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Weathering, Erosion & Soil A. Weathering Processes: - chemical weathering: minerals in a rock are chemically altered or dissolved. produce clays and soils change chemical composition of natural waters - physical weathering: rock becomes fragmented by physical processes. - chemical and physical weathering enhance each other chemical weathering weakens the rock for easier physical disintegration. physical weathering increases the surface area for fast chemical reaction. Figure 5.3 (surface area) B. Erosion: - the set of processes that loosen and move earth material downhill or downwind - erosion carries away weathered material and exposes fresh rock surface to weathering - weathering enhances erosion by breaking down rock into smaller and weak fragments. C. Physical Weathering - This process tends to be more pronounced along zones of weakness: e.g. bedding plane of sedimentary rocks, fractures called joints in igneous rocks (they form when rocks cool and contract). - Frost wedging is the most efficient mechanisms for widening cracks and break the rock: as water freezes and turns into ice, the volume increases by about 9%. Figure 5.4 insert - Alternating heat (expansion) and cold (contraction) could result in breakage of rock. - Exfoliation: a large scale physical weathering that produces flat or curved sheets of rock (Fig. 5.5) Spheroidal weathering: similar to exfoliation, but at a smaller scale. (some newer books, including your textbook, define spheriodal weathering as rounding effect of chemical weathering.) - No generally accepted explanation for either exfoliation or spheroidal weathering. One possibility is that they are products of uneven expansion and contraction due to alternating cooling and heating, and perhaps with the removal of overlying rock that reduces the pressure. D. Chemical Weathering: Chemical reactions dissolve mineral and create new chemical compounds. e.g. K-feldspar weathered to kaolinite Feldspar + carbonic acid + water 2KAlSi3O8 2H2CO3 H2O kaolinite + dissolved+potassium+ bicarbonate silica Al2Si2O5(OH)4 4SiO2(aq) ion 2K+ 2 HCO3- Where did the carbonic acid come from? Kaolinite: a common weathering product of silicate minerals in most environments. Bauxite, an ore composed of aluminum hydroxide and a major source of aluminum Many iron silicates, such as iron pyroxene (FeSiO3), could be altered to iron mineral, such as hematite (Fe2O3) by a process called oxidation. With a few exceptions (e.g. halite), calcite is the most easily soluble mineral: calcite+carbonic acid ca. ion+bicarbonate CaCO3 H2CO3 Ca+2 HCO3- Stability of common minerals: minerals formed under lower temperature tend to be more stable than those crystallized at higher temperature. (Bowen’s reaction series) E. Factors that affect the extent of weathering - parent rock: solubility of minerals in the rock Fig. 5.11 granite vs. limestone - climate: rainfall & temperature: hot & humid environments enhance weathering freezing of water may speed up physical weathering - soil: medium for life (microbes and plants); water retainer. - Residual Soils - Transported Soils acid environment enhances chemical weathering - time Transported soil and residual soil F. Soil profiles: A product of weathering A soil profile in South Dakota (Figure 5.17) Figure 5.18: Idealized soil profile (humid climate) G. Weathering makes the raw material of sediment