Revision:Deserts

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• Definition: An area with little/no vegetation
• Characterised by aridity (low rainfall and high evaporation usually <250mm p.a.)
• 1/3 of the world surface is desert

Location of Deserts

Agents of Earth Sculpture in Deserts

• The main agents are wind and water.

The Action of Wind in Deserts

• Wind is the most important agent of denudation and deposition in deserts.
• The actions of wind in deserts are known as Aeolian processes.

Wind Transport

Also known as Aeolian Transport
There are three different types of Aeolian Transport. The type varies depending on the size of the load that is being carried and, of course, the velocity of the wind.

Suspension

Approximately <0.1mm in size.

• The finest material is picked up and carried by the wind.
• An example of this process occurring is 'red rain' that often falls in the British Isles. This sand originates from the Saharan Desert and it is thought that 25-35 million tonnes of it is lost per year through suspension.

Saltation

Approximately 0.1mm to 1mm in size

• Most saltation takes place close to the surface. The wind can raise particles to a height of around 1-2m.
• It takes place as a series of leaps and jumps. The sand is raised into the air at a near vertical take off. It then falls to the ground at an angle (off the normal parallel to the ground) of approximately 8-12 degrees. The impact of the particle hitting the ground disturbs others and causes them to be raised by aerodynamic lift.
• The wind is specific on the size of particle that it will pick up, hence the small range of size.

Surface Creep

Approximately >1mm in size

• Coarser particles which are too heavy to be uplifted are rolled along the surface or pushed forward as other wind blown particles hit them.

Wind Erosion

Involves the following processes:

• Abrasion: Material is carried away by the wind and sand blasts on exposed rock surfaces cause the rocks to be broken down.
• Deflation: Wind removes loose material causing a Reg or Desert Pavement. Can also form a depression, for example the Qattara depression in Egypt.
• Attrition: The breaking down of the load by particles colliding with each other.

Features Produced by Wind Erosion

1. Rock Pedestals (aka Mushroom Rocks):
   • Mushroom Rocks are specific features of abrasion. When abrasion occurs, the sand blasting it inflicts is most effective within the first 1.5m from the ground. This explains the top heavy land form that is created.
   • The rock will be made of 'hard resistant' layers of rock and softer layers of less resistant rock. Abrasion occurs more in the areas of less resistant rock, and generally the exposed rock on the top will be of a higher resistance.
   • Example of where to find Mushroom Rocks - Mushroom State Park, Ellsworth County. (USA).

1. Zeugens:
   • Occur in areas with resistant and less resistant rocks occurring in layers perpendicular to the prevailing wind.
   • The less resistant rock is more eroded than the less resistant rock producing a ridge and furrow landscape.

1. Yardangs:
   • Yardangs are aerodynamically shaped ridges.
   • They are easily remembered by noticing that they often resemble the keels of an upturned boat.
   • They can be made of soft material such as aelianite as well as more resistant material such as limestone.
   • They can take the form of ridges that lie parallel to one another to the prevailing winds.
   • There are different scales of Yardang. Mega-yardangs can be several km in length and very high - several hundreds of meters. They are common in the Sahara made from more resistant rock. A specific example is in the central areas in the Tibetsti mountains.
   • Other scales of Yardang include Meso-yardangs and micro-yardangs.
   • An example of Yardangs made from less resistant rock is in the Iranian Desert.

1. Deflation Hollows:
   • These are depressions produced by the deflation in weak rocks.
   • When the water table is reached, a swamp/oasis may be formed in these depressions.
   • The largest is the Qattara depression in Egypt.
   • Flat floored depressions are called pans.

Features Produced by Wind Deposition

1. Dunes: deposits of sand by wind in deserts.

• 1. Barchans: Crescent shaped deposits made when sand being blown by wind encounters an obstruction such as a rock or a dead camel.
  2. Seifs: Ridge-shaped deposits of sand with steep sides lying parallel to the prevailing winds.

Wind Blown Deposits in Deserts

• Wind blows fine particles out of deserts each year.
• Some are deposited in the sea.
• Others accumulated on land to form fertile land known as loess.

Features Produced by Water in Deserts

• Rain in deserts is infrequent and unpredictable. When it does occur, it does so in torrents.
• Torrential run-off occurs in small narrow valleys called rills.
• These rills may enlarge to form gullies.
• The gullies may further enlarge to form deep steep-sided valleys with wide flat floors and rocky walls called wadis.
• Torrential rains carries large quantities of materials which may turn into mudflows.
• The deposited material is called alluvial flans.
• This may be deposited into wadises. If irrigated, they form very fertile alluvial plains.
• Some valleys form good drainage basins. They may form temporary lakes known as playas or sebkhas.
• These soon dry up forming the playas into salt-flats.
• Some basins may be rimmed by uplands. The basins may join together to form a continuous feature called a bajada.
• Between the playa and bajada is a gently sloping platform called a pediment.

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