Physical Weathering *AS edition*

Physical Weathering

Freeze thaw shattering: Occurs in rocks that contain crevices and joints (e.g. joints formed in granite as it cooled, bedding planes found in sedimentary rocks, and pore spaces in porous rocks), where there is limited vegetation cover and where temperature fluctuates around 0 ⁰C. In the daytime, when it is warmer water enters the joints, but during cold nights it freezes. The process of shattering of rock is due to frost cycles i.e. fluctuating above and below 0⁰C. The process occurs with climates with rapid frost cycles, rocks with joints and rainfall.  Frost leads to mechanical breakdown in two ways:

1.     As ice occupies 9% more volume than water, it exerts pressure within the joints.

2.     When water freezes within the rock it attracts small particles of water, creating increasingly large ice crystals.

In either case the process slowly widens the joints and, in time, causes process of rock to shatter (or disintegrate) from the main rock. Where the block disintegration occurs on steep slopes large angular rocks collect at the foot of the slope as scree; if the slopes are gentle large blockfields tend to develop.

Salt crystallisation: If water entering the pore spaces or joints in rocks is slightly saline then, as it evaporates, salt crystals are likely to form. As the crystals become larger, they exert stresses upon the rock, causing it to disintegrate. This process occurs in deserts and coastal areas (areas contains sodium sulphates, magnesium sulphates and calcium chloride) where capillary action draws water to the surface and where rock is sandstone. Individual grains of sand are broken off by granular disintegration. This process also occurs on the coast with a constant supply of salt. During the day water enters the rock and is heated, water evaporates leaving salt crystals. These are large in volume and put pressure on rocks by expansion and eventually will disintegrate.

Spheroidal Weathering: In jointed rock, the weathering and heating/cooling takes place along all joints so this temperature change produces rounded boulders.

Exfoliation: Occurs in hot arid and desert climates where diurnal ranges can range up to 50⁰C (below zero to 40). It also occurs in places of high altitudes in low latitudes. These rocks are usually heated via conduction. Because the outer layers of the rock warm up faster (and expand) and cool more rapidly (and contracts) than the inner ones, stresses were set up that would cause the outer thickness to peel off (or flake off) like layers – the process of exfoliation. Changes in temperature will also cause different minerals within a rock to expand and contract at different rates. It is also theorised that water is needed for the process to be stimulated or accelerated.

Pressure Release: Many rocks have developed under considerable pressure. The confining pressure increases the strength of the rocks. If these rocks are exposed to the atmosphere, then there will be a substantial release of pressure.  The release of pressure weakens the rock allowing other agents to enter it and other processes to develop. When cracks develop parallel to the surface, a process called sheeting causes the outer layers of the rock to peel away. This process is responsible for the formation of large round rocks called exfoliation domes.

Wetting and drying: Affects less resistant rocks such as clays. The clay is porous and has the ability to absorb. When these rocks are wet they expand and when dry is contracts. Over time they disintegrate the rocks. 

Biological weathering: When tree roots penetrate and widen weaknesses in the rock until blocks of rocks become separated.