Weathering

Weathering is the mechanical disintegration and chemical decomposition of rocks by the elements of weather and climate. Little or no movement happens, so it is an in-situ (on-site) process. Climate controls both the kind of weathering and the depth of the weathering mantle.

Three groups (rarely acting alone, though one often dominates):

• Chemical weathering: solution, carbonation, hydration, oxidation and reduction decompose or dissolve rock. They need water and air (oxygen and carbon dioxide) and are sped up by heat. Decaying plants and animals add more carbon dioxide underground.
• Physical (mechanical) weathering, from applied forces: gravitational (overburden pressure, load, shearing stress), expansion (temperature change, crystal growth, animal activity) and water pressure (wetting and drying). Most of it comes from thermal expansion and pressure release. Each change is small, but repeated expansion and contraction fatigue the rock until it breaks.
• Biological weathering: organisms add or remove minerals, or break rock by growing and moving. Earthworms, termites and rodents burrow, exposing fresh surfaces to air and water; ploughing does the same; decaying matter makes humic and carbonic acids; plant roots push rocks apart with great force.

Exfoliation is a result, not a process: curved sheets of rock flake off, leaving smooth, rounded surfaces. It comes from unloading, temperature change and salt weathering. Exfoliation domes result from unloading; tors from thermal expansion.

Why weathering matters:

• It breaks rock into fragments, preparing the way for regolith and soil, and for erosion and mass movements.
• Forests, and so biomes and biodiversity, depend on the depth of the weathering mantle.
• It enriches ores of iron, manganese, aluminium and copper: when some materials are leached away, the valuable ones left behind become concentrated enough to be worth mining.

Example (new): a tree growing out of a crack in an old fort's stone steps slowly lifts and splits the slabs as its roots thicken: biological weathering at work in a single lifetime.