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Sediments and Sedimentary Rocks Chapter 6

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Sediments and Sedimentary Rocks Chapter 6 Sediments and Sedimentary Rocks Chapter 6 Weathering, Soil and Sedimentary Rocks Sediments, Soils & Sedimentary Rocks Processes of the rock cycle • Weathering (Soils) • Erosion • Transportation • Deposition (sedimentation) • Burial • Diagenesis Introduction Introduction Rocks and minerals are disintegrated and decomposed by the processes of mechanical and chemical weathering. How does weathering differ from erosion? This breakdown occurs Weathering is the mechanical and chemical because the parent alteration of Earth materials at or near the surface material reacts with its Erosion involves removing weathered materials new physical and from their place of origin-by running water or chemical environment wind, for example. transforming it into a new equilibrium state. Geo-inSight 4., p. 136 Fig. 6.2, p. 135 1 How Are Earth Materials Altered? How Are Earth Materials Altered? The products of weathering include soluble salts, Weathering and erosion take place at different rates ions in solution, and solid particles These products of weathering can be eroded and This can occur even on become sedimentary rock or modified in place to the same body of rock become soils. because rocks are not compositionally and structurally homogenous throughout, thereby producing uneven surfaces. Fig. 6.1, p. 134 Geo-inSight 9., p. 137 How Are Earth Materials Altered? How Are Earth Materials Altered? Mechanical Weathering Mechanical Weathering Frost Action Frost action When water freezes in cracks in rocks it expands and then it Pressure release contracts when it thaws, thus Thermal expansion and exerting pressure and contraction opening the cracks wider. Crystal growth Repeated freezing and Activities of organisms. thawing disaggregates rocks into angular pieces that may tumble downslope and The products of mechanical weathering are accumulate as talus. chemically the same as their parent materials. Fig. 6.9d, p. 142 Fig. 6.3a, p. 138 4. Physical weathering: frost wedging How Are Earth Materials Altered? Mechanical Weathering Pressure Release and Sheet Joints Sheet joints are fractures that more or less parallel exposed rock surfaces, especially rocks now at the surface that formed under great pressure at depth. These joints form in response to pressure release; that is, when the rocks formed, they contained energy that is released by outward expansion. Frost wedging due the expansion of freezing water can turn small cracks into large ones Fig. 6.4 a-b, p. 138 2 Mechanical /Physical weathering: exfoliation Mechanical / Physical weathering: joints in rocks Exfoliation occurs where large flat & curved sheets Breakage along natural bedding joints plus cracking of rock fracture and detach from outcrop from expansion due lowered pressure at surface How Are Earth Materials Altered? Mechanical / Physical weathering: tree roots Mechanical Weathering How do organisms contribute to mechanical and chemical weathering? Any organic activity such as tree roots growing in cracks contributes to mechanical weathering Organic acids and the tendrils of mosses and lichens aid in the chemical alteration of parent material. Fig. 6.5b, p. 139 The force of the growing roots pry the cracks apart How Are Earth Materials Altered? How Are Earth Materials Altered? Chemical Weathering Chemical weathering These processes cause a change in the chemical composition. The parent material is transformed into products Solution including ions in solution, soluble salts and clay Oxidation minerals. Hydrolysis Hot and wet environments accelerate chemical weathering. Chemical weathering occurs in all environments, except, possibly, permanently frozen polar regions. Fig. 6.7, p. 141 Fig. 6.6, p. 140 3 How Are Earth Materials Altered? Chemical Weathering Solution – rocks dissolve Carbonate Rocks Rocks such as limestone (CaCO³) are nearly insoluble in neutral or alkaline solutions, but they rapidly dissolve in acidic solutions The atoms making up the minerals dissociate, that is, they separate and the rock dissolves. Chemical weathering: Chemical weathering: carbon dioxide carbon dioxide How Are Earth Materials Altered? Chemical Weathering Oxidation – rocks rust Rocks such as sandstone may contain iron minerals that will breakdown when exposed to the atmosphere The atoms making up the minerals dissociate, that is, they separate as the rock rusts away. Geo-inSight 4., p. 136 4 Chemical weathering Chemical weathering: iron and oxygen Pyroxene dissolves, releasing silica and ferrous iron. Pyroxene (FeSiO ) ● Role of oxygen in weathering: ferrous iron. 3 from iron silicates to iron oxides Silica Ferrous ● ferric and ferrous iron Ferrous iron is oxidized, iron forming ferric iron. ● hematite, a common mineral Ferric iron precipitates Ferric iron a solid, iron oxide. ● red and brown – the colors of oxidized iron Iron oxide (hematite) Fe2O3 Chemical weathering: red means iron How Are Earth Materials Altered? Chemical Weathering Hydrolysis – breakdown to clays Potassium Feldspar During hydrolysis hydrogen ions react with and replace positive ions in potassium feldspar The result is clay minerals and substances in solution such as potassium and silica. Chemical weathering: the Chemical weathering: the disintegration of granite disintegration of granite Granite is made up of several minerals that decay at different rates. Feldspar Magnetite Biotite Quartz Mr. Granite 5 Chemical weathering: the Chemical weathering: the disintegration of granite disintegration of granite The decay progresses, Granite is made up Granite is made up and the rock weakens of several minerals of several minerals and disintegrates. that decay at that decay at Cracks form along Cracks form along different rates. different rates. crystal boundaries. crystal boundaries. Feldspar Feldspar Magnetite Magnetite Biotite Biotite Quartz Quartz How Are Earth Materials Altered? Chemical weathering: the role of Chemical Weathering increasing surface area 24 sq cm Factors That Control the Rate of Chemical Weathering Mechanical weathering enhances chemical weathering by breaking material into smaller pieces, thereby increasing the surface area for chemical reactions. 2 cm Because chemical weathering is a surface process, the more surface exposed, the faster the weathering. 2 cm Fig. 6.8 a-c, p. 141 Chemical weathering: the role of Chemical weathering: the role of increasing surface area 24 to 48 sq cm increasing surface area 24 to 48 sq cm 2 cm 2 cm 1 cm 1 cm 2 cm 1 cm 2 cm 1 cm Large rocks have less surface area for chemical weathering… 6 Chemical weathering Chemical stability: a speed control for 2 cm weathering 1 cm 2 cm 1 cm • Solubility (halite high, quartz low) • rate of dissolution (feldspar higher than quartz) • relative stability of common rock- forming minerals (halide to iron oxide) Large rocks have less …than small rocks do, surface area for chemical so smaller rocks weather weathering… more quickly. Weathering factors A. duration of weathering B. bedrock type - stability of minerals C. climate i. water & temperature >>> chemical weathering; ii. lower temperature >>> mechanical weathering; iii. more acidity >>> chemical weathering D. topography i. steep slopes >>> mechanical/physical weathering; ii. gentle slopes >>>chemical weathering How Does Soil Form and Deteriorate? weathering The Soil Profile Soils consist of weathered materials, air, water, humus and also the plants which they support. Fig. 6.10a, p. 143 7 How Does Soil Form and Deteriorate? How Does Soil Form and Deteriorate? Factors That Control Soil Formation Climate - Certainly climate is the most important factor because The Soil Profile chemical processes operate faster where it is warm and wet. Soil formation produces horizons that are known in descending order as O, A, B, and C. These horizons differ from one another in texture, Soils known as pedalfers develop in humid climates such structure, composition and as that of the eastern United color. States and much of Canada. Soils of arid and semiarid regions are known as pedocals, and may contain hard, irregular masses of caliche (calcium carbonate) in horizon B. Fig. 6.10b, p. 143 Fig. 6.11, 6.12, p. 144-145 How Does Soil Form and Deteriorate? How Does Soil Form and Deteriorate? Factors that Control Soil Formation Laterite is a deep red soil typical of the tropics where Other Factors That Control Soil Formation chemical weathering is intense. Laterites are made up of clays and the most Parent material insoluble compounds Organic activity that were present in the Relief and slope parent material. Time Fig. 6.12, p. 145 Fig. 6.7, p. 141 How Does Soil Form and Deteriorate? How Does Soil Form and Deteriorate? Soil Degradation - Any soil losses, physical Soil Degradation changes, or chemical alteration is called soil Soil erosion is caused mostly by sheet and degradation, and all lead to reduced soil productivity. rill erosion. Causes include erosion, compaction, and any kind It is a problem in some areas, especially of chemical pollution that inhibits plant growth. where accelerated by human activities such as construction, agriculture, ranching, and deforestation. Fig. 6.14, p. 147 Fig. 6.13, p. 146 8 How Does Soil Form and Deteriorate? Soil Degradation The Dust Bowl – An Nutrient depletion American Tragedy Loss of nutrients is most prevalent in areas of land overuse. Improper disposal of chemicals and concentrations of insecticides can destroy soil. Geo-Focus Fig. 1 a-c, p. 149 Fig. 6.14, p. 147 Weathering and Resources Sedimentary rocks are produced by surface processes in the
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