Weathering Processes (AQA A Level Geography)

• Hot arid and semi-arid environments generally experience slow rates of weathering
• This is largely due to the lack of water, which means that the chemical breakdown of rock is slow
• Rock breakdown is, therefore, mainly due to physical weathering, which can be quite rapid due to the absence of soil and plant cover exposing bedrock over large areas
• Two main forms of mechanical or physical weathering in hot deserts are:
  • Thermal fracture
  • Pressure exfoliation
• Thermal fracturing: Also called thermal exfoliation and onion skin weathering - is due to extreme, daily temperature variations
• During the day rocks absorb insolation and expand
• At night heat is released and the rock contracts
• This process continues (cyclical process 1 in the diagram below) until eventually, fractures form along the surface
• These fractured pieces expose the rock beneath and the process continues (cyclical process 2)
• Thermal expansion and contraction occur at different rates on different parts of the rock

Image of cyclical thermal fracturing in hot deserts

• Pressure exfoliation is also known as pressure release exfoliation
• This is where overburden is removed through weathering and erosion and the rock beneath is gradually exposed
• The removal of the weight of the overburden releases the pressure on the rock beneath and the outer layer splits/fractures apart
• Once the fractures develop, water enters and chemical weathering takes place, leading to the formation of new low-density minerals
• This enhances the fractures and encourages slabs of rock to detach from the rock surface

Image showing pressure release exfoliation over time

Freeze-thaw weathering

• Freeze-thaw weathering is possible where temperatures fluctuate above and below freezing and where there is sufficient  moisture
• Generally found in semi-arid mid-latitude environments

Crystal growth

• Crystal growth is the major cause of weathering, especially in porous, sedimentary rock such as sandstone
• Coastal, fog bound deserts are particularly prone to this type of weathering
• High temps draw saline groundwater to the surface and water evaporates, leaving behind salt crystals
• Salt crystals grow between pores and joints and lead to granular and block disintegration

Granular disintegration 

• Found with rocks of different coloured minerals
• Darker minerals absorb more heat than lighter ones
• Rock will break by grain to produce sand-sized material

Block disintegration

• Well jointed and bedded limestone break into blocks along the natural weaknesses

Hydration

• Minerals in rocks expand when they absorb water, putting the surrounding rocks under more stress
• This build up of stress causes the rocks to snap along their joints