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.
  • Off shore trade winds are often bathed in cold currents which produces a desiccating (dehydrating) effect, hence moisture is not easily condensed into precipitation.

Deserts Landforms

CanyonAlluvial FanMesa
ButteInselbergSalt Pan
OasisBlowoutsSand Dunes
YardangZeugenMushroom rocks
 

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