10 Weathering

Definition and types

• Weathering is the breakdown of rocks of the crust at the surface by mechanical and chemical factors; it is the first step of the rock cycle and the source of sediments.
• Physical (mechanical) weathering breaks rock into pieces without changing its chemical composition.
• Frost wedging: water freezes in cracks and joints and the ice expands.
• Temperature changes: repeated heating and cooling cracks the rock.
• Biological action: plant roots widen cracks.
• Chemical weathering changes the minerals into new substances.
• Oxidation: iron-bearing minerals rust.
• Carbonation: rain containing dissolved carbon dioxide dissolves limestone, forming caves.
• Hydrolysis: feldspar in granite reacts with water and changes into the clay mineral kaolinite.
• Quartz resists chemical weathering because it is chemically stable silicon dioxide.