7 WIND AND ITS GEOLOGICAL IMPACTS

Meenal Mishra

1. Objectives

• Discuss the geological processes of wind
• Elucidate the erosive action of wind and resulting erosional landforms
• Describe the action of wind as transport agent
• Elaborate on the depositional landforms formed due to wind action

2. Concept Map

3. Description

3.1 Introduction

The geological processes, active on the surface of the Earth are known as exogenous processes. They are capable of creating both constructive and destructive landforms. The geological agents play a significant role in sculpturing the Earth’s surface. The geological processes like weathering, mass-wasting, erosion, transportation and deposition play a significant role.

The chemical and mechanical weathering disintegrates the solid rocks exposed on Earth’s surface into small loose fragments so that they can be picked up and transported by wind. Wind is a powerful and effective agent capable of eroding, transporting and depositing the surface materials. Sand and dust storms are a regular phenomenon in the deserts. Environments in the desert are characterized by very high daily and annual range of temperature, very low mean annual rainfall, absence of vegetation, dominance of sand and high velocity winds. Aeolian landforms are produced by either destructive or constructive action of the wind. The word “Aeolian” is derived from “Aeolus”, in Greek meaning, the God of the wind. Eolian term is also used for all processes powered by wind.

3.2 Erosive Action of Wind

Wind is controlled by the seasonal variations resulting in the uneven distribution of solar heat over the surface of the Earth. Thus the circulation of air makes the wind to blow and moving air is termed as wind. The action of wind can transport huge quantities of windblown material to great heights and many kilometres. Wind is a turbulent fluid and transports sediment. We can compare wind action with running water though wind flows at a greater velocity than water. Sand, clay and silt particles can be carried as the suspended load by the wind as it has lower density. While the larger particles move along the ground as bed load. Wind is considered as a very effective agent of erosion and deposition like water. The wind exerts the same kind of force on particles on the land surface that a river exerts on its bed.

The wind velocity and surface roughness are the two factors that primarily control the power of wind to erode surface particles. The erosive force increases exponentially with the increase in wind  velocity. As the result the fast winds are capable of causing much more erosion than slow winds. The roughness of the surface at ground level plays an important role in controlling the nature of wind erosion. The moisture affects and slows down the action of the wind. Therefore the water table is regarded as base level for erosion of wind. Boulders, trees, buildings, shrubs, and even small plants like grass and herbs can increase the frictional roughness of the surface and reduce wind velocity. Vegetation can also reduce the erosional effects of wind by binding the soil particles to the roots of plants. Eolian or aeolian erosion are the terms used for wind erosion. It is performed by processes, viz. deflation, abrasion and attrition. Wind transports sediment and modifies the landscape through these processes of abrasion and deflation.

Deflation: The term deflation is originated from the Latin word “Deflare” means “to blow away”. The surface of any desert is covered by diverse type of fragments of rock, sand, soil and dust. Deflation involves the picking up or lifting, blowing and removal of fine and dry particles of loose sand, silt and soil from one place to the other by wind. The blowing wind picks up the sandy and fine grained materials, and leaves the coarser fragments behind to form stony and rocky deserts.

Abrasion: This involves mechanical scraping, rubbing, grinding, polishing and abrading of a rock surface by friction between rocks and moving particles during their transport by wind. In this process the individual grains may shed off some of its weight and shape and attain more roundness on grinding off their angularity. The particles transported by wind can strike the obstructing materials along its path called corrosion. The process of corrosion can carry out grooving, scarring, polishing and grinding of rocks. Its intensity depends on the size and density of carrying materials, and also the Earth’s surface rocks.

Attrition: When the load sediments undergo wear and tear due to mutual impacts during the process of transportation by wind resulting in further reduction in size is known as attrition.

3.3 Erosional Landforms

We will discuss the various erosional landforms resulting due to above mentioned processes.

• (i) Desert pavement: It is a typical desert surface covered with closely packed, interlocking angular or rounded rock fragments of pebble and cobble size. They are formed by gradual removal of sand, dust and other fine grained material by wind and intermittent rain. Wind blows away finer particles like clay, silt and sand on deflation leaving behind the bigger fragments. Boulders or pebbles are too large for wind to transport. Accumulated coarser material (having fairly smooth shinning surface) are left behind as desert pavement (Figure 1) also known as lag deposits. It marks the maximum limit of aeolian activity by deflation. Desert pavement also known as stone pavement forms a coarse gravelly ground surface that protects the soil or sediments below from further erosion.

Figure 2 Fine-grained particles are blown by wind. Wind blows away finer particles leaving behind pebbles and boulders resulting in desert pavement

• (ii) Deflation basins: They are depressions formed in the deserts due to removal of sands to a greater extent. Deflation by strong wind scoops out sand sized particles producing shallow depressions of varying shapes. This may cause depression sometimes with its base touching quite depth. These evacuated depressions are up to 1.5 Km long and a few meters deep (Figure 2). These blowouts when intersect the water horizon, they get partially filled with water, resulting in development of swamp or oases. Deflation basins also commonly develop where calcium carbonate cement, in sandstone formations, is dissolved by groundwater, leaving loose sand grains that are picked up and transported by the wind. Large deflation basins, covering areas of several hundred square kilometers, are associated with the greatest desert areas of the world, particularly in Sahara desert.

• (iv) Pedestal rocks: They are the exposures having broad upper part and narrow base carved by the action of wind over an existing standing rock mass. The heavier and larger particles remain near the ground and only smaller lighter particles move above with the wind at higher level which results in the formation of pedestal rock. When the larger particles strike the homogeneous rocks they differently abrade the upstanding rock mass such that rock head or cap balances over a very slender neck resulting into a pedestal fan like shape. Differential erosion is the term used for varying resistance to the agencies of erosion wherein parts of softer rock are removed to a greater rate than the harder one. Thus wind abrasion plays a significant role in shaping pedestal rocks and is also called as Mushroom rocks (Figure 4).

Figure 4 Sketch showing differential erosion resulting in formation of pedestal rock; Field photographs of Pedestal rock from Thar Desert

• (v) Yardangs: Yardangs are sculptured landforms extending up to tens of meters high and kilometers long. They are streamlined hill, carved from bedrock or any consolidated or semi consolidated material by double action of wind abrasion, and deflation (Figure 5). When viewed from above, yardangs resemble like the hull of a boat exhibiting commonly asymmetrical shape and elongated leeward tails. This is perhaps because of slight variation in wind direction. Yardangs are composed of cohesive silts and clays, sandstone, or limestone and develop in regions with strong unidirectional winds. Yardangs are classified as mega-yardangs, meso-yardangs and micro-yardangs on basis of size. Mega-yardangs can be of several kilometers long and hundreds of meters high. Meso-yardangs are generally a few meters high and 10 to 15 meters long. Micro-yardangs are only a few centimeters high.

• (vii) Blow-outs: These are broad-shallow caves in hills formed due to wind erosion. The sand grains are the most effective agents of abrasion. Rocks exposed to the sand blasting by prevailing winds become pitted, grooved, and polished (Figure 7).

• (viii) Desert varnish: It is a dark coating on rocks found in arid regions, composed dominantly of fine-grained clay minerals containing black manganese oxide and red iron oxide. Desert varnish forms only on physically stable rock surfaces that are no longer subject to frequent precipitation, fracturing or sandblasting.

3.4 Transporting Power of Wind

The stages of transportation depend on the wind velocity and sediment load. Wind has the ability and force to lift the Earth’s unconsolidated sand, silt and dust sized particles by the mechanisms mentioned like saltation or rolling and traction and suspension (Figure 8).

Saltation: This process involves the lifting of heavier and coarser sediments such as sand grains, pebbles and gravels etc. only for short distance and for smaller heights above the ground periodically during high wind velocity. This movement takes place close to the Earth’s surface and the picked up and lifted materials may be dropped and picked up again and again during the process of transportation. The lifted particle fall and they transmit an impact to another stationary particle resting on the ground. The size and shape of the grain control the height and distance to which these sediments are transported in one cycle.The heavier particles and massive rock fragments are transported by dragging or rolling along the ground called traction as bedload.

Suspension: The silt and clay sized particles are carried in suspension, above the ground surface. The suspended grains travel thousands of kilometres and reach to great altitudes. The force of the wind lifts clay and silt particles of lighter density from the ground and carries high up to upper layers. This is called suspension because the particles once lifted are not allowed to rest on the ground again till the velocity of wind in those upper layers is checked.

Figure 8 Suspension, saltation and traction or rolling

3.5 Deposition by Wind

Wind carries forward sediments and particles once picked by it from any source on the surface for a varying distance depending on its carrying capacity. Deposition of material carried away by wind can take place at any place and time. The settling and deposition of wind borne material takes place when there are obstructions. All these create constructive aeolian landforms. Wind is an effective agent for sorting of materials according to their shape, size and weight. Decrease in velocity of wind results in deposition of windblown sediments which are highly sorted in nature.

Wind deposition can take place due to following reasons: obstruction in movement of wind like hill, mountains, buildings, forest belts, vegetation, etc; reduction in wind velocity; and increase in moisture content and precipitation (rainfall and snowfall)  nature is easily eroded and therefore loessic areas are characterized by steep cliffs and rapid lateral and headward stream erosion.

3.6  Depositional Landforms

Let us discuss the following depositional landforms formed by wind action:

1. i) Loess: Loess is aeolian sediment, clastic in nature formed by accumulation of windblown sediments predominantly of silt size. Clayey along with silty fractions are deposited as loess. Loess is pale yellow or buff in colour, homogeneous, friable, porous, slightly coherent, typically non-stratified and often calcareous (Figure 9). Its grains are angular with little polishing or rounding and are composed of quartz, feldspar, mica, calcite and other minerals. Loess is mainly derived from three main sources: deserts, river floodplains in semiarid regions and Pleistocene glacial outwash deposits. Loess is formed by accumulation of wind-blown dust picked from outwash plains, exposed by repeated advances and retreats of continental glaciers. Loess is stabilized by moisture and vegetation in order to accumulate. These deposits create rich soil, e.g. loessic deposit in Northern China which is about 300 meters thick. Loess ridges that have formed are aligned along direction of prevailing winds during last glacial period, maximum in several areas of the world and are called as paharidges in America and greda ridges in Europe. Their formation has been explained by a combination of wind and tundra conditions. Loess as a result of its unconsolidated

Figure 9 Loessic sediments deposited over quartzites of Delhi ridge

1. ii) Playa: Water sometimes collects in flat area called playa at bottom of desert basin resulting in formation of an oasis. When playa deposits have more saline matter and salts then it is known as

salina. As already mentioned oasis may be produced when bottom of a basin intersects with groundwater.

• iii) Sand Dunes: When you think of sand dunes and dune fields, you are reminded of deserts (Figure 10a). Dunes are of variable height such as 100 to 300 feet or more depending on transporting power and available material. Many dunes grow up to the height as much as 300m. The conditions required for formation of dunes are:

• abundant supply of loose sediments, i.e. sand;
• sufficient energy to move sediment, i.e. wind velocity;
• obstacle around which sand accumulates i.e. cliff, amount of vegetative cover; and dry or arid climate.

Sand dune is a mound or ridge of sand deposited with a crest (Figure 10b). Dunes are not permanent structures. A sand dune often moves or migrates forward windward from one place to the other in desert region. However grasses anchor the dunes with their roots and hold them temporarily in place. If dune is devoid of vegetation, winds can regularly change its form and location. An ideal dune has a gentle windward slope of about 10 to 15°and much steeper leeward slope of 30 to 35°. Wind lifts sand grains from windward side and drops them on leeward side also known as slip side and consequently the migration takes place. Dune advances by movements of individual grains of sand as been observed in Thar Desert. Structures of static and migrated dunes are shown in Figure 11.

There are following three types of desert:

• (i) Rocky desert consists largely of barren, hard and rocky plateaus and plains that have very little amounts of sand e.g. Sahara.

(ii)Stony desert constitutes rocky waste e.g. near shrikolayatji, Bikaner.

(iii)Sandy desert is essentially composed of sands; poor vegetation e.g. Pokharan, Rajasthan.

TYPES OF DUNES: Dunes are classified into five principal types, namely barchan, longitudinal, transverse, parabolic and star dune. The size, shape, and arrangement of dunes depend on interaction of factors as sand supply, direction and velocity of prevailing wind, and amount of vegetation.

• (i) Barchan dunes are crescent-shaped dunes with two tapering horns pointing windward and steep slip face on concave side (Figure 12). They are generally flat with dry surface and little vegetation, a limited supply of sand, and a nearly constant wind. Barchan dune consists of well sorted, very fine to medium sand. They form when wind blows constantly in one direction. These dunes migrate when sand slides down the leeward slope and sand moves up the windward side. Such movement of sand grains produces a series of crossbeds that slope in direction of wind movement. Barchan are usually small, with the largest reaching about 30 m high. Barchan are the most mobile type of major dune types.

• (ii) Longitudinal dunes (also called seif dunes) consists long, linear or slightly sinuous sand ridges aligned, generally much longer than wide, parallel to direction of prevailing winds. Long axis of sand ridges is parallel to direction of net sand movement. They form where sand supply is somewhat limited (Figure 13). Sediments in longitudinal dunes are well sorted with grain size varying from very fine to medium grained sand. Longitudinal dunes result when winds converge from slightly different directions to produce the prevailing wind. They range in height from about 3 m to more than 100 m, and some stretch for more than 100 km. The longitudinal dunes cover extensive areas in Saudi Arabia, Egypt, and Iran.

• (iii) Transverse dunes form long, wavy, linear ridges perpendicular or transverse to the prevailing wind direction in areas that have abundant sand and little or no vegetation (Figure 14). In plan view transverse dunes have a wavelike appearance and are therefore called sand seas. The dunes may be as wide as 3 km with crests of as high as 200 m. The intermediate form of dunes known as barchanoid dunes develop when barchan form separate into individual barchan dunes along edges of dune field. Sediments in transverse dunes are very fine to medium sand and are well-sorted. Wind speed should be constant, moderate and unidirectional.

• (iv) Parabolic dunes are most common dunes occurring in coastal areas where there is abundant sand supply, strong onshore winds, and a partial cover of vegetation (Figure 15). Parabolic dunes have a crescent shape like dunes with their tip pointing upwind. Parabolic dunes form when there is abundant supply of sand and vegetation cover is broken and erosive action by deflation creates a deflation hollow or blowout. As the wind transports sand out of depression, it builds up on convex downwind dune crest. Central part of dune is excavated by wind, while vegetation holds the ends and sides fairly well in place. Sediments comprising parabolic dunes are very fine to medium and well-sorted.

(v) Star Dune is commonly found in the deserts of North Africa and Saudi Arabia. Star dune is so named because of its resemblance to a multipointed star (Figure 16). It is pyramid shaped with three or more arms radiating from a peaked centre, and they develop where wind direction is variable.They grow taller rather than migrating. Star dunes are sometimes more than 100 m above the surrounding desert plain. Star dune comprises very fine to medium sand. Star dune for its formation requires wind blowing from several different directions.

4. Summary

In this lecture we learnt about: Processes controlling wind erosion and resulting erosional landforms Transportation of wind borne sediments Depositional landforms like loessic deposits and sand dunes. Types of sand dunes