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Page1of5 WeatheringInstructionalCase:Aseriesofstudent-centeredsciencelessons TeacherBackground Weatheringisthephysicalbreakdownandchemicalalterationofrocksandmineralsatornearthe Earth'ssurfaceduetointeractionswithlivingorganisms,waterandtheatmosphere.Weathering occursinplaceanddoesnotinvolvethemovementortransportofsediment.Erosiondiffersfrom weatheringinthatitisthephysicaltransportofsediment(theproductsofweathering)bymobile agentssuchaswater,windorice.MassWasting(commonlycalledlandslides)isthetransferof rock,soilandsedimentdownslopeundertheinfluenceofgravity. Therearetwomajortypesofweatheringprocesses: 1.Physical(alsoknownasMechanical)Weatheringistheresultofphysicalforcesthatbreak rockintosmallerandsmallerpieceswithoutchangingtherock'smineralcomposition(itis stillcomposedofthesameminerals). 2.ChemicalWeatheringinvolvesachemicaltransformationofrockintooneormorenew mineralsorothercompounds. Weatheringisarock’sresponsetoachangingenvironment.Forexample,plutonic(intrusive)rocks formunderconditionsathighpressuresandtemperaturesdeepintheEarth’scrust.Theserocks maybeupliftedtothesurfacewheretheyarenotasstableastheconditionsunderwhichthey originallyformed.Inresponsetotheenvironmentalchange,theygraduallyweather(transformto mineralsthatarechemicallystableatthesurface).Physicalandchemicalweatheringwork simultaneouslyandaidoneanother. Physical/MechanicalWeathering Therearefourcommonphysicalprocessesthatleadtophysicalweathering: 1.FrostWedging 2.Unloading(releaseofpressure) 3.ThermalExpansion 4.BiologicActivity Physicalweatheringincreasesthesurfaceareaavailableforchemicalweathering.Increasedsurface areausuallyresultsinmorerapidchemicalreactions(chemicalweathering).Theagentsoferosion (water,wind&ice)mayalsocontributetophysicalweatheringby"breakingrocks”duringthe movementofsediment.Thefigureshowsthatthesurfaceareaofacubemayincreasedramatically asitiscutintosmallercubes. Page2of5 1cubex6sidesx1m 8cubesx6sidesx.5m 64cubesx6sidesx.25m 2 2 =6m surfacearea =24m surfacearea =96m2surfacearea 1.FrostWedgingiscausedbyrepeatedcyclesoffreezingandthawing.Waterhastheunique propertyofexpanding(~9%increaseinvolume)whenitfreezes(that’swhyicefloatsinliquid water).Inthegeologicenvironment,waterworksitswayintocracksinrock,anduponfreezing, expandsandenlargestheseopenings.Aftermanyfreeze-thawcycles,therockisbrokeninto angularfragments.Frostwedgingismostprevalentinmountainousregionswhereadailyfreezethawcycleiscommon.Itisalsofrostwedgingthatcausespotholesinroadsduringthewinter. 2.Unloadingiswhererocksthatformdeepin theEarth’scrust(likegranite)willbeginto expandwhentheyreachtheEarth'ssurface (decompression).Thiscanresultinthe generationofonion-likelayersorsheetsofrock thatbegintoseparate. ThephotototherightshowsHalf-Domein YosemiteNationalParkwherethesheet-like structuresareduetounloadingattheEarth's surface. WeatheringduetounloadingonHalf-Dome 3.ThermalExpansioniscausedbydailyand ScottMarshall,AppalachianStateUniversity seasonalchangesintemperature.Asthe temperaturechanges,thermalexpansionand contractionofindividualmineralscanexertdestructiveforcesonthecohesionofarock.Thisis especiallytrueindesertenvironmentswherethechangeintemperatureduringthedaymaybeas greatas30°C.Thismechanismisprobablytheleasteffectiveofalltheweatheringprocesses. 4.BiologicalActivity Physicalweatheringcanbeaccomplishedbytheactionsoforganismssuchastreesandburrowing animals.Plantrootsgrowintofractures,andastheygrowlarger,theywedgetherockapart.A commonexampleofthiscanbeseenwheretreerootsupliftanddamageasidewalkasrootsunder thesidewalkgrowlarger. Page3of5 Theprimaryagentsoferosion(water,windandice)mayalsocontributetothephysicalweathering ofrocks. 1.Watermaytransportrocks,knockingthemtogetherandbreakingthemintosmallpieces. Commonly,rocksfoundinstreamsareroundedbyphysicalweatheringwhilebeing transportedbywater. 2.Ice(glaciers)grindsrockasglaciersmoveacrossthesurfaceoftheEarth.Rockflouris producedbythegrindingactionofglacierswheretherockispulverizedintoafinedust. 3.Windmayblowparticlessuchassandthatcanabraderocks(similartosandblasting). ChemicalWeathering Thechemicalweatheringofrocksandmineralsisduetoavarietyofchemicalprocessesand reactionsthatresultinthetransformationoftheoriginalmineralsintonewmineralsthatarestable atsurfaceconditions.Chemicalweatheringmayalsoinvolveputtingmineralcomponentsinto solutionsuchasdissolutioninwater. Wateristhemostimportantagentinthethreedifferentprocessesofchemicalweathering: 1.Dissolution 2.Oxidation 3.Hydrolysis Generally,chemicalreactionsoccurmorerapidlyathighertemperatures.Thusitisclearthat chemicalweatheringofrocksandmineralsismoreintenseinwarmerenvironments(lowlatitudes) thanincolderenvironments(highlatitudes). Dissolutioniswhenmineralsdissolveinwaterby disintegrationofthemineralintoseparateions.Wateris aneffectivesolvent(goodatdissolvingawidevarietyof solids)becausethemoleculehasabentshapewith differentelectricalchargesattheendsofthemolecule (polar).Inawatermolecule,theoxygenatomhasa slightnegativechargeandthehydrogenatomshave Diagramshowingstructureandcharge slightpositivecharge. distributionofwatermolecules. Thisunevenchargedistributiononthewatermoleculemaydisrupttheattractiveforcesholdinga mineraltogether.Thechargedendsactaslittlewedgestotakeamineralsurfaceapart-dissolveit. Althoughpurewateractsasasolvent,thepresenceofevenasmallamountofacidinwater dramaticallyincreasestheabilityofwatertodissolvedifferentmaterials.Carbonicacidoccursin rainwaterandsurfacewaterswhereitcontainsdissolvedatmosphericCO2.Othernaturally occurringacidsinclude: organicacidfromthedecayoforganicmatter sulfuricacidfromtheweatheringofsulfidemineralslikepyrite. Page4of5 Themineralcalciteisparticularlysusceptibletodissolutionbyacid(remembertheacidtestfor calcite). CaCO3+2H+→Ca2++CO2+H2O Inthisprocess,calciteisdissolved.Thisreactionisimportantfortheformationofcavesin limestone. 2.Oxidationisachemicalprocesswheresomeatomsloseelectronstoanotherelement(achemical reactionthatinvolvesthetransferofelectronsbetweendifferentchemicalcomponents).Oxygenis themostcommonoxidationagentandisresponsibleforoxidationreactionsthatformrustfrom iron. 4Fe+3O2→2Fe2O3 Theoxidationofironisgreatlyacceleratedinthepresenceofwater.Manyigneousmineralscontain iron(suchasolivine,pyroxeneandamphibole)andoxidizeinthepresenceofwateratsurface conditions.Thechangeistheoxidationstateofironinthesemineralsresultsinthedisintegrationof theoriginalmineral.Theoxidationofironisresponsibleformanyrockshavingrustyorredcolors. 3.Hydrolysisisachemicalreactionwherechemicalbondsmaybebrokenbytheadditionofwater. Asanexample,feldsparschemicallyalter(hydrolysis)toformclaymineralssuchaskaolinite. Althoughfeldsparmineralscontainnowaterintheircrystalstructure,claymineralssuchaskaolinite containwaterthatisbondedtothecrystalstructureofthemineral.Typically,naturalwatersatthe Earth’ssurfacecontainsomedissolvedionsthatacceleratethechemicalprocessofhydrolysis. Claymineralsareaverycommonproductofchemicalweatheringbyhydrolysis.Theclayminerals areverystableattheEarth’ssurfaceandrepresenttheendproductsoftheweatheringofigneous silicateminerals(thataregenerallyunstableattheEarth’ssurface). Unlikeothersilicateminerals,quartzisparticularlyresistanttoweathering.Quartzsandtendsto accumulateinsediments(theproductsofweathering)suchasonbeachesandsanddunessince othermineralsintherockhaveweatheredaway. RatesofWeathering Therearedifferentfactorsthatmayinfluencethetypeandrateofweathering. 1.Rockcharacteristics.Thechemicalcompositionofrocks(asexpressedbythemineralsinthe rock)andthepresenceoffractures(thatpermittheinfiltrationofwater)maydramaticallyaffectits susceptibilitytochemicalweathering. 2.Climate.Climate(temperatureandthepresenceofwater)maydramaticallyinfluencethe weatheringofrocksandminerals.Forexample,thephysicalweatheringofrocksbyfrostwedgingis dependentuponthepresenceofsurfacewater.Inaddition,frostwedgingandthermalexpansion aredramaticallyaffectedbyclimate.Bothoftheseprocessesareenhancedinclimateswithlarge dailychangesintemperature.Lastly,chemicalweatheringchemicalweatheringisenhancedin warm,moistclimates.Tropicalenvironmentsrepresentregionswiththemostintensechemical weatheringofrocks. Page5of5 ConnectionstootherTopicsinEarthScience Uniformitarianismisakeyprincipleingeologythatstatesthatthenaturalprocessesthatare occurringtodayhaveoccurredthroughoutthegeologicpast.Thus,changestotheEarthare dominantlytheresultofsmallincrementalchangesduetoordinaryprocessessuchasweathering anderosion.Uniformitarianismreplacedtheideaofcatastrophismthatstatesthatfeaturesofthe Earth(suchasmountainsanvalleys)weretheresultofsuddenandviolentevents.Thus,the weatheringofrocksandmineralsattheEarth’ssurfacecanbeviewedasoneoftheslowgradual processesthatmayresultinlarge-scalechanges(overgeologictime)suchasthelevelingof mountainranges. Erosionisaprocessthatisrelatedtoweatheringbutisdistinctlydifferent;itiscommontoconfuse weatheringanderosion.Erosionisthephysicaltransportofsedimentbymobileagentssuchas water,windorice.Theproductsofweathering(sediments)representthematerialsthatare transportedbyerosion.Themosteffectiveagentoferosioniswaterthatisalsoanecessary componentofphysicalandchemicalweatheringprocesses.Thetransportofsedimentsduring erosioncommonlycontributestoweathering.Forexample,sedimentstransportedduringerosion maybebrokenintosmallpieces(physicalweathering). Thewatercycle(akahydrologiccycle)describesthemovementofwateratorneartheEarth’s surfaceandisessentialforprocessesthatareresponsiblefortheweatheringofrocksandminerals. Thetransferofwaterthroughthewatercycleisresponsibleforprecipitationoncontinentsthataids weatheringanderosion. Sedimentaryrocksareformedfromsedimentsthataretransportedtoanenvironmentwherethey canaccumulate.Physicalandchemicalweatheringprocessesproducetherawmaterials(sediment) forsedimentaryrocks.Theweatheringofrocksresultsintwofundamentallydifferenttypesof products: 1.solidparticles(sandgrains,clayminerals,etc.) 2.dissolvedconstituentsinwater Thesetwotypesofproductsfromweatheringresultintwodifferenttypesofsedimentaryrocks: 1.detrital(clastic)sedimentaryrocks-formedfromtransportedsolidparticles 2.chemicalsedimentaryrocks-formedbytheprecipitationofdissolvedsubstancesbyeither inorganicorbiologicprocesses Soilmaybedefinedasalayerofweatheredmineraland/ororganicmaterialthatiscapableof supportingplantlife.Theformationofsoilbeginswiththeweatheringofrock.Theproductsof weatheringsuchasquartzsandgrainsandclaymineralsarecommonconstituentsofsoils.In regionswherethechemicalweatheringofrocksisgreatest,thereisgenerallygreatersoilformation. Inaddition,theamountofchemicalweatheringthatiscontrolledbyclimatealsodramatically affectsthecompositionofthesoilanditsproductivity.