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
Image of cyclical thermal fracturing in hot deserts
Exam Tip
Always remember to tell the examiner that weathering is cyclical and takes time. It doesn't happen in an instant!
- 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