Desert Processes

After studying this section you should be able to understand:

• the processes of weathering in deserts
• that water and wind processes cause differing landforms
• that the origin of some landforms is difficult to ascertain

With regard to desert landforms, controversy has raged for many years over the part played by water and wind (aeolian effects) in forming desert landform features.

Processes

Desert surfaces

A range of desert surfaces exist:

• Ergs – sandy deserts/sand seas, common only in about 30% of deserts. Their distribution seems to be climate linked (i.e. less than 150 mm of rain).
• Pavements, gibber plains or reg – form as a result of wetting and drying. They are hard, rock covered areas.

Weathering

Desert weathering is a controversial topic.

Chemical weathering is limited by:

• the lack of water
• low rates of penetration into the rocks
• the amount of capillary action
• the alkaline nature of the chemicals taken into rocks creating no aggressive acids.

Other weathering processes include:

Salt weathering

Rocks in deserts often contain efflorescent salts which set up stresses in the rock and produce fractures. This process is seen in porous and poorly cohesive rocks.

Exfoliation

Granular disintegration and chemical rotting also have an effect.

Aeolian and fluvial processes (see below for more details)

Aeolian processes

Winds that blow across deserts often produce an effect similar to fluid in motion.

The lack of vegetation reduces surface roughness permitting smoother wind/land contact. The wind produces particulate sand, which is transported or deposited.

Wind erosion

• Abrasion – occurs when small particles are hurled by the wind against rock surfaces. This is only a minor erosional force and mostly occurs slightly above the ground. Ventifacts, rocks smoothed by wind abrasion, are common in deserts.
• Deflation – wind blows away rock waste and lowers the desert.
• Attrition – rock particles rub against each other and wear away.

Aeolian transport

• Saltation moves small particles in the direction of the wind in a series of short hops and skips. It normally lifts sand-size particles no more than onecentimeter above the ground, and proceeds at one-half to one-third the speed of the wind. A saltating grain may hit other grains that jump up to continue the saltation. The grain may also hit larger grains that are too heavy to hop,but that slowly creep forward as they are pushed by saltating grains. This is called surface creep. Velocity is of course an important variable, a critical velocity has to be reached before particles will move (see below).

Image

Deposition

Three processes have been recognised:

• sedimentation – settling occurs and there is no further effect on other sand particles
• accretion – occurs when sand grains come to a rest
• encroachment – the process of continued growth of sand accumulations.

Once sand has accumulated it traps more and more sand, ripples turn into dunes, and dunes into ‘draa’.

Fluvial processes

Three main types of river are found in desert areas:

• exogenous rivers – sources outside the desert
• endoreic rivers – these form near the desert and never show beyond it
• ephemeral rivers – these flow for only part of the year.

Desert lakes are generally ephemeral and are called playas. They vary in size from a few metres to several thousand km². They are very salty.

Drainage systems

The mountain areas of deserts and the lowland deserts have hugely different drainage systems.

KEY POINT - Mountainous areas - A high amount of scouring occurs producing very rocky beds and lots of debris/sediments in the upper areas of mountains. As slope decreases and sediments concentrate in lower areas so there is a rise in the deposition of alluvial fans at changes in slope.

Lowland areas - The nature of the surface over which water flows determines the drainage pattern. Few permanent rivers exist; where they do, they are shallow, sandy, straight and lack sinuosity. In flood conditions they are choked with sediment.