Desert

• A desert is a place with little precipitation and hostile living conditions for plants and animals.
• Lack of vegetation exposes the ground to denudation processes.
• About one-third of the world’s land surface is arid or semi-arid, including the Polar Regions.
• Deserts can be classified based on precipitation, temperature, causes of desertification, or geographical location.
• About 1/5 of the world’s land is made up of deserts.
• True deserts are barren and have no vegetation.
• Insufficient and irregular rainfall, high temperature, and rapid evaporation cause desert aridity.
• Most deserts lie within 15-30 degrees latitude of the equator known as trade wind or tropical deserts.
• Trade wind deserts are found on the western parts of continents.
• Moisture is not easily condensed into precipitation in offshore trade winds, which are often bathed in cold currents, resulting in a desiccating effect.
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Types of desert

• Hamada/Rocky Desert: large stretches of bare rocks smoothed and polished by wind, highly sterile.
• Reg/Stony Desert: extensive sheets of angular pebbles and gravels, accessible for large herds of camels.
• Erg/Sandy Desert: vast stretches of undulating sand dunes deposited by winds in one direction, also known as the sea of sand.
• Badlands: gully and ravines formed on hill slopes and rock surfaces by water action, not suitable for agriculture or survival, leading to abandonment of the region.
• Mountain Deserts: found on highlands, erosion has dissected the desert highland into rough chaotic peaks and uneven ranges, with steep slopes consisting of Wadis (dry valleys) with sharp and irregular edges carved by frost.
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Mechanism of Desert/Arid Erosion

• Weathering:
  • Weathering is a potent factor in reducing rocks to sand in arid regions.
  • Rainwater, despite being small in amount, penetrates into rocks and sets up chemical reactions in various minerals.
  • Intense heating during the day and rapid cooling during the night by radiations set up stresses in already weakened rocks, causing cracks.
  • Water freezes in cracks of a rock at night as the temperature drops below freezing point and expands by 10% of its volume.
  • Successive freezing prises off fragments of rocks that accumulate as screes.
  • Heat penetrates rock and its outer surface heats up and expands, leaving its inner surface comparatively cool.
  • The outer surface prises itself from the inner surface and peels off.
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• Action of Wind:
  • Efficient in arid regions as there is little vegetation or moisture to bind loose surface materials.
  • Carried out in three ways:
    • Deflation: involves lifting and blowing away of loose materials from the ground, and results in the lowering of the land surface to form large depressions called Deflation hollows.
    • Abrasion: sandblasting of rock surfaces by the wind when they hurl sand particles against them, and results in rock surfaces being scratched, polished, and worn away.
    • Attrition: when wind-borne particles roll against each other in the collision, they wear each other away, and the grains become greatly reduced in size and rounded into millet seed sand.
  • Blowing capacity depends largely on the size of the material lifted from the surface.
  • Finer dust and sands may be removed miles away from their place of origin and may get deposited even outside the desert margins.
  • Abrasion is most effective near the base of the rocks where the amount of material the wind is able to carry is greatest.
  • Telegraph poles in the deserts are protected by a covering of metal for a foot or two above the ground to prevent abrasion.
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Desert Landforms by Wind Erosion

• Rock pedestals/Mushroom rocks:
  • These are formed by the sandblasting effect of winds against any projecting rock masses
  • This process wears down the softer layer leading to the formation of irregular edges on alternate bands of softer & harder rocks
  • Grooves & hollows cut in the rock surfaces carve them into grotesque-looking pillars known as rock pedestals
  • Such rock pillars will be further eroded near their bases where friction is greatest
  • This process of undercutting produces rocks of mushroom shape called mushroom rocks
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• Messa and Butte:
  • Mesa means “table” in Spanish.
  • It is a flat, table-like landmass with a resistant horizontal top layer and steep sides.
  • The hard surface layer resists denudation by wind and water, protecting underlying rock layers from erosion.
  • Mesas may form in canyon regions (e.g. Arizona) or on fault blocks (e.g. Table Mountain, Cape Town).
  • Continued denudation over time may reduce mesas in the area, forming isolated flat-topped hills called buttes.
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• Zeugen:
  • Tabular masses with a layer of soft rocks beneath a resistant surface layer
  • Difference in erosional effect of wind on soft and resistant rock surfaces leads to ridge and furrow landscape
  • Mechanical weathering initiates formation by opening joints of surface rocks
  • Wind abrasion further eats into underlying softer layer, developing deep furrows
  • Hard rock stands above furrows as ridges or Zeugen, 10 to 100 feet above sunken furrows
  • Continuous wind abrasion gradually lowers Zeugen and widens furrow
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• Yardangs:
  • Similar to Zeugen but with vertical bands of hard and soft rocks
  • Rocks aligned in direction of prevailing winds
  • Wind abrasion excavates bands of softer rocks into long, narrow corridors
  • Steep-sided overhanging ridges of hard rock between corridors called Yardangs.
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• Isenberg (Island Mountain)
  • Isolated residual hills rising abruptly from the ground level
  • Characterized by steep slopes and rounded tops
  • Often composed of granite or gneiss
  • Probably relics of an original plateau which has been almost entirely eroded away
• Ventifacts & Dreikanter
  • Pebbles faceted by sand-blasting and polished by wind abrasion
  • Shaped like Brazil nuts with characteristic flat facets and sharp edges
  • Dreikanter are ventifacts with three wind-faceted surfaces
  • Form the desert pavement, a smooth, mosaic-like region covered by numerous rock fragments and pebbles
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• Deflation Hollows
  • Small depressions formed by wind blowing away unconsolidated materials
  • Minor faulting can also initiate depressions
  • Eddying action of oncoming winds wears off weaker rocks until the water table is reached
  • Water seeps out forming oasis or swamps in the deflation hollows or depressions
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  • Great Dust Bowl
  • Large areas in the western USA stripped of natural vegetation for farming
  • Strong winds moved materials as dust storms, laying waste crops and creating the Great Dust Bowl

Deserts Landforms by wind deposition

• Materials eroded & transported by winds eventually settle down.
• Finest dust can travel for long distances in the air, sometimes as long as 2300 miles.
• Dust from the Sahara desert is sometimes blown across the Mediterranean to fall as blood rains in Italy or on the glaciers of Switzerland.
• Dust that settles in the Hwang Ho basin from the Gobi desert has been accumulated over past centuries to a depth of several hundred feet.
• Coarser sands will be too heavy to be blown out of desert limits and remain as dunes or other depositional landforms within deserts themselves.
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• Dunes:
  • Dunes are hills of sand formed by accumulation of sand and shaped by wind movement
  • Dunes may be active/live or inactive/fixed
  • Erg desert has the most representation of dunes
  • Two most common types of dunes: Barchan & Seifs
  • Barchan:
    • Barchan dunes are crescent or moon-shaped, and advance steadily before winds from a particular direction
    • Barchans occur transversely to wind, with convex windward and concave leeward sides
    • Crest of the sand dune moves forward as more sand accumulates
    • Migration of barchans may be a threat to desert life, long-rooted trees and grasses planted to prevent devastation
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  • Seif:
    • Seif dunes are long, narrow ridges of sand, often over a hundred miles long
    • Seifs lie parallel to the direction of prevailing winds and rise and fall in alternate peaks and saddles
    • Prevailing winds blow straight along the corridor between the lines of dunes, dropping sand to form the dunes
    • Crosswinds tend to increase height and width of dunes
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• Loess:
  • Fine dust blown beyond desert deposits as loess on neighboring lands
  • Loess is a yellow, friable material, fine loam rich in lime, porous, and fertile
  • Loess may lead to badland topography and deep valleys cut by streams
  • Roads constructed through a loess region sink and their walls rise steeply
  • The largest deposit of loess is found in northwest China in the loess plateau of the Hwang-Ho basin
  • Loess in China is called ‘Hwangtu’
  • The term loess comes from a village in Alsace, France, where such deposits occurred
  • Similar deposits called Limon occur in some parts of Germany, France, and Belgium, blown from material deposited at the edge of ice sheets during the Ice Ages
  • Loess in parts of the Mid-West, USA, is derived from the ice sheets which covered northern North America and is termed adobe.
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Landform of water actions in desert

• Deserts have irregular showers and thunderstorms that can bring several inches of rain within a few hours
• Flash floods and sudden raging torrents can cause devastating effects, drowning people and flooding mud-baked houses
• Deserts have little vegetation to protect surface soil, leading to large quantities of rock wastes being transported in flash floods
• Flash floods cut deep gullies and ravines and form bad-land topography
• Masses of debris deposited at the foot of hills or valleys form alluvial cones, fans or ‘dry deltas’
• Alluvial deposits subjected to rapid evaporation and downward percolation of water can dry up quickly, leaving mounds of debris
• Deserts have many larger dry channels or valleys

• Temporary lakes (Playas):
  • Formed in arid or semi-arid areas by intermittent streams flowing into depressions
  • Contain high percentage of salts due to high evaporation and lower precipitation
  • The playa plain covered up by salts is called alkali flats
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• Bajada and Pediment:
  • Bajada is a depositional feature made up of alluvial material lay down by intermittent streams
  • Pediment is an erosional plain formed at the base of the surrounding mountain scarps
  • Bajada is formed by the coalescence of alluvial fans
  • Pediments are gently inclined rocky floors close to the mountains at their foot with or without a thin cover of debris
  • Pediments form through the erosion of mountain front through a combination of lateral erosion by streams and sheet flooding
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• Alluvium fans:
  • Cone-shaped heaps of sand that are deposited on the exit of a wadi or valley
  • Wadi is a narrow dry valley with ephemeral water flow
  • Alluvium is deposited as the wadi terminates into an open space, forming a fan shape
• Canyons/Gorges:
  • Deep narrow valleys that are excavated and eroded vertically by rivers that flow along deserts
  • The Grand Canyon in Arizona, USA was formed by vertical erosion of sedimentary strata by the Colorado River for millions of years.
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