GEOLOGICAL WORK OF WIND - ENGINEERING GEOLOGY (StudyCivilEngg.com)

 GEOLOGICAL WORK OF WIND

SUBJECT : ENGINEERING GEOLOGY

• No part of the Earth's surface is completely free from the forces of wind and its impact is capable of producing profound changes on the earth's surface. In most cases, the speed of the wind ranges from very mild, moderate (32-48 kms/hr) to high speed winds, in the range of hundreds of kms/hr.
• In the deserts, very high velocity winds become violent while sweeping across the plains, unfettered by vegetation and then carry large clouds of dust to very great heights and also transport sand particles closer to the ground. Wind arises due to the normal heating of the earth's surface. Thus, at the place of maximum heating (at the equator), it gives rise to a belt of warm, rising air, marked by a low pressure.
• However, at the poles, the low temperatures cause the air to settle down and to create a high-pressure zone. These low and high pressures zones on the earth's surface make the air to move away from the zones of high pressure towards the zones of low pressure.
• Further, if it were not for the rotation of the earth, these surface winds would have blown either directly north or south. This fact was recognised by the French mathematician G.G. Coriolis, in the 19th century when he said that the earth's rotation was an inducing factor, known as the Coriolis effect, which influences everything that moves across the face of the earth, such as the ocean currents or the winds.
• The geological action of wind is most effective and pronounced in the deserts. As is well known, a desert represents an area on the earth, which receives little or no rainfall during the year and which, therefore, is devoid of vegetation. The temperatures swing from a hot extreme during the day to a very low temperature at night, within a span of few hours. The prominent deserts of the world include the Sahara (north Africa), Arabian (middle East), Victoria (Australia), Kalahari (south Africa), Mexico, Arizona and California; and the deserts of Afghanistan, Baluchistan and in the northwestern India, the Thar desert of Rajasthan.
• Moving air is called wind. At low velocities, wind has a laminar movement which becomes turbulent at much higher velocities. The wind velocities increase rapidly with increasing height above the ground. The wind moves forward across the land surface, but the movement within the air may be upward and downward and also from side to side.

Observations made in the deserts and also in the laboratory show that loose particles are transported by wind in 3 ways, viz as (1) suspended load, (2) saltation load and (3) traction load.

1. The suspended load consists mainly of dust or clay size particles (less than 0.2 mm in size), which are in continuous suspension due to the turbulence of the wind.
2. Most sand grains move forward in a series of leaps and jumps, commonly referred to as saltation. The sand grains bounce up into the air under the impact of another particle.
3. Some sand grains, particularly the larger ones, are unable to rise into the air due to their size, even under the impact of other grains. They simply roll forward along the ground and this movement is called as traction.

Wind Transport

Basically wind carry fine, coarse material along it, according mode of transportation are classified as saltation, suspension, traction. fine, dry low density material lifted by wind from ground and carry in to high air. Its moves along with air, this mode of transportation saltation known as suspension. Heavier and coarse sediment like sand, pebble, gravel are transported by wind and heavier particle (sand) are jump according velocity of wind but it never lift greater than height 2 meter, always move along ground surface knows saltation.

Traction

In this made of transportation, heavier coarse particle with increased momentum and made them move in the direction of wind

Wind is a potent geological force, capable of sculpturing the landform either by its erosional activity or by the depositional processes.

Erosional Features Formed by Wind

Wind erosion is effected in two ways viz. (1) Deflation and (2) Abrasion.

Deflation

Deflation (meaning 'to blow away' in Latin) is a process in which loose and dry particles are removed from a place. Deflation is more effective in dry regions, with little or no rainfall and scanty or no vegetation. As the wind blows over such regions, the loose, dry soil and sand particles are readily lifted and carried away by the wind. The erosional features developed by the wind due to deflation are

(i) deflation hollows and (i) deflation armour.

(i) Deflation Hollow : Deflation scoops out hollows or deflation basins in a dry and unconsolidated deposit which may be a few feet to several kilometers in diameter, e.g., southern Wyoming deflation basins in Egypt have depths of about 50 mts. In India, such deflation basins exist in the state of Rajasthan, especially in the Thar Desert. The process of deflation continues till it reaches the water table and then it ceases. The deflation basins may later develop as an oasis.

(ii) Deflation Armour : In some deserts, larger rock particles are found mixed along with the sand

Deflation Armour developed along the deserts

This is so because when the wind blows over the area, the sand gets deflated, leaving the surface enriched with rock fragments which are too large to be removed by the wind.

The concentration of these rock fragments may grow quite high thereby protecting the sand below it from getting deflated, as it forms a deflation armour. This surface is called a desert pavement, e.g., floor of the Death valley in California.

Abrasion

This process consists of sand blasting action where the hard and resistant sand grains (usually of quartz) are carried away by the wind and strike against the exposed rocks in its path. Sand blasting or abrasion is most effective 2 to 3 meters above the ground and its effectiveness decreases further upwards.

Important erosional features developed by the abrasive action of the wind include mushroom rock and yardangs.

(i) Mushroom Rock: Abrasion begins at the base of an obstructive rock when it comes in the way of sand-laden wind. If the wind direction keeps changing, the rock gets undercut from all the sides due to sand blasting. This may continue for many thousands of years and the rock may acquire the shape of a mushroom and is described accordingly

Mushroom Rock

(ii) Yardangs: This feature was first described from the Taklinmankan desert in China. The gently dipping sedimentary rocks, made up of alternating bands of soft, unconsolidated and hard bands, are sand blasted by the wind, resulting in the soft unconsolidated sediments getting scoured and creating long passage ways or ho ows while the harder sediments stand out as hanging ridges which are called Yardangs

Yardangs formed in Deserts

Rock Column

Blowing wind strikes on obstacle which is vertical pillar like shape which consists number of joints. Winds starts eroding soften part of obstacle and it cause blocks fall down one by one and form number of individual column in that obstacle it is known as Rock column.

Rock Column

Pinnacles

The phenomenon of selective abrasion leads to the overturning of rock column nevertheless the column which endorse are known as pinnacles.

Pinnacle

Polished Rock

Limestone surface will be shine and more smooth, due to continuously impact of sand blasting.

Pitted Rock

The rock obstacle lose their soft part due to sand blasting and retain as small pit and hard part remain as it is that structure look like ‘honey comb’ structure known as pitted rock.

Pitted Rock

Ventifacts

It is small size rock fragments having two or more polished surface that polished surface developed due to sand blasting. Obstacle change their position due to high or strong winds and again next face also subjected to sand blasting and form new polished surface so polished surface and facetted fragment known as ventifacts.

Ventifacts

Depositional Features Formed by Wind

Whenever the wind starts blowing over a patch of loose, dry and unconsolidated sediments, the dust particles in the sediments (0.03 mm) get carried in suspension, and the sand-sized grains get carried along the surface in a series of leaps and jumps, described as saltation. The sand particles on impact with other particles get thrown into the air. Whenever a deposit laden wind loses its velocity, the deposits are dropped back to the ground.

Any obstacle in the path of the wind (like boulders, hills, ridges, fences, buildings, bushes and trees) diverts the moving air and causes the load to be dropped, resulting in the accumulation of deposits on both the windward and leeward sides of the obstruction. The two main types of depositional features formed by the wind are (1) sand dunes and (2) loess.

Sand Dunes

• As the sand keeps accumulating, it itself acts as an obstruction to the wind, causing a continued deposition of sand. Such accumulations of sand mounds are referred to as dunes. A sand dune shows a profile having a gentle windward slope and a steep leeward slope. As the wind blows over the sand dune, the sand grains roll along the windward slope towards the crest of the dune and then it gets dropped onto the leeward slope. The lee side thus, assumes an angle of repose for the sand of around 300 to 340 (which results from the equilibrium between the pull of gravity on the sand grains and the force of friction between the grains).
• Many dunes grow to heights of 30 to 100 mts. The height to which a dune can grow is governed by the upward increase in the wind velocity so as to transfer the sand grains off the top of a dune as fast as they arrive there, by climbing the windward slope. The migration of a dune takes place by the transfer of sand from the windward side to the dune crest where the sand rolls or slides down the slip face in the leeward side. Thus, as the sand accumulation continues with periodic sliding of the sand grains along the slip face, the dune slowly migrates in the direction of the wind.
• A dune may travel as much as 20 meters on an extremely windy day.
• Sand dunes are described according to their shape or form as : (i) barchan dunes, (it) transverse dunes, (it) parabolic dunes and (iv) longitudinal dunes.

Formation of Sand Dune

Barchan Dunes

• A barchan dune is an isolated crest shaped dune. It forms mainly because the wind is largely resisted at the central part and less along the flanks.
• Thus, the flanks are like two points or 'horns' with a concave sand slope, called the slip face.
• The wind energy transfers the sand from the windward side towards the slip face, where it slips or slides down the slip face at a rate of several centimeters to about 15 m/year. Barchans grow as high as 30 mts with a width upto 400 mts. Sometimes, a number of barchans may occur together like a swarm of barchans, e.g., North African and Arabian deserts.

Parabolic Dunes

Parabolic dunes are most commonly observed in the coastal areas. These are convely bowed in the downwind direction. In other world, parabolic dunes look more or less like reversed direction, i.e. their horns point upwind rather than in the downwind direction. The sparse vegetation cover permits limited migration of the sand along these dunes

Different types of Dunes

Transverse Dunes

Transverse dunes are typically developed along the sea coasts or lakes where there is a large supply of sand and a constant wind direction. The wind driven sand accumulates nearly parallel to the coastline or gets oriented across the direction of the flow of the wind and hence, called transverse dunes. In majority of cases, the sand gets transported from the windward side towards the leeward side, by virtue of which the dune migrates or advances inland, which is of great concern to the nearby villages. However, in majority of the cases, it can be stabilised by growing vegetation such as runners and creeper plants on the dunes.

Longitudinal Dunes

These dunes are developed in very dry regions where sand supply is scanty but the wind is strong. Under such conditions, these dunes occur in the form of long ridges of sand which are parallel to the general wind direction. These dunes vary in height from less than 8 mts to around 100 mts and their lengths range from 60 mts to several kilometers, e.g., longitudinal dunes of Libyan desert.

Loess

As strong wind starts blowing over the desert, a large quantity of dust particles (< 0.01 mm) are taken in suspension and are carried for long distances. Ultimately these particles settle down to form thick, fertile, blanket-like deposits called loess, e.g., deserts of China and Asia. The constituting material is yellowish or light buff coloured, very fine grained and in most cases, is devoid of stratification. This is because the grain size is too small to produce layering and the plants, worms and other organisms churn up the sediments as they are deposited. These are known for their high potassium content and exceptionally thick deposits of about 60 mts are observed in China 

Loess Deposits in China

FAQs COVERED IN THIS POST

What are the Geological Works of Wind?
What are the ways loose particles are transported by wind?
How are particles transported through Wind Transport?

What is Traction in Geological Works of Wind?
What are the Erosional Features formed by Wind?
What is Deflation?
What is Deflation Hollow?
What is Deflation Armour?
What is Abrasion?
What is Mushroom Rock?
What are Yardangs?
What are Rock Columns?
What are Pinnacles?
What is polished rock?
What are Pitted Rocks?
What are Ventifacts?
What are the depositional features formed by wind?
What are Sand Dunes?
What are Parabolic Dunes?
What are the types of Parabolic Dunes?
What is a Loess?

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