Weathering

Physical and Chemical Weathering

Physical Weathering is the disintegration of rock into smaller particles by mechanical processes but without any change in the chemical composition of that rock. It is more likely to occur in areas devoid of vegetation, such as deserts, high mountains and arctic regions. Physical weathering usually produces sands.

Chemical Weathering is the decomposition of the rock resulting from a chemical change. It produces changed substances and solubles, and usually forms clays. Chemical weathering is more likely to take place in warmer, more moist climates where there is an associated vegetation cover.

It should be appreciated that although in any given area either of these two processes may be locally dominant, both processes usually operate together rather than in isolation.

Freeze thaw Weathering is the most widespread form of physical weathering. It occurs in rocks that contain crevices and joints (GRANITE) where there is little vegetation cover and where temperatures fluctuated around 0⁰c.

When it is warmer during the daytime water enters the joints, but freezes during cold nights causing frost which leads to physical breakdown in two ways:

1 As ice occupies 9pc 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

These therefore widen the joints and cause pieces of rock to shatter from the main body, gathering at the foot of the slope as SCREE or TALUS.

Salt Weathering if water entering the pore spaces in rocks is slightly saline then, as it evaporates, salt crystals are likely to form. As the salt crystals become larger, they exert stresses upon the rock, causing it to disintegrate. This process occurs in hot deserts where capillary action draws water to the surface and where the rock is SANDSTONE.

Pressure Release many rocks have developed under considerable pressure. The confining pressure increases the...