Soil refers to the weathered and fragmented surface layer of the earth's crust. It is formed initially through disintegration and decomposition of rocks by physical and chemical processes and is influenced by the activity and accumulated residues of numerous species of microscopic and macroscopic animals. The physical weathering process which brings about the disintegration of rocks into small fragments includes expansion and contraction caused by alternating heating and cooling, stresses resulting from freezing and thawing of water and the penetration of roots, and scouring or grinding by abrasive particles carried by moving ice or water and by the wind. The chemical processes tending to decompose the original minerals in the parent rocks include hydration, oxidation, reduction, solution and dissociation, immobilization by precipitation or removal of components by volatilization or leaching, and various physicochemical exchanged reactions. The loose products of these weathering processes are often transported by running water, glaciers, or wind, and deposited elsewhere.