Lecture 12 Weathering and Sedimentary rocksQuestions to ponderPowerPoint PresentationThe role of water in chemical weathering1. DissolutionSlide 6Slide 7Slide 82. HydrolysisHydrolysisSlide 11Slide 12Many silicates from Bowen’s Reaction Series weather into a variety of clays3. Chemical Weathering - OxidationChemical Weathering - OxidationOxidationSlide 17Weathered Fe3+ is insolubleWeathering is differentiation process because different minerals weather at different ratesChemical weathering affects minerals differentiallySlide 21Slide 22Interaction of lithosphere, hydrosphere, atmosphere and biosphereSlide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Fine-grained rocks – All tend to form fine bedsSlide 54Slide 55Lecture 12Weathering and Sedimentary rocksQuestions to ponder•What are sedimentary rocks•What are their textures•What types of minerals are found in sedimentary rocks•How do we classify the siliciclastic rocksB. Chemical Weathering•Minerals are destroyed or altered by chemical reactions–Dissolution–Hydrolysis–Oxidation•Organisms also contribute to chemical weathering•Secondary minerals often formThe role of water in chemical weathering- Transports ions to the site where chemical reactions take place - Carries away the dissolved products of chemical weathering reactions- Water can combine with other chemicals to form acidswater + carbon dioxide = carbonic acidwater + sulfur dioxide = sulfuric acid1. Dissolution•Dissolution–Dissolving away of materials•e.g., Halite – NaCl,•Gypsum - CaSO4•Calcite - CaCO3 –Minerals dissolve into constituent ions–Ions removed with water by leachingLimestone undergoing dissolutionCa2+ and HCO 3 ions are removed by water when CaCO3 (CALCITE) is Dissolved.Caves = “Dissolution”featuresIn the case of cave formations (stalagmites; stalactites), these are re-precipitated2. HydrolysisHydrolysis: chemical bonding of hydrogen ions (H+) or hydroxyl ions (OH-) with the ions of a mineralThe chemical formula for water can be represented asH2OOr, it can be represented asHOHWhen water forms ions (i.e., disassociates) these ions are H+ and OH-When the small H+ ions replace the positive ions in a mineral, a new mineral is formed, and some products are removed by solutionAcidAcidBaseBaseHydrolysis•CO2 mixes with water to produce carbonic acid, H2CO3•H+ attacks minerals by replacing other ions in the mineral structure•Promotes dissolution–Calcite hydrolysis by carbonic acid solution–CaCO3 + H2CO3 = Ca+2 + 2HCO3-Hydrolysis•New “secondary” minerals may be created by this process–H+ ion replaces the K+ ion in the feldspar structure–K+ ion goes into the water solution–Kaolinite, a clay mineral, formed2KAlSi3O8 + 2H2CO3 + 9H2O =2K+ + 2HCO3- + 4H4SiO4 + Al2Si2O5(OH)4Many silicates from Bowen’s Reaction Seriesweather into a variety of clays3. Chemical Weathering - Oxidation•Valence state of metals (Fe, Mn, Mg, etc) increases (Larger positive number)–Often associated with free O2 in the environment•Iron is usually found as the Fe+2 ion in silicate minerals•Exposed to the atmosphere it will oxidize to the Fe+3 ionChemical Weathering - Oxidation•Change in valence state disrupts crystal structure•Oxidation works in combination with hydrolysis and dissolutionEx) Fe silicate2FeSiO4 + 4H2O + O2 = 2 Fe2O3 + 2H4SiO4Fe2+ Fe3+Ex) PyriteIron sulfide + oxygen + water = iron oxide(FeS2) (O2) (H2O) (Fe2O3)OxidationFe3+ is not very soluble,Results in deposits enriched in Fe3+Olivine deposits oxidized to reddish colorWeathered Fe3+ is insolubleWeathering is differentiation process because different minerals weather at different ratesMost stable Silicate mineralChemical weathering affects minerals differentiallyGraniteRocks and minerals areDifferentiated by Weathering5. The effects of biota…..Interaction of lithosphere, hydrosphere, atmosphere and biosphereWeathering can actually consume CO2Interaction of lithosphere, hydrosphere, atmosphere and biosphereWeathering of material can affect climateProcessesMaterialsThe Rock CycleWhat are Sedimentary Rocks•Detrital sediment or chemical sediment deposited into discrete layers called “Beds”–Sediments are lithified (turned into rock) by compaction, dewatering, and cementation•Formations of similar beds are called Strata (for “stratified”)•Sed. Rocks classified (named) according to 1) Texture and 2) CompositionLayered StrataWhy are sedimentary rocks important?•Of the three rock types, these are volumetrically the least abundant, but…SedimentaryRocks compose75 % of surfaceexposureWhy are sedimentary rocks important?•Sedimentary rocks–These are the rocks most often encountered–Contain the fossil record–Source for fossil fuels–Records of the environments in which they formed–Used for building materials–Often contain needed minerals and resourcesClassifying Sedimentary rocks•Two types of Sedimentary rock Textures:–1) Detrital or Clastic RocksProduct of Sediments weathered and eroded from pre-existing rocks,• Sediments transported and lithified•Fragments of pre-existing rocks•Organic remains of Plants and Animals can also be sediment–2) Chemical rocksSediments from the Precipitation of Ions from water column and/or BiotaMay appear as crystalline materialOr as biotic detritalOr as a combination of crystalline and detrital materialRock CycleTemperatureAnd PressureUplift, Exposure and Weathering create detrital and chemical sedimentsRock fragments, organics, and dissolved materials are:1) TransportedWind Glaciers Water – streams, groundwater, oceans2) Deposited (or precipitated, if chemical)3) LithifiedGeneralized diagramshowing the Relationship ofsediment formation, transportation and sedimentary rock1. Detrital (clastic) sedimentary rock•Often called siliciclastic because silicate minerals dominateMost stable silicateVery common material In detrital rocksQuartz and Clays are both silicates. Quartz is resistant to weathering,Clays are a chemical weathering productSediment is transportedTransportation modes and energy affects the type of sediment as well as additional weathering of the materials•Wind – smaller particles•Glaciers – All sizes of sediments•Water – Depends on the velocity of waterDistance