06 Weathering
Weathering is the breakdown and decomposition of rocks under the action of atmospheric agents at the surface. It is the first stage of the rock cycle: rocks crumble, then agents of transportation carry and deposit the debris to form sedimentary rocks. It is divided into mechanical and chemical weathering.
Mechanical weathering
- Breaking the rock into smaller pieces without changing its chemical or mineral composition.
- Freezing and thawing of water in rock cracks: blocks separate and talus (scree) slopes form.
- Thermal expansion caused by the difference in temperature between night and day in the desert.
- Unloading: by erosion, buried rocks rise upward, expand and peel off in sheets, as in granite.
Chemical weathering
- Decomposition and change of the mineral and chemical composition of rocks by water and air, producing new, weaker minerals suited to surface conditions.
- Oxidation: dissolved oxygen acts on iron-rich rocks such as basalt, so they rust.
- Carbonation: carbonic acid forms when CO₂ dissolves in rainwater; it dissolves limestone and forms chasms and caves.
Comparison
- Chemical and mineral composition — mechanical: unchanged · chemical: changed, new minerals form.
- Agents — mechanical: freeze–thaw, thermal expansion, unloading · chemical: oxidation, carbonation, water and air.
- Result — mechanical: smaller pieces of the same rock · chemical: weaker and purer minerals.
- Example — mechanical: exfoliation of granite · chemical: rusting of basalt, limestone caves.