teacher background · rock, soil and sediment downslope under the influence of gravity. there are...

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Weather ing Instruct iona l Case : A ser ies o f s tudent-centered sc ience lessons

TeacherBackgroundWeatheringisthephysicalbreakdownandchemicalalterationofrocksandmineralsatorneartheEarth'ssurfaceduetointeractionswithlivingorganisms,waterandtheatmosphere.Weatheringoccursinplaceanddoesnotinvolvethemovementortransportofsediment.Erosiondiffersfromweatheringinthatitisthephysicaltransportofsediment(theproductsofweathering)bymobileagentssuchaswater,windorice.MassWasting(commonlycalledlandslides)isthetransferofrock,soilandsedimentdownslopeundertheinfluenceofgravity.Therearetwomajortypesofweatheringprocesses:

1.Physical(alsoknownasMechanical)Weatheringistheresultofphysicalforcesthatbreakrockintosmallerandsmallerpieceswithoutchangingtherock'smineralcomposition(itisstillcomposedofthesameminerals).2.ChemicalWeatheringinvolvesachemicaltransformationofrockintooneormorenewmineralsorothercompounds.

Weatheringisarock’sresponsetoachangingenvironment.Forexample,plutonic(intrusive)rocksformunderconditionsathighpressuresandtemperaturesdeepintheEarth’scrust.Theserocksmaybeupliftedtothesurfacewheretheyarenotasstableastheconditionsunderwhichtheyoriginallyformed.Inresponsetotheenvironmentalchange,theygraduallyweather(transformtomineralsthatarechemicallystableatthesurface).Physicalandchemicalweatheringworksimultaneouslyandaidoneanother.Physical/MechanicalWeatheringTherearefourcommonphysicalprocessesthatleadtophysicalweathering:

1.FrostWedging2.Unloading(releaseofpressure)3.ThermalExpansion4.BiologicActivity

Physicalweatheringincreasesthesurfaceareaavailableforchemicalweathering.Increasedsurfaceareausuallyresultsinmorerapidchemicalreactions(chemicalweathering).Theagentsoferosion(water,wind&ice)mayalsocontributetophysicalweatheringby"breakingrocks”duringthemovementofsediment.Thefigureshowsthatthesurfaceareaofacubemayincreasedramaticallyasitiscutintosmallercubes.

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1.FrostWedgingiscausedbyrepeatedcyclesoffreezingandthawing.Waterhastheuniquepropertyofexpanding(~9%increaseinvolume)whenitfreezes(that’swhyicefloatsinliquidwater).Inthegeologicenvironment,waterworksitswayintocracksinrock,anduponfreezing,expandsandenlargestheseopenings.Aftermanyfreeze-thawcycles,therockisbrokenintoangularfragments.Frostwedgingismostprevalentinmountainousregionswhereadailyfreeze-thawcycleiscommon.Itisalsofrostwedgingthatcausespotholesinroadsduringthewinter.2.UnloadingiswhererocksthatformdeepintheEarth’scrust(likegranite)willbegintoexpandwhentheyreachtheEarth'ssurface(decompression).Thiscanresultinthegenerationofonion-likelayersorsheetsofrockthatbegintoseparate.ThephotototherightshowsHalf-DomeinYosemiteNationalParkwherethesheet-likestructuresareduetounloadingattheEarth'ssurface.3.ThermalExpansioniscausedbydailyandseasonalchangesintemperature.Asthetemperaturechanges,thermalexpansionandcontractionofindividualmineralscanexertdestructiveforcesonthecohesionofarock.Thisisespeciallytrueindesertenvironmentswherethechangeintemperatureduringthedaymaybeasgreatas30°C.Thismechanismisprobablytheleasteffectiveofalltheweatheringprocesses.4.BiologicalActivityPhysicalweatheringcanbeaccomplishedbytheactionsoforganismssuchastreesandburrowinganimals.Plantrootsgrowintofractures,andastheygrowlarger,theywedgetherockapart.Acommonexampleofthiscanbeseenwheretreerootsupliftanddamageasidewalkasrootsunderthesidewalkgrowlarger.

WeatheringduetounloadingonHalf-DomeScottMarshall,AppalachianStateUniversity

1cubex6sidesx1m=6m2surfacearea

8cubesx6sidesx.5m=24m2surfacearea

64cubesx6sidesx.25m=96m2surfacearea

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Theprimaryagentsoferosion(water,windandice)mayalsocontributetothephysicalweatheringofrocks.

1.Watermaytransportrocks,knockingthemtogetherandbreakingthemintosmallpieces.Commonly,rocksfoundinstreamsareroundedbyphysicalweatheringwhilebeingtransportedbywater.2.Ice(glaciers)grindsrockasglaciersmoveacrossthesurfaceoftheEarth.Rockflourisproducedbythegrindingactionofglacierswheretherockispulverizedintoafinedust.3.Windmayblowparticlessuchassandthatcanabraderocks(similartosandblasting).

ChemicalWeatheringThechemicalweatheringofrocksandmineralsisduetoavarietyofchemicalprocessesandreactionsthatresultinthetransformationoftheoriginalmineralsintonewmineralsthatarestableatsurfaceconditions.Chemicalweatheringmayalsoinvolveputtingmineralcomponentsintosolutionsuchasdissolutioninwater.Wateristhemostimportantagentinthethreedifferentprocessesofchemicalweathering:

1.Dissolution2.Oxidation3.Hydrolysis

Generally,chemicalreactionsoccurmorerapidlyathighertemperatures.Thusitisclearthatchemicalweatheringofrocksandmineralsismoreintenseinwarmerenvironments(lowlatitudes)thanincolderenvironments(highlatitudes).Dissolutioniswhenmineralsdissolveinwaterbydisintegrationofthemineralintoseparateions.Waterisaneffectivesolvent(goodatdissolvingawidevarietyofsolids)becausethemoleculehasabentshapewithdifferentelectricalchargesattheendsofthemolecule(polar).Inawatermolecule,theoxygenatomhasaslightnegativechargeandthehydrogenatomshaveslightpositivecharge.Thisunevenchargedistributiononthewatermoleculemaydisrupttheattractiveforcesholdingamineraltogether.Thechargedendsactaslittlewedgestotakeamineralsurfaceapart-dissolveit.Althoughpurewateractsasasolvent,thepresenceofevenasmallamountofacidinwaterdramaticallyincreasestheabilityofwatertodissolvedifferentmaterials.CarbonicacidoccursinrainwaterandsurfacewaterswhereitcontainsdissolvedatmosphericCO2.Othernaturallyoccurringacidsinclude:

organicacidfromthedecayoforganicmattersulfuricacidfromtheweatheringofsulfidemineralslikepyrite.

Diagramshowingstructureandchargedistributionofwatermolecules.

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Themineralcalciteisparticularlysusceptibletodissolutionbyacid(remembertheacidtestforcalcite).

CaCO3+2H+→Ca2++CO2+H2OInthisprocess,calciteisdissolved.Thisreactionisimportantfortheformationofcavesinlimestone.2.Oxidationisachemicalprocesswheresomeatomsloseelectronstoanotherelement(achemicalreactionthatinvolvesthetransferofelectronsbetweendifferentchemicalcomponents).Oxygenisthemostcommonoxidationagentandisresponsibleforoxidationreactionsthatformrustfromiron.

4Fe+3O2→2Fe2O3

Theoxidationofironisgreatlyacceleratedinthepresenceofwater.Manyigneousmineralscontainiron(suchasolivine,pyroxeneandamphibole)andoxidizeinthepresenceofwateratsurfaceconditions.Thechangeistheoxidationstateofironinthesemineralsresultsinthedisintegrationoftheoriginalmineral.Theoxidationofironisresponsibleformanyrockshavingrustyorredcolors.3.Hydrolysisisachemicalreactionwherechemicalbondsmaybebrokenbytheadditionofwater.Asanexample,feldsparschemicallyalter(hydrolysis)toformclaymineralssuchaskaolinite.Althoughfeldsparmineralscontainnowaterintheircrystalstructure,claymineralssuchaskaolinitecontainwaterthatisbondedtothecrystalstructureofthemineral.Typically,naturalwatersattheEarth’ssurfacecontainsomedissolvedionsthatacceleratethechemicalprocessofhydrolysis.Claymineralsareaverycommonproductofchemicalweatheringbyhydrolysis.TheclaymineralsareverystableattheEarth’ssurfaceandrepresenttheendproductsoftheweatheringofigneoussilicateminerals(thataregenerallyunstableattheEarth’ssurface).Unlikeothersilicateminerals,quartzisparticularlyresistanttoweathering.Quartzsandtendstoaccumulateinsediments(theproductsofweathering)suchasonbeachesandsanddunessinceothermineralsintherockhaveweatheredaway.RatesofWeatheringTherearedifferentfactorsthatmayinfluencethetypeandrateofweathering.1.Rockcharacteristics.Thechemicalcompositionofrocks(asexpressedbythemineralsintherock)andthepresenceoffractures(thatpermittheinfiltrationofwater)maydramaticallyaffectitssusceptibilitytochemicalweathering.2.Climate.Climate(temperatureandthepresenceofwater)maydramaticallyinfluencetheweatheringofrocksandminerals.Forexample,thephysicalweatheringofrocksbyfrostwedgingisdependentuponthepresenceofsurfacewater.Inaddition,frostwedgingandthermalexpansionaredramaticallyaffectedbyclimate.Bothoftheseprocessesareenhancedinclimateswithlargedailychangesintemperature.Lastly,chemicalweatheringchemicalweatheringisenhancedinwarm,moistclimates.Tropicalenvironmentsrepresentregionswiththemostintensechemicalweatheringofrocks.

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ConnectionstootherTopicsinEarthScienceUniformitarianismisakeyprincipleingeologythatstatesthatthenaturalprocessesthatareoccurringtodayhaveoccurredthroughoutthegeologicpast.Thus,changestotheEartharedominantlytheresultofsmallincrementalchangesduetoordinaryprocessessuchasweatheringanderosion.UniformitarianismreplacedtheideaofcatastrophismthatstatesthatfeaturesoftheEarth(suchasmountainsanvalleys)weretheresultofsuddenandviolentevents.Thus,theweatheringofrocksandmineralsattheEarth’ssurfacecanbeviewedasoneoftheslowgradualprocessesthatmayresultinlarge-scalechanges(overgeologictime)suchasthelevelingofmountainranges.Erosionisaprocessthatisrelatedtoweatheringbutisdistinctlydifferent;itiscommontoconfuseweatheringanderosion.Erosionisthephysicaltransportofsedimentbymobileagentssuchaswater,windorice.Theproductsofweathering(sediments)representthematerialsthataretransportedbyerosion.Themosteffectiveagentoferosioniswaterthatisalsoanecessarycomponentofphysicalandchemicalweatheringprocesses.Thetransportofsedimentsduringerosioncommonlycontributestoweathering.Forexample,sedimentstransportedduringerosionmaybebrokenintosmallpieces(physicalweathering).Thewatercycle(akahydrologiccycle)describesthemovementofwateratorneartheEarth’ssurfaceandisessentialforprocessesthatareresponsiblefortheweatheringofrocksandminerals.Thetransferofwaterthroughthewatercycleisresponsibleforprecipitationoncontinentsthataidsweatheringanderosion.Sedimentaryrocksareformedfromsedimentsthataretransportedtoanenvironmentwheretheycanaccumulate.Physicalandchemicalweatheringprocessesproducetherawmaterials(sediment)forsedimentaryrocks.Theweatheringofrocksresultsintwofundamentallydifferenttypesofproducts:

1.solidparticles(sandgrains,clayminerals,etc.)2.dissolvedconstituentsinwater

Thesetwotypesofproductsfromweatheringresultintwodifferenttypesofsedimentaryrocks:1.detrital(clastic)sedimentaryrocks-formedfromtransportedsolidparticles2.chemicalsedimentaryrocks-formedbytheprecipitationofdissolvedsubstancesbyeitherinorganicorbiologicprocesses

Soilmaybedefinedasalayerofweatheredmineraland/ororganicmaterialthatiscapableofsupportingplantlife.Theformationofsoilbeginswiththeweatheringofrock.Theproductsofweatheringsuchasquartzsandgrainsandclaymineralsarecommonconstituentsofsoils.Inregionswherethechemicalweatheringofrocksisgreatest,thereisgenerallygreatersoilformation.Inaddition,theamountofchemicalweatheringthatiscontrolledbyclimatealsodramaticallyaffectsthecompositionofthesoilanditsproductivity.