Weathering and Soil-forming Processes

Weathering occurs because rocks and minerals become exposed to physical and chemical conditions different from those under which they formed. Weathering in the pedosphere takes place close to the surface where the overburden pressure and temperature are low, and H2O is plentiful. Because most soils harbor large numbers of microorganisms, their metabolic activity results in the consumption of O2 and production of CO2. The partial pressure of CO2 in soil may be 10-100 times greater than that of the atmosphere (Holland, 1978). Under waterlogged conditions, where gas diffusion between the soil and the atmosphere is impeded, the oxygen partial pressure may approach zero and the soil solution may contain CO2, CH4, and H2S. Under these conditions, Fe and Mn may become soluble. 

The inorganic solid phase of any soil consists of a number of minerals displaying different degrees of weathering susceptibility. The extent of weathering of these minerals depends on the stabilities of the minerals and the physical and chemical environment in which the minerals are immersed in the soil, including the supply of water and the removal or transport of the products of weathering (see Chapter 6 Carrels and Christ, 1965 Kittrick, 1977 Colman and Dethier, 1986). 

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Dubroeucq D, Geissert D and Quantin P (1998) Weathering and soil forming process under semi-arid conditions in two Mexican volcanic ash soils. Geoderma 86 99-122... 

Mica is a very common constituent of soils other than laterites. The earth s crust contains 1.4% muscovite and 3.8% trioctahedral micas (Ahrens [1965]). On the average, soil clays and silts probably contain more mica. The percentage of mica in the silt and clay fraction of the average soil is probably closer to 10%, and this is largely dioctahedral mica. Soils are derived more from sedimentary rocks than from other rock types. Since mica is a common component of shale, and shale is a common sedimentary rock, natural rock weathering and soil-forming processes have tended to favor the concentration of mica in the silt and clay fractions of soils. 

C - unconsolidated material underlying the solum (A and B). It may or may not be made of the same parent material from which the solum formed. It is the least weathered and also little affected by the soil-forming process, although it may be weathered R - underlying consolidated rock... 

Aluminum, a silver-white, malleable, and ductile metal, is the most abundant metallic element in the lithosphere, comprising about 8% of the earth s crust. It is never found free in nature, but occurs combined with other elements, most commonly as aluminosilicates, oxides, and hydroxides in rock, minerals, clays, and soil. It is also present in air, water, and many foods. Bauxite, a weathered rock consisting primarily of aluminum hydroxide minerals, is the primary ore used in aluminum production. Aluminum enters environmental media naturally through the weathering of rocks and minerals. Anthropogenic releases are in the form of air emissions, waste water effluents, and solid waste primarily associated with industrial processes, such as aluminum production. Because of its prominence as a major constituent of the earth s crust, natural weathering processes far exceed the contribution of releases to air, water, and land associated with human activities. 

Erosion from wind, water, or glaciers picks up materials from weathering rocks and deposits them as sediments or soil. A process called lithification describes the conversion of sediments to sedimentary rocks. In contrast to the parent igneous rocks, sediments and sedimentary rocks are porous, soft, and chemically reactive. Metamorphic rock is formed by the action of heat and pressure on sedimentary, igneous, or other kinds of metamorphic rock that are not in a molten state. 

Erosion processes ultimately redistribute the weathering products and new sediments are formed. When buried they undergo diagenesis to form the sedimentary rocks. Ultimately, these sedimentary rocks will be exposed and will themselves undergo weathering and form the parent materials of new soils. 

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