desert weathering processes (chemical and physical)

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

thermal-fracturing-in-hot-deserts

Image of cyclical thermal fracturing in hot deserts

  • 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

pressure-exfoliation

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

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