WINDS

INTRODUCTION

As in humid regions, running water and in colder latitudes moving ice are the chief agents of denudation, similarly in arid regions - in the hot and temperate deserts of the world – wind is the chief agent of denudation. The chief factor that limit the action of the wind or which define the limits of the region where wind action is specially powerful are scarcity of rainfall and absence of vegetation cover. Besides these, excessive insolation and evaporation are the other controlling factors. In the absence of roots that bind the loose earth particles and the humidity of the soil that keeps it from being blown off, the wind attains special force.

WORK OF THE WIND

The wind carries with it thousands of dust particles. These dust particles collide with each other and become smaller. With the help of these sand particles the wind erodes the earth’s surface but it can do so when its velocity is very high. As soon as the speed of wind declines, it feels difficulty in carrying these particles. Consequently they are deposited in the form of hillocks (heaps).

WIND EROSION

Moving air, like moving water is capable of picking up loose debris and moving it to another location. Although wind erosion is not restricted to arid and semi-arid regions, it does its most effective work in these areas. In humid regions, moisture binds particles together and vegetation anchors the soil so that wind erosion is negligible. For wind to be effective, dryness and scanty vegetation are important prerequisites. When such circumstances exist, wind may pick up, transport and deposit great quantities of fine sediment.

Difference b/w Wind Erosion & Stream Erosion
Wind erosion differs from stream erosion in two significant ways. First, wind has a low density compared to water; thus it is not capable of picking up and transporting coarse materials. Second, because wind is not confined to channels, it can spread over large areas, as well as high into the atmosphere.

Ways in which erosion takes place

1. Deflation
This involves the lifting and blowing away of loose materials from the ground. Such unconsolidated sand and pebbles may be carried in the air or rolled along the ground depending on the grain size. The finer dust and sand may be removed kilometers away from their place of origin, and be deposited even outside the desert margins. Deflation results in the lowering of the land surface to form large depressions called deflation hollows. The Qattara Depression of the Sahara Desert lies almost 135 m below sea level. (GOH 69)

2. Abrasion
The sandblasting of rock surfaces by winds, when they hurl sand particles against them, is called abrasion. The impact of such blasting results in rock surfaces being scratched, polished and worn away. Abrasion is most effective at or near the base of rocks, where the amount of material, the wind is able to carry, is greatest. This explains why telegraph poles in the deserts are protected by a covering of metal, a meter or two above the ground. A great variety of desert features are produced by abrasion.

3. Attrition
When wind-borne particles role against one another in collision, they wear each other away so that their sizes are greatly reduced and grains are rounded into millet seed sand. This process is called attrition.

Erosional Landforms

1. Blowouts or Deflation Hollows
The most noticeable result of the deflation in some places are shallow depressions called blowouts or Deflation Hollows. In some places, layers and layers of loose dry sediments are removed by deflation. The result is a blowout. In the Great Plains Region, from Texas north to Montana, thousands of blowouts can be seen. They range in size from small dimples (depressions) less than 1 m deep and 3 m wide to depressions that are over 45 meters deep and several kilometers across. In wet years, they may be covered by grasses, some may even fill with runoff, forming shallow lakes. In dry years, however, the grasses die and the lakes dry up. Then the wind continues to deflate the bare soils. (Strahler 567, GOH 69)
Similarly, minor faulting can also initiate depression and the eddying action of oncoming winds will wear off the weaker rocks until the water table is reached. Water then seeps out forming oases or swamps, in the deflection hollows or depressions. The Faiyum Depression in Egypt lies 40 m below sea level. Large areas in western USA, stripped-off their natural vegetation for farming were completely deflated when strong winds moved materials as dust storms, laying waste crops and creating what is now known as the Great Dust Bowl.

2. Desert Pavement
In portions of many deserts the surface is characterized by a layer of coarse pebbles and gravel. Such a layer, called Desert Pavement, is created as the wind removes fine material, leaving the coarse particles behind. Once desert pavement becomes established, a process which may take hundred of years, the surface is effectively protected from further deflation. In North Africa such a pebble-covered surface is called reg. The desert pavement of quartzite fragments was formed by action of both wind and water on the surface of an alluvial fan in the desert of southeastern California.

3. Mushroom Topography
The sandblasting effects of winds against any projecting rock masses wears back the softer layers so that an irregular edge is formed on the alternate bands of hard and soft rocks. Grooves and hollows are cut in the rock surfaces, carving them into fantastic and grotesque-looking pillars called Rock Pedestals. Such rock pillars will be further eroded, due to abrasion, near their bases where the friction is greatest. This process of undercutting produces rock of mushroom shape called mushroom rock or gour as in the Sahara. (GOH 70)

4. Zeugen
These are tabular masses, which have a layer of soft rocks lying beneath a surface layer of more resistant rocks. The sculpting effects of wind abrasion wear them into a weird-looking “ridge and furrow” landscape. Mechanical weathering initiates their formation by opening up joints of the surface rocks. Wind abrasion further eats into the underlying softer layer so that deep furrows are developed. The hard rocks then stand above the furrows as ridges or zeugen, and many even overhang. Such tabular blocks of zeugen may stand 3 to 30 meters above the sunken furrows. Continuous abrasion by wind gradually lowers the zeugen and widens the furrows. (GOH 70)
Sometimes the pedestals of softer rocks or pillars are completely removed by undercutting so that the whole mass is overturned or turned turtle. This feature is also known as Zeugen. (Gupta 347)

5. Yardangs
Quite similar to the “ridge and furrow” landscape of zeugen are the steep-sided yardangs. Instead of lying in horizontal strata upon one another, the hard and soft rocks of yardangs are vertical bands and are aligned in the direction of prevailing winds. Wind abrasion excavates the bands of softer rock into long, narrow corridors, separating the steep-sided over hanging ridges of hard rock, called yardangs. They are commonly found in the Atacama Desert, Chile, but the more spectacular ones with yardangs rising 8 to 15 m are best developed in the interior deserts of Central Asia where the name originated.

6. Inselberg
This is a German word meaning “island-mountain”. They are isolated residual hills rising abruptly from the level ground. They are characterized by their very steep slopes and rather rounded tops. They are often composed of granite or gneiss, and are probably relics of an original plateau which has been almost entirely eroded away. Inselbergs are typical of many desert and semi-arid landscapes in old age, e.g., those of northern Nigeria, western Australia, and the Kalahari Desert in Botswana. Sometimes these hills are pyramid like with a cap of hard rock and sometimes they are dome shaped. These represent an island like emergence from a rock plain. (GOH 71)

7. Dreikanter or Ventifacts
These are pebbles faceted by sand blasting. They are shaped and thoroughly polished by wind abrasion to shapes resembling Brazil nuts. Rock fragments, mechanically weathered from mountains and upstanding rocks, are moved by wind and smoothed on the windward side. If wind direction changes, another facet is developed. Such rocks have characteristic flat facets with sharp edges. Among the ventifacts, those with three wind-faceted surfaces are called dreikanter. These wind faceted pebbles form the desert pavement, a smooth, mosaic region, closely covered by the numerous rock fragments and pebbles.

8. Hamada
In desert areas the exposed bedrock surfaces become the sides of the attack by sandblast and as the matter is moved to and fro, the less resistant parts gradually reduce to sand and conveyed beyond the limits of the region. Ultimately what remains behind is the rock pavement made of resistant rock stuff over which are spread dreikanter-shaped pebbles. Such a rock flow represents the base level of erosion in deserts – the last stage of works in the cycle of dessert erosion. This flat, bare rock floor is called Hamada. The best known rocky deserts are those of the Sahara Desert, e.g., the Hamada el Homra, in Libya, which covers an area of almost 52,000 km2.

9. Earth Pillars
It is seldom that wind action goes all alone. The water does intervene to modify the results of wind action. The desert areas of the world have occasional cloud bursts which produce sheet floods near the isolated mountainous borders and swell the wadis. These temporary masses of moving water are quite ephemeral (brief, temporary) in their existence and they carry a load which is much more than their capacity to carry. The result is that these under-fit streams are suffocated with the material they carry and produce very characteristic profiles of deposition. But due to its terrific nature, the rainwater helps in erosion and produce earth pillars each having at its cap some pebbles. These are also known as Hoodos or Demoiselles (like female child). These earth pillars are round or many faceted, smooth or rough accordingly as the wind has attacked them from one side or from all the sides steadily. (Gupta 349)

10. Castellated Chimneys
The rain water tends to sharpen the yardangs so that they begin to look like pointed needles or a series of chimneys known as Castellated Chimneys (built in the style of castle with battlements).

Wind Deposition
Although wind is relatively unimportant as a producer of erosional landforms, wind deposits are significant features in some regions. Accumulation of wind blown sediments are particularly conspicuous landscape elements in the world’s dry lands and along many sandy coasts.

Types of Wind Deposits
Wind deposits are of two types:

1. Loess
The fine dust carried to the borders of the desert or beyond and deposited on neighboring lands is called loess. It is yellow, friable (flaky) material and usually very fertile. The name loess comes from a village in Alsace (France) where such deposits occurred. Loess is infact, fine loam, rich in lime, very coherent and extremely porous. It is very pervious to water and thus water streams cut deep valleys. In this way Badland Topography may develop.
The thickest and most extensive loess deposits occur in western and northern China, where accumulations of 30 m are not uncommon and thickness of more than 100 m have been measured. It is this fine, buff colored sediment, which comes from Gobi Desert (Magnolia), and gives the Yellow River (Huang-He) and the adjacent Yellow Sea their names. The source of China’s 800,000 km2 of loess are the extensive desert basins of Central Asia.
In the United States, deposits of loess are significant in many areas, including South Dakota, Nebraska, Iowa, Missouri, and Illinois as well as portions of the Columbia plateau in the Pacific Northwest.
In Germany, France, Belgium, it is called Limon. In USA, loess is derived from ice-sheet called Adobe.

2. Sand Dunes
Like running water, wind releases its load of sediments when its velocity falls and the energy available for transport diminishes. Thus sand begins to accumulate where ever an obstruction across the path of the wind slows the movement of the air. Unlike deposits of loess, which from blanket like layers over large areas, winds commonly deposit sand in mounds or ridges called Dunes. Dunes may be live or fixed. (Strahler 569)

Process of Formation

As moving air encounters an object, such as a dump of vegetation or a rock, the wind sweeps around and over it leaving a shadow of more slowly moving air behind the obstacle as well as a smaller zone of quieter air just infront of the obstacle. Some of the salting sand grains moving with the wind come to rest in these wind shadows. As the accumulation of sand continues, increasing efficient wind barrier forms to trap even more sand. If there is a sufficient supply of sand and the wind blows steadily long enough, the mound of sand grows into a dune.
Although often complex, dunes are not just random heaps of sediments, rather they are accumulation that usually assumes surprisingly consistent patterns.

a. Barchan or Crescent Dunes
Solitary sand dunes shaped like crescents and with their tips pointing downwards are called Barchan Dunes. They are also spelled barcan, barkhan or barchane. These dunes form where supplies of sand are limited and the surface is relatively flat, hard and lacking vegetation. They migrate slowly with the wind at a rate of upto 15 meters/year. Their size is usually modest with the largest barchans reaching a height of about 30 m while the maximum spread between their horns approaches 300 m. When the wind direction is nearly constant, the crescent form of these dunes is nearly symmetrical. However, when the wind direction is not perfectly fixed, one tip becomes longer than the other. (GOH 72, Strahler 569)

b. Transverse Dunes
In regions where vegetation is sparse or absent and sand is very plentiful, the dunes form a series of long ridges that are separated by troughs and oriented at right angles to the prevailing wind. Because of this orientation, they are termed Transverse Dunes. Typically, many coastal dunes are of this type. In addition, they are common in arid regions where the extensive surface of wavy sand is sometimes called Sand Sea.

c. Longitudinal Dunes
These are long ridges of sand that generally form parallel to the prevailing wind and where sand supplies are limited. Apparently the prevalent wind direction must vary somewhat, but not by more than about 90o. Although the smaller types are only 3 or 4 meters high and several tens of meters long, in some large deserts, longitudinal dunes can reach great size. For example, in portions of North Africa, Arabia and Central Australia, these dunes may approach a height of 100 m and extend far distances of more than 100 m. (GOH 73, Strahler 573)

d. Parabolic Dunes
Unlike the dunes that have been described thus far, parabolic dunes form where vegetation partly covers the sand. The shape of these dunes resembles the shape of barchans except that their tip points into the wind rather than downwind. Parabolic dunes often form along coasts where there are strong onshore winds and abundant sand. If the sand’s vegetative cover is disturbed at some spot, deflation creates a blowout. Sand is than transported out of the depression and deposited as a curved rim which grows higher as deflation enlarges the blowout. (Strahler 571)