Geography – Soils in rainforests and savannas | e-Consult

Weathering is the breakdown of rocks, soils, and minerals through contact with the Earth's atmosphere, water, and biological organisms. It is a crucial initial stage in soil formation, transforming parent material into the components of soil. Weathering can be broadly classified into physical (mechanical) and chemical weathering.

Physical Weathering: This involves the breakdown of rocks without changing their chemical composition. Examples include:

• Freeze-thaw weathering: Water enters cracks in rocks, freezes and expands, widening the cracks. This process repeats, eventually breaking the rock apart. This is particularly effective in areas with frequent freeze-thaw cycles.
• Thermal expansion and contraction: Repeated heating and cooling cause rocks to expand and contract, leading to stress and eventual fracturing.
• Exfoliation: The outer layers of rocks expand and crack due to decreasing pressure as overlying material is removed. This results in peeling off of thin sheets.
• Biological weathering: Plant roots growing into cracks, or the action of animals burrowing, can physically break down rocks.

Physical weathering increases the surface area of the parent material, making it more susceptible to chemical weathering. It also contributes to the size of soil particles.

Chemical Weathering: This involves the alteration of the chemical composition of rocks. Examples include:

• Solution: Soluble minerals (like halite) dissolve in water and are carried away. This is common in areas with high rainfall.
• Hydrolysis: Minerals react with water, often involving acids, to form new minerals. For example, the breakdown of feldspar into clay minerals.
• Oxidation: Minerals react with oxygen, often in the presence of water, leading to the formation of oxides. This is responsible for the reddish colour of some soils.
• Carbonation: Carbon dioxide in rainwater reacts with rocks containing calcium carbonate (like limestone) to form calcium bicarbonate, which is soluble and carried away.

Chemical weathering breaks down rocks into smaller particles and alters their chemical composition, releasing nutrients that contribute to soil fertility. The type of weathering that dominates in a particular environment will influence the mineral composition of the resulting soil.