Deserts

Deserts


* 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
Location Reason
On the Western sides of continents Cold ocean currents exist off the Western sides of continents. Winds blowing over these currents lose their moisture over the sea and are dry by the time they get to sea.
In the interior of continents Winds blowing over continents are dry by the time they get to the interior of continents.
The leeward side of tall mountains Moisture carrying winds are blocked by the side (windward side) of tall mountains. The other side (leeward side) is consequently dry, and deserts may be formed there.
Depressions/basins in the interior of continents The surrounding highlands around depressions/basins cause a rainshadow effect leaving the depression dry.
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
* Involves the following processes:
1. Suspension: The movement of particles held up by river turbulence.
2. Saltation: The bouncing of particles on the bed of the river
3. Surface Creep: When particles carried through saltation dislodge and push others forward.
Wind Erosion
* Involves the following processes:
1. Abrasion/Corrasion: The wearing away of the sides and the bed of a river by the impact of the load.
2. Deflation: When wind blows away rock waste leaving a depression in deserts. An example is the Qattara depression in Egypt.
3. Atrrition: The breaking down of the load by particles hitting against each other.
Features Produced by Wind Erosion

1. Rock Pedestals:
* A tower-like mass of rock made up of alternating layers of resistant and less resistant rock.
* The less resistant layers are eroded away faster than the more resistant layers leaving a mass of rock with projections protruding from it.
2. 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.
3. Yardangs:
* Occur in areas with resistant and less resistant rocks occurring in layers parallel to the prevailing wind.
* The less resistant rock is more eroded than the less resistant rock producing a ridge and furrow landscape.
* An example is near Komombo in Egypt and near InSalah in Egypt
4. 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

* 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 od 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 continous feature called a bajada.
* Between the playa and bajada is a gently sloping platform called a pediment.