Physical and chemical weathering

Weathering is the breakdown of rocks in place by physical and chemical processes.

Physical (mechanical) weathering breaks rock into smaller pieces without changing its mineral composition. Main processes:

• Frost wedging: water freezes in joints and expands about 9%, prying the rock apart. Common in the high Himalaya.
• Thermal expansion and contraction (insolation): repeated day-night heating and cooling stresses the surface minerals.
• Exfoliation (unloading): removal of overlying rock releases pressure, so sheets peel off massive rocks such as granite.
• Salt crystallisation: salts growing in pores push the grains apart (arid and coastal areas).
• Biological action: plant roots widen cracks; burrowing animals expose fresh rock.

Chemical weathering changes the minerals of the rock into new, more stable minerals (mostly clay) and soluble salts. It is fastest in warm, wet climates. Main processes:

• Solution: soluble minerals such as rock salt and gypsum dissolve in water.
• Carbonation: CO$_2$ in rain forms carbonic acid, which dissolves limestone: $CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2$. This produces karst.
• Hydrolysis: feldspar reacts with water to form clay: $2KAlSi_3O_8 + 2H^+ + H_2O \rightarrow Al_2Si_2O_5(OH)_4 + 4SiO_2 + 2K^+$ (orthoclase to kaolinite).
• Oxidation: iron minerals combine with oxygen and turn into rust-coloured oxides, e.g. $4FeO + O_2 \rightarrow 2Fe_2O_3$.
• Hydration: minerals take up water and swell, e.g. anhydrite to gypsum, hematite to limonite.

Differences