Weathering and Sedimentary Rocks Lecture
Weathering and Sedimentary Rocks Lecture GEOL 1005
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This 3 page Class Notes was uploaded by Frances Cooke on Sunday November 15, 2015. The Class Notes belongs to GEOL 1005 at George Washington University taught by Glenn Havelock in Fall 2015. Since its upload, it has received 19 views. For similar materials see Environmental Geology in Geology at George Washington University.
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Date Created: 11/15/15
Weathering and Sedimentary Rocks Weathering gradual destruction of rock under surface conditions 0 natural response of Earth to new environments 0 Physical processes mechanical weathering like a river 0 Actual components stay the same 0 Breaking rocks into smaller pieces 0 Frost wedging splitting of rocks due to rain getting into cracks alternate between freezing and melting I Liquid water expands about 9 when freezing I Enlarges openings I Mountain areas of midlatitude where freezethaw cycle exists 0 Talus scree slope slope of loose material disintegrated rocks I Don t slide down these even though it s fun 0 Sheeting entire slabs of intrusive igneous rocks break loose I Topsoil erodes gt mass of rock expands and sheets off I Exfoliation dome I Granite especially 0 Biological activity I Plant roots wedge rocks apart I Burrowing animals move fresh material to surface gt subject to new physical environment o Chemical weathering 0 Water the main agent in chemical weathering carbonic acid I Rain groundwater bodies of water I Dissolves limestone feldspar into clay o Limestone subject to weathering I Rare formations of limestone pavement o Alters internal structures of minerals I Elements removed or added I Original rock altered into new stable material 0 Products of chemical weathering I Clay minerals I Limestone dissolves into calcium and bicarbonate ions gt taken away by groundwater I Silica dissolved from some minerals o Quartz resistant to chemical weathering Sedimentary rocks 0 Form from sediment o Weathering breaks down rocks 0 Erosional agents river wind gravity glaciers transport and deposit sediment 0 Loose sediment transformed into solid rock by lithification o Compaction and cementation silica calcite o 75 of continental rocks are sedimentary 0 Important Geologists use them to reconstruct Earth s history 0 Clues to past environments lake river delta marine area 0 Info about sediment transport stream velocity 0 Rocks often contain fossils 0 Economic importance 0 Coal sedimentary rock burned in power stations to create electrical energy 0 Sandstoneothers contains petroleum and natural gas in voids eg sandstone 0 Sources of iron and aluminium 0 Construction esp limestone to concrete and cement o Classifying sedimentary rocks 0 Two groups based in source of material I Detrital clastic Material is solid particles derived from weathering erosion Classified by particlegrain size 0 Four size categories Shale very fine grained 0 Most abundant fossils bc formed from clay things don t move around much preserves the fossils o Siltstone fine grained o Sandstone 0 Very fine gt coarse gravel o Conglomerate breccia sharp edges or rounded edges 0 Sediment transport rounding o Sorting size distributionvariation of particles I Chemical 0 Derived from material that was once in solution in water 0 Precipitated to form sediment o Directly precipitated eg rock salt left after saltwater evaporates 0 Biochemical origin gt coral and shellfish precipitate calcite to produce their shells and skeletons Limestone most abundant chemical rock 0 90 is biogenic calcite o Fossiliferous made from whole fossils o Crystalline chemical evaporated Chalk made up entirely of hard parts of microscopic organisms biochemical 0 England chalk cliffs o Formed mainly during Cretaceous period Chert precipitated quartz flintagate o Microcrystalline quartz dense hard rocks Evaporites rock salt gypsum precipitated Coal 0 Formation burial gt peat partially altered plant material not oxidized because not exposed to decayair o Compaction forms lignite soft brown coal gt bituminous coal soft black coal which is used to open fire place 0 Metamorphism due to more compaction and stress gt anthracite hard black coal used in power plants 0 Ideal condition swamp stagnant water or oxygendeficient area 0 Salt gt salt flats o Sedimentary structures in sedimentary rocks 0 O 0 Shows processes over periods of time Features that form while sediment is being transporteddeposited Bedding lamination I Strata represents discrete increments of time I Low level events where no deposition occurs isn t preserved I High level events mudslides floods Crossbedding internal structure showing movement flowing current moving air I Unidirectional river current 0 Grains build up on top of ridge fall down slope and form another layer indicative line I Sand dunes river bed Ripple marks indications of ripple movement build up crossbedding I Symmetrical equal slopes from waves oscillating like in ocean I Asymmetrical ripples dunes or river beds from unidirectional movement Mud cracks shrinkage cracks from wet then dry Groove marks from saltmarsh plants preserved in change in water level
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