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04_weathr_sed_09_post - 4 Weathering Sedimentary Rocks...

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Lecture 4: Lecture 4: 1 Sedimentary rock : rock that formed at or near the Earth’s surface by precipitation of minerals from water solutions, by growth of skeletal material in organisms, or by cementing together of shell fragments or loose small pieces (grains) derived from preexisting rocks. Sediment : loose (unconsolidated) fragments. 4: Weathering & Sedimentary Rocks Sedimentary layers in Grand Canyon. Photo: P.L. Kresan Weathering of granite produces sediment. Photo: S. Marshak. Lecture 4: Lecture 4: 2 Why this is important Weathering is the first step in the process of Weathering is the first step in the process of wearing down what tectonics builds up. Photo: P.L. Kresan Lecture 4: Lecture 4: 3 Why this is important Weathering produces soils and concentrates certain mineral resources. Photo: L. Lefever Sedimentary rocks contain fossil fuels & many non-metallic mineral resources. Photo: Chevron Corp. Lecture 4: Lecture 4: 4 Why this is important Sedimentary rocks are the prime recorders of geologic history. Photo: P.L. Kresan Only rock type that contains fossils, the record of the history of life. Photo: P.L. Kresan
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Lecture 4: Lecture 4: 5 Origin of sediment Fig. 7.16 Weathering : mechanical disintegration and chemical decomposition of rock to produce sediment. Erosion : removal of (generally weathered) rock or rock by-products. Transportation : movement of weathered material by moving air, water, ice. Deposition : accumulation of previously transported sediment. Lecture 4: Lecture 4: 6 Two types of weathering Mechanical (physical) weathering : reduces the size of rock and mineral fragments without changing their composition Chemical weathering : results in a change in chemical composition. Lecture 4: Lecture 4: 7 Mechanical weathering Frost wedging : water in cracks expands by 9% when it freezes Fig. 7.6a Taylor Valley, Victoria Land, Antarctica. Photo: M. Hambrey Analogy: freezing water bursts Analogy: freezing water bursts pipes and breaks water bottles pipes and breaks water bottles placed in freezer. placed in freezer. Lecture 4: Lecture 4: 8 Mechanical weathering Root wedging & burrowing: mechanical weathering produced by plants and animals See Fig. 7.6b Photo: P.L. Kresan
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Lecture 4: Lecture 4: 9 Chemical weathering Dissolution : some minerals (halite, gypsum, calcite) dissolve in water, especially acidic water. Lecture 4: 10 Chemical weathering Oxidation : elements combine with oxygen (iron rusts) Arches National Park, Utah. Photo: P.L. Kresan Lecture 4: 11 Chemical weathering Hydrolysis : hydrogen ions in water replace ions of silicate minerals and convert them to insoluble residue (commonly clay minerals like kaolinite) and dissolved ions. Lecture 4: 12 Weathering Fig. 7.9 Chemical weathering aids physical weathering by weakening the Chemical weathering aids physical weathering by weakening the attachments between mineral grains so that rock attachments between mineral grains so that rock “ falls apart falls apart”; chemical weathering then attacks the crumpled pieces.
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