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Classroom presentations to accompany Understanding Earth, 3rd edition prepared by Peter Copeland and William Dupré University of Houston Chapter 6 Weathering and Erosion Weathering and Erosion Tim Davis/Photo Researchers Weathering • Physical and chemical changes that occur in sediments and rocks when they are exposed to the atmosphere and biosphere. • Not the same as erosion. Factors Controlling Rates of Weathering Chemical weathering • This process occurs because minerals formed deep in the earth’s interior are not stable under surface conditions. • Stability is generally the reverse of Bowen’s reaction series. • The principle agent of chemical weathering is water. Chemical weathering of silicates • Quartz: very stable • Feldspars: form clay minerals • Mafic minerals: decompose to oxides Chemical weathering of carbonates • Easily soluble in water (especially with some acid present) • Ca and Mg taken into solution Weathering rates of gravestones Marble Slate R. Siever Fig. 6.1 Etched and corroded feldspar in the soil zone Berner & Holden, 1977 Fig. 6.2 Fig. 6.3a Fig. 6.3b Fig. 6.3c Analogy of weathering: making coffee fresh grounds + water = coffee + residue (a solution) K-feldspar + water = K+ + kaolinite (a clay mineral) Weathering and Making coffee Fig. 6.4 Weathering by solution •The complete breakup of minerals into ions in solution •NaCl (halite) is the best example, but is geologically unimportant •calcite (limestone) = CaCO3 •CaCO3 + H2CO3 = Ca2+ + 2HCO3•Mafic silicates dissolve much more slowly Mechanical Weathering Changes the Surface to Volume Ratio Fig. 6.5 Fig. 6.6 Fig. 6.6 CO2 and the Atmosphere, Weathering, and the Climate Fig. 6.7 Fig. 6.8 Weathering Oxides Provide Color to the Desert Landscape Betty Crowell Fig. 6.9 Weathered Limestone Ric Ergenbright Fig. 6.10 Joint-controlled Weathering Jeff Foott/DRK Fig. 6.11 Mechanical weathering Frost — water expands by 9% when it freezes Thermal expansion — differential thermal expansion of minerals creates stress in rocks Organic activity — tree roots to microorganisms Mechanical abrasion — things go bump Role of Organisms in Weathering Peter Kresam Fig. 6.12 Gneiss Boulder Fractured by Frost Action Michael Hambrey Fig. 6.13 Exfoliation Dome in Yosemite Tony Waltham Fig. 6.14 Spheroidal Weathering Michael Follo Fig. 6.15 Weathering terms Bedrock: Regolith: Soil: unaltered rock of any kind a layer of broken pieces of rock and slightly altered rock that overlies the bedrock a layer of altered mineral material usually mixed with organic material Weathering, Soil Formation, and Erosion Fig. 6.16 Weathering, Soil Formation, and Erosion Fig. 6.16 Change in U.S. soil erosion rates Soil Profile Fig. 6.17 Laterite Fig. 6.18a Pedalfer Fig. 6.18b Pedocal Fig. 6.18c Sand Rex Elliot Fig. 6.19