Second, the leeward side keeps slumping, and wind eddies remove loose particles from the dune.

It is to be noted here that sand particles can achieve stability only on a surface with a slope of 34 degrees or less. As soon as the slope exceeds this critical point, the sand becomes unstable and shows a tendency to roll down. This critical angle is called the ‘angle of repose’ for sand and plays an important role in all Aeolian features.

The processes active on the windward and leeward sides continue, and gradually the dunes move towards the slip-face side. The balance between incoming sand on the windward side and the sand removed by eddies from the slip face maintains the size of the dune as it moves ahead. Strong winds may add bulk to the dune, while gentle winds only rearrange its mass.

Barchan

Barchans are crescent-shaped dunes. They are either single or may form groups. They migrate in the downwind direction but maintain their shape as they move. They have the following distinct characters:

Slip-face is a downwind steep slope across which sand particles roll down when steepness is more than 34 degrees.
The gently sloping windward slope that receives a fresh amount of sand. The sand here is compacted and not loose.
A sharp lip marks the meeting line of the two slopes.
Two limbs or horns are gradually tapering and pointing downwind.
They develop on rocky or lag deposit surfaces.
Their height ranges from 0.5 to 100 metres.
Their width ranges from 30 to 300 metres.
They form under unidirectional winds.
They can move at a rate of 40 metres per year.
When several barchans join their horns, they form transverse dunes.

Seif

Seif is a linear dune with its axis aligned parallel to the prevailing strong winds. Its crest runs along its length and is marked by a sharp edge; hence, it is also called a sword dune.

There are several theories to explain it. One theory believes that seif is a result of a bi-directional wind pattern. “The longitudinal or seif dune occurs when the wind regime is such that the strong winds blow from a quarter other than that of the general drift of sand caused by the more persistent gentle winds” – Bagnold.

Seif is formed when gentle winds collect sand to form a normal, crescent-shaped dune, which is intermittently disturbed by strong winds, and the shape and mass of the initial dune are modified. The two winds take turns to develop a seif in the following stages:

The steady gentle wind forms a dune with two horns.
When the strong seasonal wind approaches, it adds a lot of new sand to the windward limb b and disturbs the balanced growth of the two horns.
Again, during the gentle-wind phase, the development and arrangement of all sand are aligned parallel to the wind flow. Thus b’ is created.
Alternatively, the seasonal wind again works on the windward side of the dune, adding more supply to b’’ and pushing it to the lee of its flow.
The two winds work on the seif turn by turn, but the axis of the dune is controlled and kept parallel to its flow by the gentle wind, giving it a linear shape.
Slip face on a seif is always to the lee of the prevailing wind; therefore it changes according to the changes in the wind direction. In general, the slip face runs on the two sides of the crest parallel to the axis of the dune.

Seif dunes have several summits; their number depends on the height of the seif. In lower seifs, summits may be as close as 20 metres, while on higher seifs summits may be 500 metres apart. These dunes are capable of maintaining their straight alignment to winds for several kilometres. They can run across low cliffs or moderate depressions without losing their straightness.

Southern Iran has examples of the highest seif dunes. Here, from base to crest they reach a maximum height of 210 metres. Their width usually is six times their height.

There are other explanations for linear dunes that develop parallel to the wind direction. According to one theory if the surface has some pre-existing linear features, like bands of raised resistant rocks or hills, then sand deposition is controlled by them to give shape to linear dunes.

In such cases, the wind sweeps sand from the lower areas and moves all sand particles towards the raised features. This movement of the wind results from hot air currents that tend to move from the central lower zone to the bordering higher features and rise along them. The winds form a circular cell between the hills and maintain the mass and shape of the linear dunes (Fig.).

Other Important Types of Dunes are:

Parabolic Dunes

These are typical of moist regions like the seashore. In these dunes, the horns of the crescent are fixed because shallow sand in horns allows vegetation growth, which stabilizes the sand. The dune’s higher, dry, central part keeps moving forward in the downwind direction. The shape of the parabolic dune looks like an inversion of barchans, as its horns point towards the windward direction (Fig.).

Star Dune

These dunes are formed in multi-directional wind regions. They have several limbs joined along a crest. Star dunes are fixed and have been at one site for several years.

Sand Ripples

Ripples are small-scale Aeolian features. They are 1 to 30 centimetres high and a few centimetres to some metres apart. They develop perpendicular to the wind direction. Their shape changes very quickly.

Sand Ridges

Ridges are long, undulating aeolian features parallel to the wind direction. The main process responsible for their formation is saltation.

In the beginning, there are windward and slip-face activities, just like in the formation of sand dunes. Particles on the leeward side are protected from the impact of the wind. The depression continually gets deeper as particles are removed from here rapidly.

Due to saltation, large grains are pushed up along the windward slope to the crest of the feature. The crest receives grains faster than it loses them. On the other hand, depressions lose grains faster than they receive and hence get hollowed.

Whalebacks, Dunefields and Sand Sea

In the Sahara desert, vast sandy areas are called ergs. These areas with a level surface made of coarse grains are called ‘plinth’ by Bagnold. These are remnants of old dunes and seifs that have migrated from here. Whalebacks are “Coarse-grained residues or platforms built up and left behind by the passage of a long-continued succession of seif dunes along the same path” – Bagnold

Chains of transverse and seif dunes, barchans and other small-scale features develop on these whalebacks to make dune fields. In Western Egypt, the sand sea extends for 600 kilometres (Fig.).

Sand Shadow

The formation of this feature depends on the presence of an obstacle in the path of the wind. The velocity of wind dips in the lee of such an obstacle while the flow circumventing the obstacle maintains its force.

As a result, weak flow fails to remove any sand particle that arrives on the leeward side of the obstacle; this allows sand to collect and form a depositional feature called the Sand Shadow of the obstacle. It is formed close to the obstacle in its shelter (Fig.).

Sand Drift

This feature is related to the presence of gaps in landforms that allow wind to blow as a channelized strong stream.

In such cases, the rest of the landform obstructs wind and transportation of sand while the gap allows unobstructed flow. All sand accumulating against such obstacles is directed to the gap and moves forward through it. Close to the gap, there is no deposition because here, the force of the channelized wind is strong and transports its entire load.

As the wind moves farther from the gap and loses its force, it drops the sand it is carrying. Right in the line of the gap, a mound builds up. Later this mound forces the wind to slow down and deposit more sand here.

Loess

Loess is very fine soil that wind has transported and deposited in thick layers far away from the place of its origin.

Aeolian Processes and Landforms: Erosional & Depositional

Aeolian Environment