Wind and water in deserts: erosional landforms

Detail

1. Why erosion is strong in hot deserts

• Bare ground heats fast. Hot deserts have little plant cover, so nothing shades or holds the ground.
• In the day, the bare floor gets very hot very quickly.
• The air just above it becomes hot and unstable. This causes turbulence (mixed-up, uneven air movement), eddies (small whirling currents of air) and strong winds.

• These winds pick up loose material and wear down rock.

• Aeolian processes are all the work done by wind: erosion, carrying material, and dropping it. ("Aeolian" comes from Aeolus, the Greek god of wind.)

• Wind is an important agent of erosion in deserts. Running water also does a lot of the work (see Section 3).

2. The three ways wind erodes

Process	What it means	What it does	Landforms it makes
Deflation	Wind lifts and removes loose dust and fine particles	Lowers the ground and leaves coarse stones behind	Deflation hollows, blowouts, desert pavement
Abrasion	Wind-blown sand and silt act as cutting tools that grind rock	Strongest within 1–2 m of the ground, where sand grains are carried	Mushroom rocks, ventifacts, yardangs
Impact	Moving sand hits rock with force, like sand-blasting	Breaks off and chips rock surfaces	Pitted and grooved rock faces

• Deflation: wind removes loose, fine-grained particles through the swirling (eddy) action of turbulent air [4].
• Abrasion works like sandpaper. It is heaviest near the ground because sand grains are heavy and cannot rise high. So rocks are worn most at their base.

3. Running water in deserts: the sheet flood

• Rain in deserts is rare but very heavy.
• Rare, torrential rain falls on bare ground with no plants or soil to soak it up.
• The water spreads out as a sheet flood: a broad, shallow sheet of water flowing over the land, not kept inside one channel.

• The sheet flood strips away weathered rock debris over large areas.

• Desert stream channels are broad, shallow and short-lived. They carry water only for a few hours or days after rain.

• Key point for exams: much of the erosion in deserts is done by running water (sheet floods), not only by wind. Wind mainly moves fine material. Water does most of the general wearing down.

4. Types of desert surface: erg, reg and hamada

All three are found in the Sahara (North Africa).

Surface	Meaning	How it forms	Example
Erg	"Sand sea": large area covered with sand dunes	Sand blown away from other areas collects here	Great Eastern and Great Western Ergs (Algeria)
Reg	Gravel plain, stony desert	Wind removes fine particles; gravel and pebbles stay	Large parts of the Sahara; called serir in Libya
Hamada	Bare rocky plateau	Wind sweeps away all loose material, so bare bedrock is exposed	Hamada du Draa (Morocco–Algeria)

• Most desert land is not covered by sand. Sand sheets make up only about 40% of aeolian depositional surfaces (surfaces built by wind-dropped material) [4].
• Almost half of Earth's desert surfaces are stony deflation zones [4].

5. Pediments, parallel retreat, inselbergs and pediplains

Pediment

• A pediment is a gently sloping rocky floor at the foot of a desert mountain. It may be bare rock or have a thin cover of debris.
• How it forms:
• Lateral erosion (sideways cutting) by streams wears away the base of the mountain.
• Sheet floods sweep the debris away.
• Result: a smooth, gently sloping rock surface forms in front of the mountain.

Parallel retreat of slopes

• Parallel retreat of slopes (also called backwasting): the slope moves backwards but keeps the same steepness.
• First, a steep wash slope and a free face form on the mountain front. (Free face: a steep, bare rock face.)
• Erosion wears this face back. It does not wear it down, and its angle does not change.

• As the front moves back, the pediment in front of it becomes wider.

• So the pediment grows at the mountain's expense.

• Contrast with Davis: in the cycle of erosion proposed by W.M. Davis (youth → maturity → old age), slopes are worn downward and become gentler over time, ending in a low plain called a peneplain. In L.C. King's pediplanation model, slopes retreat backward at the same angle, ending in a pediplain.

Inselberg

• An inselberg (German for "island mountain") is the steep-sided leftover hill that remains after the mountain has retreated almost fully. It rises sharply from a flat plain.
• World example — Uluru (Ayers Rock), Northern Territory, central Australia:
• Rises over 300 m above the flat sandplain around it [3].
• Made of hard arkosic sandstone (sandstone rich in feldspar). Its colour changes as the sun's height changes [3].

• Part of Uluru-Kata Tjuta National Park, a UNESCO World Heritage Site: listed for natural values in 1987, and again as a Cultural Landscape in 1994 [3].

• Inselbergs are also common in the dry parts of Africa, such as Namibia and the Sahel.

Pediplain

• A pediplain is a low, flat plain with no major features. It forms when many pediments grow and join together.
• It is the final stage of King's desert erosion cycle. In Davis's humid-region cycle, the final stage is the peneplain.

Feature	Peneplain (Davis)	Pediplain (King)
Climate	Humid	Arid/semi-arid
Slope change	Wears down and becomes gentler	Moves back at the same angle
Leftover hills	Monadnocks	Inselbergs

6. Playas and alkali flats

• Playa forms in basins with inland drainage, where rivers flow into a closed hollow and never reach the sea.
• Streams from the mountains around the basin carry sediment towards its centre.
• Over time, this sediment builds a nearly level plain at the centre: the playa.
• After rain, a shallow lake forms on the playa.

• The heat quickly evaporates the water. Dissolved salts are left behind.

• A playa covered with a salt crust is an alkali flat.

• Indian examples:
• Sambhar Lake, Rajasthan: India's largest inland saline lake. It is a large saline lake fed by four streams and covers about 24,000 ha [2]. It was designated a Ramsar site on 23 March 1990 [2]. It is a major centre of salt production. It hosts flamingos in large numbers, second only to the Rann of Kachchh in India [2]. Its main threats are siltation, soil salinisation and sewage discharge [2].
• Dhands of the Thar: small, shallow, salty lakes between dunes in western Rajasthan. Examples are Didwana, Pachpadra and Lunkaransar.

• Rann of Kachchh, Gujarat: wide salt flats. They flood in the monsoon and dry into a white salt crust.

• World examples: playas of the Mojave Desert, USA; the Chotts (salt lakes) of Tunisia and Algeria.

7. Deflation hollows and caves

• Deflation hollow: a shallow depression scooped out by persistent winds blowing from one direction.
• Wind keeps removing loose fine material from one spot.
• The ground there sinks lower and forms a hollow (also called a blowout).

• Blowouts are usually small, but they can be several kilometres wide [4].

• When deflation and abrasion attack the side of a rock outcrop, blowouts can deepen into caves.

8. Residual rocks: mushroom, table and pedestal rocks

• Mushroom rock has a slender stalk and a broad, rounded cap.
• Sand-laden wind hits the base of a rock hardest, because abrasion is strongest near the ground.
• The lower part wears away faster and becomes thin.

• The upper part is barely touched, so it stays wide and forms the cap.

• Table rock: a rock with a flat top, left standing after the softer rock around it was worn away.

• Pedestal rock: a rock that rests on a narrower base, like a statue on a stand.
• All three are resistant remnants: harder rock that survives while softer rock around it is removed.
• Indian setting: such rocks are seen in the Thar Desert of western Rajasthan.

9. Yardangs, ventifacts and desert pavement

• Yardang: a streamlined rock ridge that lies parallel to the main wind direction.
• Bands of hard and soft rock run along the wind's path.
• Wind-blown sand scours out the softer rock. The harder bands stand up as ridges, with furrows between them.
• The windward end is worn by abrasion. The middle and the far end are shaped by deflation and by air flowing back around the ridge [5].

• Yardangs can be tens of metres high and kilometres long. In the Lut Desert, Iran, they reach almost 100 m of relief [4].

• Ventifact: a stone cut, faceted and sometimes polished by wind abrasion (sandblasting) [4]. The large chalk formations of the White Desert, Egypt were carved by wind over thousands of years [6].

• Desert pavement: a surface of closely packed stones left behind after wind and water remove the fine particles. It acts as a protective layer that slows further deflation [4].

Prelims Hooks

• Process–landform pairs: deflation → deflation hollow, blowout, desert pavement; abrasion → mushroom rock, ventifact, yardang; sheet flood + lateral erosion → pediment.
• Erg = sand sea; reg = gravel plain; hamada = bare rocky plateau. All three are found in the Sahara.
• Uluru (central Australia) is an inselberg of arkosic sandstone, rising over 300 m. It became a UNESCO World Heritage Site in 1987 (Cultural Landscape in 1994) [3].
• Pediplain is the end stage of the arid cycle (King). Peneplain is the end stage of the humid cycle (Davis). Inselberg goes with pediplain; monadnock goes with peneplain.
• Trap: in parallel retreat, the slope angle does not change. In Davis's cycle, slopes become gentler.
• Trap: much desert erosion is done by running water (sheet floods), not wind alone.
• Yardangs run parallel to the wind direction, not across it.
• Sambhar Lake (Rajasthan): India's largest inland saline lake, about 24,000 ha, Ramsar site since 1990 [2]. A salt-covered playa is called an alkali flat.
• Wind abrasion is strongest within 1–2 m of the ground. That is why mushroom rocks have thin stalks.

Mains Points

• Wind and water work together in deserts (GS-I). Bare floors heat up, causing turbulence and deflation and abrasion. Rare torrential rain causes sheet floods that strip debris. Together they make pediments, inselbergs and pediplains. Use the Sahara, central Australia and the Thar as examples.
• Davis vs King: two models of how landscapes change (GS-I). Davis says slopes are worn down to a peneplain in humid climates. King says slopes retreat backward, keeping their angle, to form a pediplain in dry climates. Inselbergs such as Uluru support King's model.
• Playa lakes as resources and fragile wetlands (GS-III). Sambhar supports salt production and flamingo habitat. It is also a Ramsar site under pressure from siltation, salinisation and sewage [2]. Salt mining and wildlife conservation need to be balanced.
• Deflation and land degradation (GS-III). When plant cover or desert pavement is disturbed by overgrazing, ploughing or vehicles, deflation speeds up. This causes dust storms and spreads desertification in western Rajasthan and the Sahel. Protective measures include shelterbelts and dune stabilisation.