Definition: Weathering is the gradual destruction of rock under surface conditions. Weathering may involve physical processes (mechanical weathering) or chemical activity (chemical weathering). Some workers also include the actions of living things (organic weathering), although these can also be classified as mechanical or chemical or a combination of both.
Weathering can range from a change in color all the way to the complete breakdown of minerals into clay and other surface minerals. Weathering creates deposits of altered and loosened material, called residue, that are ready to undergo transportation and thus be eroded. Erosion means weathering plus transportation at the same time. Weathering is necessary for erosion, but a rock may weather without undergoing erosion.