Weathering is defined as the mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate.
It is responsible for breaking down the rocks into smaller fragments and preparing the way for the formation of not only regolith and soils, but also erosion and mass movements, which ultimately result in the creation of various landforms.
Weathering also leads to the enrichment and concentration of certain valuable ores such as iron, manganese, aluminium, copper, etc.
Following are the three major types of weathering processes:
• Chemical weathering processes:
- Solution: Rock-forming minerals like nitrates, sulphates, potassium, etc., are readily soluble in water. They leach out without leaving any residue in rainy climates and accumulate in dry regions. Decaying organic matter creates carbonic acid, which helps in dissolving rocks such as limestone.
- Carbonation: Carbon dioxide from the atmosphere and soil air is absorbed by water to form carbonic acid. This acid dissolves rocks containing calcium and magnesium carbonates, resulting in cave formation.
- Hydration: Minerals take up water and expand; this causes an increase in the volume of the rock. Continued repetition causes fatigue and leads to disintegration or fracturing.
- Oxidation and Reduction: Rocks containing iron, manganese, and sulphur get oxidised with access to oxygenated water. In oxygen-deficient environments (e.g., below water tables), reduction occurs instead.
• Physical weathering processes:
- Unloading and Expansion: Removal of overlying rock due to erosion causes vertical pressure release, expanding upper rock layers and forming exfoliation domes.
- Temperature Changes and Expansion: Minerals expand with rising temperature and contract with cooling. Drastic diurnal changes in dry or elevated regions cause rock fatigue, forming features like tors.
- Freezing, Thawing and Frost Wedging: Ice growth within rock pores and cracks during freezing-melting cycles breaks the rock. Common in mid-latitude high elevations.
- Salt Weathering: Rocks with salts (calcium, sodium, magnesium, potassium, barium) expand due to thermal action, hydration, and crystallisation—causing granular disintegration and polygonal cracks.
• Biological weathering:
- Burrowing and Wedging: Organisms like earthworms, termites, and rodents expose fresh rock surfaces to chemical action and improve moisture-air penetration.
- Human Action: Activities like ploughing and cultivation mix soil layers, enhancing contact between air, water, and minerals.
- Decaying Organic Matter: Produces acids (humic, carbonic, etc.) that enhance chemical breakdown and solubility.
- Plant Roots: Exert mechanical pressure, breaking rocks apart.
Conclusion:
Weathering processes rarely operate in isolation. However, a dominant weathering type is often observable depending on environmental conditions.