Mechanical weathering or Physical Weathering:
Physical or mechanical weathering processes depend on some applied forces. The applied forces could be:
- Gravitational forces such as overburden pressure, load and shearing stress;
- Expansion forces due to temperature changes, crystal growth or animal activity;
- Water pressures are controlled by wetting and drying cycles.
Many of these forces are applied both at the surface and within different earth materials leading to rock fracture. Most of the physical weathering processes are caused by thermal expansion and pressure release. These processes are small and slow but can cause great damage to the rocks because of continued fatigue the rocks suffer due to repetition of contraction and expansion.
Chemical weathering is the weakening and subsequent breakdown of rocks by chemical reactions. A group of weathering processes viz; solution, carbonation, hydration, oxidation and reduction act on the rocks to decompose, dissolve or reduce them to a fine clastic state through chemical reactions by oxygen, surface and/or soil water and other acids. Water and air (oxygen and carbon dioxide) along with heat must be present to speed up all chemical reactions. Over and above the carbon dioxide present in the air, the decomposition of plants and animals increases the quantity of carbon dioxide underground. These chemical reactions on various minerals are very much similar to the chemical reactions in a laboratory.