10.2 Processes producing desert landforms

Winds

Deserts are high-wind-energy environments. This is partly due to the lack of vegetation and so less friction between the wind and the surface.

Climate change in deserts

During the Pleistocene Ice Age high latitudes contained more ice (30% of the world surface) than today (10% of the world surface), whereas low-latitude deserts experienced prolonged periods of increased rainfall, known as a pluvials. Some deserts, however, received less rainfall – these dry phases are known as interpluvials. There is widespread evidence for pluvial periods in deserts: l shorelines marking higher lake levels around dry, salty basins l fossil soils associated with more humid climates, including horizons containing laterite l spring deposits of lime, called tufa, indicating higher groundwater levels l river systems now blocked by sand dunes l animal and plant remains in areas that are now too arid to support such species l evidence of human habitation, including cave paintings

The evidence for interpluvials includes sand dunes systems in areas that are now too wet for sand movement to occur. Dunes cannot develop to any great degree in continental interiors unless the vegetation cover is sparse enough to allow sand movement. Satellite imagery and aerial photographs have shown that some areas of forest and savanna, with 750–1500mm of rain, contain areas of ancient degraded dunes.

4 Explain the term rainfall variability.

Answer on p.219

Tested

Weathering

Weathering in deserts is superficial and highly selective. It is greatest in shady sites and in areas within reach of soil moisture; water is important for mechanical weathering, especially exfoliation. Chemical weathering is important and is enhanced in areas that experience dew or coastal fog.

Salt crystallisation causes the decomposition of rock by solutions of salt. When water evaporates, salt crystals are left behind. As the temperature rises, the salts expand and exert pressure on rock. Sodium sulfate (Na 2 SO 4 ) and sodium carbonate (Na 2 CO 3 ) expand by about 300%. This creates pressure on joints, forcing them to crack.

Thermal fracture refers to the break up of rock as a result of repeated changes in temperature over a prolonged period of time. Disintegration or insolation weathering is found in hot desert areas where there is a large diurnal temperature range. In many desert areas daytime temperatures exceed 40°C whereas night-time ones are little above freezing. The rocks heat up by day and contract by night. As rock is a poor conductor of heat, stresses occur only in the outer layers. This causes peeling or exfoliation to occur. In some instances rocks may be split in two. Block disintegration is most likely to result from repeated heating and cooling.

Revised

Expert tip

Desert processes do not operate in isolation – weathering, erosion, mass movement and deposition are likely to be affecting the same areas

A more localised effect is granular disintegration. This occurs due to certain grains being more prone to expansion and contraction than others – this exerts great pressure on the grains surrounding them and forces them to break off.

Wind action

Movement of sediment is induced by drag and lift forces, but reduced by particle size and friction. Drag results from differences in pressure on the windward and leeward sides of grains in an airflow.

There are two types of wind erosion: l Deflation is the progressive removal of small material, leaving behind larger materials. This forms a stony desert or reg. In some cases, deflation may remove sand to form a deflation hollow. l Abrasion is the erosion carried out by wind-borne particles. They act like sandpaper, smoothing surfaces and exploiting weaker rocks. Most abrasion occurs within a metre of the surface, as this is where the largest, heaviest, most erosive particles are carried. Examples of erosional features carved out by abrasion include yardangs, zeugens and ventifacts.

Transport

Sand-sized particles (0.15–0.25mm) are moved by three processes: l suspension –particles light enough to be carried by strong winds l saltation – a rolling sand particle gains sufficient velocity for it to leave the sand surface in one or more ‘jumps’ l surface creep – larger grains are dislodged by saltating grains

Revised

Deflation is lowering of a surface by wind blowing away loose, unconsolidated material.

Now test yourself

5 Outline the ways in which wind can erode desert surfaces.

Answer on p.219

Tested

The work of water

There are a number of sources of water in deserts: l rainfall may be low and irregular, but it does occur, mostly as low-intensity events, although there are occasional flash floods l deflation may expose the water table to produce an oasis l rivers flow through deserts; these can be classified as exotic (exogenous), endoreic and ephemeral: –Exotic or exogenous rivers are those that have their source in another wetter environment and then flow through the desert. –Endoreic rivers are those that drain into an inland lake or sea. –Ephemeral rivers are those that flow seasonally or after storms. Often they are characterised by high discharges and high sediment levels. Even on slopes as gentle as 2°, overland flow can generate considerable discharges. This is a result of an impermeable surface (in places), limited interception (lack of vegetation) and rain splash erosion displacing fine particles, which in turn seal off the surface and make it impermeable.

Revised

Features in the arid landscape Butte

An isolated peak, often the remnant of a former mesa or plateau

Plateaux

Large, flat areas at high altitude

Mesa

A relatively large area of plateau that has become isolated from the main plateau either by erosion or through slope retreat

Canyons

Very deep gorges, usually dry. If there is a river, it is usually exotic. The Fish River Canyon in Namibia is an excellent example

Wadis

Dry gullies that have been eroded by flash floods. Heavy rainstorms (10–250 mm) create rushing torrents on steep slopes

Pediments

Shallow slopes formed at the base of a cliff or steep hill

Wind

Inselbergs

Also known as kopjes or bomhardts, are isolated domes of resistant rock. They can be formed by parallel retreat of slopes (pediplanation) or by exhumation where the basal surface is exposed a result of surface stripping

Yardangs

Salt pans (playas)

Sites of former or occasional lakes

Zeugens

Alluvial fans

Cones of sediment that are deposited by rivers owing to an abrupt loss of energy as they leave their mountain channels and enter a plain

Oasis

Formed when the waterbearing rocks are exposed at the surface. This is commonly done by deflation

Figure 10.4 Desert landforms

Canyons are very deep gorges, and are usually dry. If there is a river it is usually exotic. The Fish River Canyon in Namibia is an excellent example. Wadis are dry gullies that have been eroded by flash floods. Heavy rainstorms (100–250mm) create rushing torrents on steep slopes. Inselbergs are isolated domes of resistant rock. They can be formed by parallel retreat of slopes (pediplanation) or by exhumation, where the basal surface is exposed as a result of surface stripping. Oases are formed when water-bearing rocks are exposed at the surface. This is commonly done by deflation. Sand dunes are formed where wind speed is high, constant and there is a large supply of sand. Pediments are shallow slopes formed at the base of a cliff or steep hill. Plateaus are large, flat areas at high altitude. Buttes are isolated peaks, often the remnants of a former mesas or plateaus. Mesas are relatively large areas of plateau that have become isolated from the main plateau either by erosion or through slope retreat. Alluvial fans are cones of sediment that are deposited by rivers because of an abrupt loss of energy as they leave their mountain channels and enter a plain. Yardangs and zeugens are wind-eroded landforms where the softer rock strata are removed, leaving the more resistant layers to form either mushroom-shaped features (zeugens) where strata are horizontal, or long ridges (yardangs) where

Hard rock Soft rock

Barchan or crescent dunes

Sand dunes are formed where wind speed is high and constant and where there is a large supply of sand