3. UNIT-12 WIND

UNIT-12 WIND

The Earth’s atmosphere is mainly composed of air, and wind is air in motion. Winds move due to differences in air pressure caused by unequal heating of the Earth by the Sun. Warm air rises while cooler air flows in to replace it, creating wind circulation.

Objectives

Ø  After studying this unit, you should be able to:

Ø  Explain wind as a geological agent

Ø  Understand erosion, transportation, and deposition by wind

Ø  Identify desert landforms

Ø  Describe the role of wind in landscape formation

 

Geological Work of Wind (Aeolian Processes)

Wind transports moisture, dust, and fine rock particles from one place to another. Since these materials are derived from land, the geological action of wind is more significant over land, especially in arid and semi-arid regions where vegetation is sparse.

Wind acts as a geological agent through three main processes:

1.     Erosion

2.     Transportation

3.     Deposition

However, wind is less effective than running water as an erosion agent.

2. Wind Erosion

Wind erosion mainly occurs in dry regions with little vegetation. It involves the removal of loose rock particles from the Earth's surface.

There are two main processes of wind erosion:

1. Deflation : Deflation is the removal of loose particles such as dust and fine sand by wind.

Features:

·       Occurs in arid regions with sparse vegetation

·       Wind picks up loose material and carries it away

·       Leaves behind coarser particles and desert pavement

 

Causes of deflation:

·       Strong winds

·       Whirlwinds and eddies

·       Lack of vegetation cover

 

2. Abrasion (Corrosion) : Abrasion or corrosion occurs when wind-blown sand acts like a natural sandblast and wears down rock surfaces.

Characteristics:

·       Sand particles strike rock surfaces

·       Rocks become polished, grooved, or fluted

·       Most effective close to the ground where sand concentration is highest

Common landforms produced by abrasion: Mushroom rocks, Pedestal rocks, Table rocks, Undercut hills

Example: Windows exposed to desert winds may lose their glass polish due to sand abrasion.

Ventifacts: Stones shaped and polished by wind abrasion are called ventifacts.

Types:

1.     Einkanters – stones with one polished face.

2.     Dreikanters – stones with three faceted faces formed by rotation of pebbles.

3. Wind Transportation

Wind transports materials depending on particle size, shape, and wind velocity.

Modes of Transportation

1. Suspension

Very fine particles like dust and clay are carried high in the air and transported over long distances.

Example: Dust storms, Volcanic ash transport

2. Saltation: Sand grains move in short jumping or bouncing movements.

Characteristics:

·       Most common method of sand transport

·       Particles move in a series of hops

·       Important in the formation of sand dunes

 

3. Surface Creep (Traction): Large sand grains and pebbles roll or slide along the ground due to wind pressure and collision with saltating particles.

Special Wind Movements

Whirlwind (Dust Devil) : A rotating column of air that lifts dust particles into the atmosphere.

Tornado: A violent rotating column of air capable of lifting heavy objects.

4. Wind Deposition

Wind deposition occurs when wind velocity decreases and it can no longer carry its load.

Deposited materials are mainly:  Sand, Dust, Clay

Deposits show sorting, with heavier particles settling first and finer particles travelling farther.

5. Sand Deposits and Sand Dunes

Wind deposits sand in heaps or ridges called sand dunes.

Formation of Dunes: Dunes begin forming when wind encounters:

Rocks, Vegetation, Surface irregularities

These obstacles reduce wind velocity and cause sand accumulation.

Structure of Sand Dunes: A typical dune has two slopes:

Windward slope

Gentle slope

Angle: 10°–15°

Leeward slope (Slip face)

Steeper slope

Angle: 20°–30°

Sand moves up the windward side and falls down the slip face, causing the dune to migrate slowly in the direction of wind.

Migration rate: Usually a few meters per year.

6. Types of Sand Dunes

1. Barchan Dunes: Barchan dunes are crescent-shaped dunes formed by winds blowing in one direction.

Characteristics:

·       Gentle windward slope

·       Steep leeward slope

·       Horns point downwind

Conditions:

·       Limited sand supply

·       Hard ground

·       Sparse vegetation

2. Seif (Longitudinal) Dunes : Seif dunes are long narrow ridges of sand.

Characteristics:

·       Formed by winds from slightly different directions

·       Parallel to wind direction

·       Common in deserts of Arabia and Sahara

3. Transverse Dunes : These dunes form perpendicular to the prevailing wind direction.

Characteristics:

·       Formed where sand supply is abundant

·       Occur in deserts and coastal regions

·       Appear as long ridges separated by troughs

7. Dune Structure: Sand dunes show cross-bedding due to continuous migration.

Typical features:

·       Well sorted sand

·       Mostly quartz grains

·       Layers inclined in different directions

Cross-bedding is a key indicator of ancient desert environments in sedimentary rocks.

8. Ripple Marks: Ripple marks form due to wind friction on sandy surfaces.

Characteristics:

·       Small ridges and troughs

·       Coarse grains accumulate at crests

·       Fine grains collect in troughs

They are common on:

·       Sand dunes

·       Beaches

9. Dust Deposits : Fine particles carried by wind settle when wind speed decreases.

Deposition may occur through: Rain, Snow, Calm atmospheric conditions

Examples: Mud rain, Dust-colored snow

These deposits are called Aeolian deposits.

10. Loess Deposits

Loess is a fine wind-blown deposit of silt.

Characteristics:

·       Yellow or buff coloured

·       Non-stratified

·       Composed mainly of quartz, feldspar, hornblende, and calcite

Thickness: Few meters to several hundred meters

Major regions:  China (largest deposits), Central United States, Europe (Rhine basin)

Importance of Loess

1.     Forms very fertile soils

2.     Can stand in vertical cliffs

3.     Associated with glacial deposits

4.     Indicates wind deposition during glacial periods

Deserts – Geological Processes

1. Desert Environment

Desserts are arid regions where wind acts as the main geological agent of erosion and deposition. These regions usually have clear skies, very dry atmosphere, and large daily temperature variations. Rainfall is extremely low and vegetation is very sparse or absent.

2. Rainfall and Surface Runoff

Although rainfall is rare in deserts, it often occurs as sudden cloudbursts. The heavy rain produces strong torrents that flow rapidly down slopes, causing intense erosion of the land surface. Many deserts are closed drainage basins, where water does not flow to the sea.

3. Alluvial Fans and Playa Lakes

Water flowing from gullies carries a large amount of sediment and spreads across the basin floor. The sediments are deposited in fan-shaped accumulations called alluvial fans. Temporary lakes called playa lakes form at the base of slopes where water collects and later dries up due to evaporation.

4. Wind Action in Deserts

Dry channels known as wadies act as natural pathways for wind movement. Wind carrying sand particles polishes and erodes the walls of these channels. Wind also transports fine dust and sand across long distances.

5. Formation of Sand Dunes and Desert Pavement

Wind may deposit sand in sand dunes, while finer particles are blown away from the surface. The remaining coarse fragments such as sand and pebbles are called lag stones. Over long periods, these fragments accumulate to form a desert pavement.

6. Desert Varnish

Lag stones often develop a dark shiny coating of iron and manganese oxides known as desert varnish, which forms due to chemical reactions on the rock surface.

7. Weathering in Desert Regions

Weathering in deserts is mainly mechanical (physical) weathering due to temperature changes and lack of moisture. As a result, the regolith is thin, discontinuous, and coarse-grained.