Role of Wind in Hot Deserts (AQA A Level Geography)

Aeolian erosion – from the Greek God of wind Aeolus

• Wind erodes arid and semi-arid surfaces and transports particles
• It does this in three ways:
  • Deflation: The carrying and removal of loose material by wind
  • Abrasion: When wind-blown sand abrades rock surfaces
  • Attrition: Loose materials collide with each other and become smaller

Deflation

• Wind removes fine sediments and lowers the desert floor
• This leaves coarser gravels, forming slag-type deposits of duricrust from the exposed bedrock
• This hardened desert floor, known as desert pavement, limits any future deflation and forms the features of a 'reg' desert
• Strong wind eddies can further hollow out the desert surface to form deflation hollows (blowouts)
• These deflation hollows are usually small but can extend several square kilometres 

Image showing the formation stages of a desert pavement through deflation

Image showing how deflation hollows form 

Abrasion

• Wind laden with sand carves and sculpts rock (sandblasting), usually within a metre of the desert floor
• The strength, duration and direction of the wind will dictate the rate and intensity of the abrasion
• The geology of the rock also factors into the rate of abrasion, with sandstone and limestone being the easiest to erode

Image showing how wind abrades rock in the desert

Attrition

• Attrition in deserts occurs during transportation and small pebbles are picked up by the wind
• These pebbles are too heavy to be carried far and attrition occurs within 0.5m of the desert floor
• As they move, the pebbles collide with each other and bits of rock are broken 
• These smaller pieces are then picked up by the wind and moved further along the desert floor colliding into each other and becoming smaller and smaller
• Eventually, these particles become part of the abrasion process 

Image showing how desert attrition occurs

• Aeolian (wind) transport is the carrying of sand particles away from an area in 3 ways:
  
  • Creep: When sanding grains (>0.25 mm) slide and roll across the surface
    • Caused by small differences in air pressure which creates a lift of no more than 2mm
  • Saltation: When the wind exceeds threshold velocity, a bouncing movement of particles begins close to the ground
    • Material transported by saltation may dislodge and push larger particles too heavy to be uplifted
    • This constant bombardment moves small stones and pebbles along the desert surface
    • Particles are too heavy to be held by the wind for long distances and only reach a height of 10 cm off the desert floor
  • Suspension- small dust particles (<0.15 mm) are carried by high velocity winds high into the atmosphere
    • The main cause of dust storms
    • These dust particles of fine silt and clay can be taken away from the desert region entirely: The dust of the Sahara has reached the UK

Image showing how material is transported along the desert surface

Deposition

• The deposition will only occur when the wind drops below the threshold velocity 
• This is dependent on the size of the particle, and the strength of the wind
• The deposited material can then be shaped by the wind
• Only 30% of all deserts are covered in wind-blown sands called 'erg' deserts
• Sand seas (great erg deserts) are some of the most distinct landscapes on Earth 
  • The Great Sand Dunes National Park, Colorado, USA contains different types of sand dunes and covers an area of over 600 km²