Weathering is the alteration and breakdown of rock minerals and rock masses when they are exposed to the atmosphere. Weathering processes occur in situ, that is, in the same place, with no major movement of rock materials involved.

Weathering is a fundamental Earth process. Weathering changes rocks from a hard state, to become much softer and weaker, making them more easily eroded.

Weathering Processes:

Two main groups of weathering processes are identified, with a third supporting group:

  • Physical weathering: the group of processes, such as frost wedging and volume changes of minerals, that result in the mechanical disruption of rocks (e.g. granular disintegration, exfoliation, joint block separation, shattering by changes in temperature or pressure).
  • Chemical weathering: the decay of rock forming minerals caused by water, temperature, oxygen, hydrogen and mild acids (e.g. solution, hydration, oxidation, carbonation).
  • Biological weathering: the group of processes that are caused by, or assisted by, the presence of vegetation, or to a lesser extent animals, including root wedging and the production of organic acids.

The type of weathering processes that occur at any particular location depend predominantly upon the climate:

  • Physical weathering: mechanical processes dominate in cold and dry climates.
  • Chemical weathering: processes of mineral decay dominate in warm and humid climates.
  • Biological weathering: vegetation, and animals, tend to be more active in warm and humid climates.

Weathering Controls:

The type, rate and extent of weathering depends upon several controlling factors:

  • Climate dictates the type of weathering processes that operate, largely by determining the amount of water available and the temperature at which the processes occur. Chemical reactions are faster at higher temperatures, while frost wedging occurs in colder climates.
  • Rock Type determines the resistance of the rock to the weathering processes that operate in that particular environment. Each rock type is composed of a particular set of minerals, which are joined together by crystallisation, chemical bonding or cementing. When the forces of plate tectonics move these rocks from the environment in which they formed and expose them to the atmosphere they begin to weather.
  • Rock Structure: highly jointed or faulted rocks present many planes of weakness along which weathering agents (e.g. water) can penetrate into the rock mass.
  • Topography: the slope angle determines the energy of the weathering system by controlling the rate at which water passes through the rock mass. Generally, higher, or tectonically active areas with steeper slopes have more dynamic weathering systems, whereas flat plains have slower weathering systems.
  • Erosion: the dynamism and efficiency of erosion determines how rapidly any weathered material is removed, how frequently fresh rock is exposed to weathering, and if deeply weathered profiles are preserved.
  • Time: the duration of the period that the same type of weathering has been operating, uninterrupted by climatic change, earth movements, and other factors, determines the degree and depth to which the rocks have been weathered.

Weathering Products:

Weathering gradually weakens rocks, and eventually produces new geological materials (rock fragments, sands, silts and clays) that are more stable in the new environment. Weathering generally produces finer and less dense rock materials, and weaker, more porous and permeable rock masses.

In the tropics and subtropics, intense weathering in the hot and humid conditions produces thick weathered profiles, which may be up to 100 metres, or more, thick.

Weathering processes penetrate down discontinuities (planes of weakness), such as faults and joints, in the rock mass and then attack the faces of the joint-bounded blocks, penetrating the solid blocks.