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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.
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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.
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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.
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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.
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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.