World Geography: Desert Landforms & Erosion Mechanism

Deserts

• About 1/5th of the world’s land is made up of deserts.
• Deserts which are absolutely barren, where nothing grows are known as true deserts.
• Insufficient & irregular rainfall, high temperature & rapid rate of evaporation are the main causes of desert’s aridity.
• Almost all the deserts are confined within 15* – 30* parallels to N – S of equator known as trade wind desert or tropical deserts.
• They lie in the trade wind belt on the western parts of the continents.
• Offshore trade winds are often bathed in cold currents which produces a desiccating (dehydrating) effect, hence moisture is not easily condensed into precipitation.

Deserts Landforms

Canyon	Alluvial Fan	Mesa
Butte	Inselberg	Salt Pan
Oasis	Blowouts	Sand Dunes
Yardang	Zeugen	Mushroom rocks

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Types of deserts

Hamada / Rocky Desert

• Consist of large stretches of bare rocks, swept clear of sand & dust by wind.
• Exposed rocks are thoroughly smoothened, polished & highly sterile.

Reg / Stony Desert

• Composed of extensive sheets of angular pebbles & gravels which the wind is not able to blow off.
• Stony deserts are more accessible than sandy deserts & large herds of camels kept there.

Erg / Sandy Desert

• Also known as sea of sand
• Winds deposit vast stretches of undulating sand dunes in the direction of winds.

Badlands

• Consists of gully & ravines formed on hill slopes & rock surfaces by the extent of water action
• Not fit for agriculture & survival
• Finally leads to the abandonment of entire region by its inhabitants

Mountain Deserts

• Deserts which are found on the highlands such as on plateaus & mountain ranges, where erosion has dissected the desert highland into rough chaotic peaks & uneven ranges.
• Their steep slopes consist of Wadis (dry valleys) with sharp & irregular edges carved due to action of frost.

Mechanism of Desert / Arid Erosion

Weathering

• Most potent factor in reducing rocks to sand in arid regions.
• Even though the amount of rain that falls in a desert is small, but manage to penetrate into rocks & sets up chemical reactions in various minerals it contains.
• Intense heating during the day & rapid cooling during the night by radiations, set up stresses in already weakened rocks, hence they eventually crack.
• When water gets into cracks of a rock, it freezes at night as the temperature drops below the freezing point & expands by 10 % of its volume.
• Successive freezing will prise of fragments of rocks which get accumulated as screes.
• As heat penetrates rock, its outer surface gets heated & expands, leaving its inner surface comparatively cool.
• Hence, outer surface prise itself from the inner surface & peels off in successive thin layers, known as exfoliation.

Action of Wind

• Efficient in arid regions as little vegetation or moisture to bind the loose surface materials
• Carried out in following ways –

Deflation 

• Involves lifting & blowing away of loose materials from the ground
• Blowing capacity depending largely on the size of the material lifted from the surface
• Finer dust & sands may be removed miles away from their place of origin & may get deposited even outside the desert margins.
• Deflation results in the lowering of the land surface to form large depressions called Deflation hollows

Abrasion 

• Sand blasting of rock surfaces by wind when they hurl sand particles against them
• This results in rock surfaces being scratched, polished & worn away
• Abrasion is most effective near the base of the rocks, where the amount of material the wind is able to carry is greatest.
• This explains why telegraphic poles in the deserts are protected by covering of metal for a foot or two above the ground.

Attrition 

• When wind borne particles roll against one another in collision, they wear each other away
• Hence their sizes are greatly reduced & grains are rounded into millet seed sand