Weathering Processes (AQA A Level Geography)

Desert Mechanical Weathering

• 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

• 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

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

Desert Chemical Weathering

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, leading to granular and block disintegration

Granular disintegration 

• Found in rocks of different coloured minerals

• Darker minerals absorb more heat than lighter ones

• The rock will break down into grains 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