Arid Processes

Arid Processes

1. Weathering Processes in hot deserts

   • Chemical and biological weathering
     • Because of the absence of water, and a lack of rotting organic matter, chemical and biological weathering are not very significant in hot deserts. Nevertheless, the presence of some moisture and solutes can contribute towards the break-up of a rocky surface prior to erosion.
   • Mechanical Weathering
     • (or physical) weathering involves the gradual disintegration of rocks without any chemical change taking place. In a desert envoirnoment, insolation weathering is probably the most important type of mechanical weathering. it involves the expansion and contraction of a rock's surface layer, due to intensetemprature fluctuations. Eventually, the outer 'skin' peels off in a process called exfoliation.
       • The rock expands when it heats up during the day, and contracts when cool at night. Deserts experience huge temperature fluctuations.
         • Scientiscts believe that the presence of some moisture helps to promote this weathering process. The moisture is most likely to involve dew, which forms at night when the temperature plummets and the air cools enough to become saturated.
   • Freeze-thaw weathering
     • Water - eg from rainfall or melting snow and ice - becomes trapped in a crack or joint in the rock. If the air temperature drops below freezing, the water will freeze and expand by 9-10 per cent putting pressure on the rock. The ice will melt when the temperature rises above freezing. If this process happens repeatedly, the rock will weaken and eventually shatter into angular fragments.
     • not very common in deserts
   • Salt crystalisation
     • When it rains, the water dissolves salts held in the soils. The high evaporation rate in the desert then draws the rainwater back to the surface by capillary action, and the salt crystals are deposited on the ground surface.
       • Over time, the salt crystals can expand -causing stresses in rocks. Salt weathering is a major problem in some developed desert environments, where it damages concrete and tarmac runaways and roads.

2. Wind processes in Deserts.

   • Wind erosion
     • Abrasion - prevailing winds pick up small particles of sand and 'sand blasting' the lowest part of exposed rock, or less resistant rock.
     • Deflation - erosion of the desert floor by the wind, which often involves the removal of light sand, leaving behind a pebbly surface. Over time, the surface is lowered and often starts to look like a cobbled pavement, because only the heavier stones are left behind.
       • strong eddies (localised winds) can hollow out the desert surface to produce a deflation hollow. Moistre that collects can increase the rate at which a deflation hollow forms, by helping to break up the surface.
   • Wind transportation and deposition
     • transports sediment in a similar way to water. Larger rocks are rolled along the desert floor by the process of traction, or surface creep. Slightly smaller particles are 'bounced' along by process of saltation. This is very common in deserts - it usually happens within 2-3cm of the ground surface, where particles constantly knock into one another. Fine dust is picked up and carried in suspension. It is then deposited.
       • The particles carried by wind become very rounded. This is because the attritional impacts are stronger in the air.
       • Wind sorts particles much more effectively than water. This means sand particles in a particular place in the desert are all likely to be the same size.

3. Water processes in deserts

   • Exogenous rivers have their sources in mountains outside desert regions. They contain enough water to flow continually, despite the high evaporation rate when they reach the desert. E.g The colorado, the Nile.
   • Endoreic rivers flow into deserts and then terminate - usually in a lake or inland Sea. e/g the River Jordan.
   • Ephemeral rivers flow intermittenly in desert regions. They might flow after storm events, or they might be fed by snowmelt from adjacent mountains in the spring. Powerful forces of erosion.
   • Rainfall tend to involve large amounts of water in a very short period of time, which can cause flash floods. overland flow can carry out significant erosion.