The Origins of Soil
Weathering of Rocks
Weathering of rocks is the name given to the process by which rocks are broken down to form soils. Rocks and geological sediments are the main parent materials of soils (the materials from which soils have formed). Rocks are weathered by the action of such natural forces as heat, water, wind, glacier, organisms, chemical reactants, etc. The changes are either physical or chemical, or both. When rocks are only changed physically without any alteration of the chemical composition; the process is known as physical weathering.
Physical Weathering
Physical weathering, also called mechanical weathering, is a process that breaks up parent rock, or primary minerals, without changing the chemical character of the minerals. The principal agents of physical weathering are heat, water, wind, glaciers, and organisms.
Temperature
The most remarkable physical weathering is caused by temperature. It may be regarded as thermal weathering. Changes in temperature cause rock to expand (with heat) and contract (with cold).
Water, Glacier, Wind, and Organisms
Alternate wetting and drying is an important process of mechanical weathering. Many rocks absorb water and swell when wetted and shrink when dried. Expansion and contraction in volume due to alternate wetting and drying create stresses that break up rocks into pieces. Water may accumulate in fractures and joints of rocks. When water freezes, it expands. The ice then works as a wedge. It slowly widens the cracks and splits the rock. When ice melts, liquid water performs the act of erosion by carrying away the tiny rock fragments lost in the split rock.
Chemical Weathering
Chemical weathering involves the change, or transformation, of primary minerals into secondary minerals. Secondary minerals are the fine materials that make up clay particles in the soil. The presence of clay in a soil signifies an active history of weathering. An example of a primary mineral weathering to form a secondary mineral is the breakdown of feldspar (primary) into clay (secondary). Chemical weathering reactions increase in warm, humid climates and are also enhanced by the presence of water and oxygen, as well as biological agents including the acids produced by microbial and plant-root metabolism.
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