Wind deposition: sand dunes

Detail

1. Wind as a sorting agent

• Aeolian deposits are deposits laid down by wind. (Aeolus was the Greek god of winds.)
• Wind sorts grains well. It can only lift grains that suit its speed.
• Strong wind → it can carry bigger and heavier grains.
• Wind slows down → the heaviest grains drop first and the finest grains drop last.

• Result → each deposit is made of grains of about the same size. Such a deposit is well sorted.

• Contrast with glacial till. Till is the mixed debris a melting glacier leaves behind. Ice carries everything at once, from clay to huge boulders. So till is unsorted.

2. How wind moves grains

Mode	What happens	Grain size	Extra facts
Surface creep / rolling	Grains roll or slide along the ground. Falling grains push them along.	Coarse sand	Creep makes up as much as 25% of grain movement in a desert [2]
Saltation	Grains make short hops and bounces. The wind lifts a grain, carries it a little way, and it falls back. When it lands, it knocks other grains into the air.	Sand	Lifts sand usually no more than 1 cm above the ground. Grains move at one-half to one-third of the wind's speed [2]
Suspension	Very fine grains stay up in the air and travel far, even across oceans.	Silt and dust	Surface winds keep particles smaller than 0.2 mm in suspension [2]

• Exam point: most sand moves by saltation. So sand stays close to the ground. That is why wind erosion cuts most at the base of rocks (mushroom rocks).

3. What a sand dune needs

• A sand dune is a hill or ridge of loose sand heaped up by the wind.
• Three conditions:
• Plenty of loose sand, from dry riverbeds, weathered rock or beaches.
• A steady wind that blows mostly from one direction.

• An obstacle, such as a bush, rock or small bump. The wind slows behind it and drops its sand.

• How a dune grows, step by step:

• The wind hits the obstacle and slows down → sand drops behind it → a small heap forms.
• The heap becomes an obstacle itself → more sand collects → the dune grows.
• The side facing the wind (windward side) has a gentle slope. Sand is pushed up this side.
• The sheltered side (leeward side) has a steep slip face. Sand slides down it.

• Sand keeps moving from the windward side to the leeward side → the whole dune creeps forward downwind.

• Where dunes form:

• Hot deserts: the Thar (India–Pakistan), Sahara, Arabian (Rub' al Khali), Namib and Australian deserts.

• Low sandy coasts: for example, parts of the Odisha, Andhra and Tamil Nadu coasts and the Konkan beaches in India.

• Moving and fixed dunes:

• Shifting dunes move every year and can bury fields, roads and railway tracks.
• Fixed (stabilised) dunes are held in place by grasses, shrubs and trees. These are often found near settlements.

4. Types of dunes

Main types (Class 11; the arrows in the textbook figure show wind direction)

Dune	Shape	Wind and sand conditions	Where seen
Barchan	A crescent (half-moon). Its horns point downwind. Gentle slope on the convex windward side; steep slip face inside the curve.	Wind is steady and moderate from one direction; the ground is flat and even; sand is limited	Thar, around Jaisalmer (Rajasthan); Sahara; Peru coastal desert
Parabolic dune	A reversed barchan: a U shape with its arms pointing upwind	Sandy ground is partly covered by plants. Plants hold the arms in place while the bare middle blows forward.	Thar margins; coastal dunes
Seif	Like a barchan but with only one wing. The single wing grows very long and high. "Seif" is Arabic for "sword" [2]	The wind changes direction	Sahara, Arabia
Longitudinal dune	A long, low ridge lying parallel to the wind	Little sand and a steady wind	Thar (western parts); Australian deserts
Transverse dune	A ridge lying at right angles to the wind	The sand source is a long feature (a beach or riverbed) at right angles to the wind	Coastal belts; deserts with plenty of sand
Star dune (Class 9)	A central peak with several arms spreading out like a star	Winds come from many directions	Sahara (Grand Erg Oriental), Namib

• Note on grouping: the US Geological Survey sorts dunes into six main types: transverse, oblique, longitudinal, barchan, star and parabolic. About 8.5% of the world's dunes are star dunes [6].
• Barchan vs. parabolic: an easy test
• Barchan → horns point downwind. The ground is bare and the sand moves freely.

• Parabolic → arms point upwind. Plants hold the arms back.

• Plenty of sand: dunes merge (coalesce) and lose their separate shapes. They form a sand sea.

5. Beyond the textbook: loess

• Loess is a thick layer of fine, yellowish, wind-blown silt. It forms like this:
• Wind carries the finest dust in suspension, far beyond the desert.
• The dust settles in layers over thousands of years.

• The deposit is soft and fertile, but it erodes easily.

• The thickest known loess is 335 m thick, on the Loess Plateau of China. The dust there came from the Gobi and the deserts of the north-west. The plateau is drained by the Huang He (Yellow River). The river gets its colour from this silt [2].

• In Europe and the Americas, loess is usually 20–30 m thick [2].
• Sorting link: sand drops close to the source and forms dunes. Silt travels farther and forms loess. Together they show that wind sorts by grain size.

6. Beyond the textbook: dunes, desertification and stabilisation in India

• The Thar Desert (Great Indian Desert) lies in western Rajasthan and covers 196,150 sq km (MoEF report to UNCCD, 2010) [3].
• Desertification means good land in dry areas slowly turns into desert-like land. Causes are overgrazing, cutting trees, poor farming and drought.
• How dunes spread desertification:
• Plants are removed → loose sand is exposed.
• Wind moves the sand → dunes re-activate and creep forward.

• Fields, canals (such as the Indira Gandhi Canal), roads and railway lines get buried.

• Wind erosion is one of the eight land degradation processes mapped in ISRO's Desertification and Land Degradation Atlas of India. The Atlas was prepared by the Space Applications Centre (SAC), Ahmedabad, an ISRO centre. It puts India's degraded land at 97.84 million hectares in 2018-19 and tracks change from 2003-05 to 2018-19 [4].

• CAZRI (Central Arid Zone Research Institute), Jodhpur:
• It began in 1952 as the Desert Afforestation Research Station, Jodhpur. Its first job was tree planting in the Thar [5].
• Its research covers desert tree planting, windbreaks and shelterbelts, and ways to fix drifting sand [5].

• It recommended roadside planting and shelterbelt planting to stop dunes from spreading [3].

• Shelterbelts are rows of trees and shrubs planted across the wind direction. Here is how they work:

• Trees slow the wind near the ground.
• Slower wind cannot lift sand by saltation.

• Sand stays in place, and the fields behind the trees are protected.

• Other ways to fix dunes:

• Fence the dune to keep grazing animals out.
• Put up micro-windbreaks (lines of brushwood) across the dune.

• Plant grasses and shrubs and hardy trees, for example Calligonum, Prosopis and Acacia.

• Results: about 4 lakh ha of sand dunes were stabilised, and the sand dune area fell by about 12% since 1980 (MoEF report to UNCCD, 2010) [3].

7. Coastal dunes as storm buffers

• Coastal dunes are formed from beach sand blown inland by sea breezes. They are often transverse dunes, because the beach (the sand source) lies at right angles to the onshore wind.
• Why they matter:
• They act as a natural wall against storm surges, cyclones and tsunamis.
• They store sand that can rebuild a beach after a storm.

• They protect freshwater layers underground from salt water.

• Example: casuarina shelterbelts on dunes along the Tamil Nadu and Odisha coasts.

Prelims Hooks

• Barchan: horns point downwind. Parabolic dune: arms point upwind because plants hold them. This is the classic "which direction" trap.
• Barchan is linked to the Jaisalmer area of the Thar in Rajasthan.
• Seif: a barchan with one wing, formed when wind changes direction. "Seif" = Arabic for "sword" [2].
• Longitudinal dune: parallel to the wind, little sand. Transverse dune: at right angles to the wind, with a long sand source.
• Star dune: many arms, winds from many directions. About 8.5% of world dunes [6].
• Saltation: short hops and bounces of sand grains. It is the main way sand moves, usually within about 1 cm of the ground [2].
• Aeolian deposits are well sorted. Glacial till is unsorted. This is an easy statement-type question.
• Loess: wind-blown silt. The thickest is 335 m, on the Loess Plateau, China [2].
• CAZRI is in Jodhpur, Rajasthan. It began as the Desert Afforestation Research Station in 1952 [5].
• Desertification and Land Degradation Atlas (ISRO-SAC, Ahmedabad): 97.84 Mha degraded in 2018-19 [4].

Mains Points

• Why dune shape depends on wind, sand and plants (GS-I):
• One steady wind and little sand → barchan or longitudinal dune.
• Changing wind → seif.
• Winds from many directions → star dune.
• Plant cover → parabolic dune.
• Plenty of sand → dunes merge into a sand sea.

• Each type tells you about the local climate.

• Shifting dunes are a hazard in western Rajasthan (GS-III, environment and disasters):

• Moving dunes bury farmland, the Indira Gandhi Canal, roads and railways.

• They speed up desertification. Wind erosion is one of the eight processes mapped in ISRO's Land Degradation Atlas (97.84 Mha degraded, 2018-19) [4].

• Fixing dunes works (GS-III, policy):

• Methods: CAZRI-designed shelterbelts, roadside planting, fencing and planting grasses on dunes.
• Result: about 4 lakh ha stabilised and dune area down by about 12% since 1980 [3][5].

• This supports India's goals under the UNCCD (UN Convention to Combat Desertification), which include land degradation neutrality.

• Coastal dunes are natural storm protection (GS-III, disaster management):

• Keeping dunes and their casuarina or grass cover on the cyclone-prone east coast lowers damage from storm surges.
• Mining dune sand or building on dunes removes this protection.