WEATHERINGandSOILS
AlessandroGrippo,Ph.D.
MechanicalandchemicalweatheringinZionCanyonZionCanyonNationalPark,Springdale,Utah,U.S.A.
©AlessandroGrippo
Weathering,Erosion,Transportation
• RocksexposedatEarth’ssurfaceareconstantlychangedbywater,air,temperaturevariationsandotherfactors
• WeatheringisthegroupofdestructiveprocessesthatchangephysicalandchemicalcharacterofrocksatornearEarth’ssurface
• Erosionisphysicalpickingupofrockparticlesbywater,ice,orwind
• Transportationisthemovementoferodedparticlesbywater,ice,orwind
CO5
Fig.5.1a
Fig.5.1b
Weathering
• Weatheringprocessesdisintegrateordecomposerocksatornearthesurface
• Whiledifferentweatheringprocessesmightworkatthesametime,theycanbebroadlycategorizedin:
• Mechanical(orPhysical)Weatheringprocesses• ChemicalWeatheringprocesses
Fig.5.2a-c
Fig.5.2d
Fig.5.3
Fig.5.4
WeatheringandEarthSystems
• Solarsystem– Earth-styleweathering(water,ice,wind)isnearlyuniquetoourplanet,atpresent.Smallamountsofweathering(primarilybywind)stilloccuronMars,andwatererosionappearstohavebeenimportantthereinthedistantpast.
• Atmosphere– Oxygenandcarbondioxidecriticaltochemicalweathering– Watercycledthroughatmosphereiscriticaltochemicalandmechanicalweatheringprocesses
– Airinsoilscontributestobiologicalactionthatcanproducechemicalandmechanicalweathering
WeatheringandEarthSystems
• Hydrosphere– Waterisnecessaryforchemicalweathering– Oxygendissolvedinwateroxidizesironinrocks– Carbondioxidedissolvedinwatercreatescarbonicacid
• Primarycauseofchemicalweathering– Runningwaterloosensandabradesparticles– Glacialiceremovesandabradesparticles
– Freeze/thawcyclingmechanicallyweathers
• Biosphere– Plantrootgrowthwidenscracks– Animalfoottrafficandhumanactivitymechanicallyweather
– Decayingorganicmatterinsoilsproducesacidicsoilmoisture
Mechanical(orPhysical)Weathering
• Frostaction(wedging)– Mechaniceffectoffreezing(andexpanding)wateronrocks
• Pressurerelease– Removalofoverlyingrockallowsexpansionandfracturing
• Biologicalaction– Plants:Growingrootswidenfractures– Animals:burrowingandbreakingrocks
• Crystalgrowth• Thermalshocks
– Largetemperaturechangesfracturerocksbyrepeatedexpansionandcontraction
Fig.5.5a
Fig.5.6ab
From:https://www.geol.umd.edu/~jmerck/geol100/lectures/12.html
Fig.5.6c
Fig.5.8a
productsofMechanical(orPhysical)Weathering
• MechanicalWeatheringbreaksdownrocksinfragments
• Fragmentsarenamednotbasedoncompositionbutbasedonsize:– Gravel(biggerthan2mm)– Sand(between2mmand1/16mm)– Silt(between1/16mmand1/256mm)– Clay(smallerthan1/256mm)
Howmechanicalweatheringhelpschemicalweathering
ChemicalWeatheringanditsproducts
• Twoagents:– Oxygen– Acids
• OxygencausesIronto“rust”• Acidsdisintegratemostmineralsinto:– Quartz– Clayminerals– Ionsinsolution
ActionofOxygen
• Freeoxygen(O2),foundinboththeatmosphereanddissolvedinwater,attacksiron(Fe)presentinmaficrocks(e.g.,basalt)
– 4Fe+3O2!2Fe2O3(Hematite)– 2Fe2O3*nH2O(Limonite)– Fe3O4(Magnetite)– Otherironoxides,suchasGoethite
– Mostimportant:Hematite,thenLimonite
– Soilandsedimentaryrocksoftenstainedwithironoxides
ActionofAcids
• Anacidisasubstancethat,whenputinwater,releasesprotons(H+)
• Protonsareveryreactive:– Lookingtosaturatetheirelectricalcharge– Theyareverysmallandwouldtryattackothermoleculestoreplacetheirpositivelychargedions
• Strongacidswouldreleasealotofprotons– Verydangerous– Onlycommonaroundvolcanicvents– Controlledsubstances– HCl,H2S,H2SO4
• Weakacidswouldreleasejustafewprotons– Vinegar,lemonjuice,andmanymore– Innature,carbonicacidisweak,butomnipresent
CarbonicAcid
• Formsnaturallyfromwaterandcarbondioxide• H2O + CO2! H2CO3
Water CarbonDioxide CarbonicAcid
• Inwater,carbonicaciddissociatesintoprotonsandbicarbonateions
• H2CO3 "! H+ + HCO3-
CarbonicAcid Proton BicarbonateIon
• Itcangofurtherbydissociatinganotherprotonandcarbonateions
• H+ + HCO3- "! 2H+ + CO3
2-
Proton BicarbonateIon TwoProtons CarbonateIon
Productsofchemicalweathering
• Oxygencausesironto“rust”• Acidsdisintegratemostmineralsinto:– Quartz– Clayminerals– Ionsinsolution
• Example:potassiumfeldspar(KAlSi3O8)– Proton(H+)replacesK+butcannotsubstituteforit,causingthestructuretocollapse
– K+goesinsolution,SiO2(quartz)forms,therestcreatesaclaymineral
• Quartzusuallybecomesaroundedparticlethesizeofsand
• Claymineralsareflatcrystalsthesizeofclay– Differencebetweentheconceptsof“claymineral”and“clayparticle”
– Generalformulaforflatsilicatecrystals: ....Al2Si2O5(OH)4
• Ionsinsolutionaredissolvedinwaterandcanprecipitatesalts
• Amongcommonminerals,calciteisanexception
• Itdoesnotyieldquartzorclay,butonlyionsinsolution
• Equilibriumreactionofcalciteinwater:– CaCO3+H2O+CO2"!Ca2++2HCO3
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• Quartz,clayminerals,andionsinsolutionarethethreebyproductsofchemicalweathering
• Quartzisusuallyreducedtothesizeofsand• Claymineralsarecomponentsofmud• Calciumionshaveaffinitywithcarbonateions
• AsaconsequencethethreemostcommonrocksatEarth’ssurfacearesedimentary,andare:– Sandstone(fromquartzsand)– Shale,ormudstone(fromclayminerals)– Limestone(fromionsinsolution)
Propertiesofclayminerals
• Claymineralsareflat• Claymineralsabsorbwaterandpositivelychargedions
• Claymineralsvirtuallypreventwaterformcirculatingthroughthem– Comparewaterinsand,percolatingfreely,vs.waterinmud,causingswelling
Fig.5.16
Intheend…
• Quartzisnotaffectedbychemicalweathering• Feldsparsandmicasyieldquartz,clays,ions• Maficminerals(olivines,pyroxenes,amphiboles,andmore)yieldthesameandhematiteand/orlimoniteinaddition
• Calcitedissolvescompletely
• Theseprocessesrequirewater,sotheyaremaximizedinwarm,moistclimates– LimestoneinArizonavs.LimestoneinMassachusetts
Soil• Soil-alayerofweathered,unconsolidatedmaterialontop
ofbedrock– Commonsoilconstituents:
• Clayminerals
• Quartz• Water
• Organicmatter
• Soilhorizons– Ohorizon-uppermostlayer;
organicmaterial– Ahorizon-darklayerrichin
humus,organicacids
– Ehorizon-zoneofleaching;fine-grainedcomponentsremovedbypercolatingwater
– Bhorizon-zoneofaccumulation;claysandironoxidesleacheddownfromabove
– Chorizon-partiallyweatheredbedrock
SoilsandClimate
• Soilthicknessandcompositionaregreatlyaffectedbyclimate– Wetclimates:
• Morechemicalweatheringandthickersoils• Soilsinmoderatelywetclimatestendtohavesignificantclay-richlayers,whichmaybesolidenoughtoformahardpan
– Aridclimates:• Lesschemicalweatheringandthinnersoils• Subsurfaceevaporationleadstobuild-upofsalts• Calcite-richaccumulationzonesmayform,cementingsoiltogetherintoahardpan
– Extremelywetclimates(e.g.,tropicalrainforest)• Highlyleachedandunproductivesoils(laterites)• Lateritescanbefurtherleachedintobauxites(aluminumore)• MostnutrientscomefromthickO/Ahorizons
pH• pHmeasurestheacidityofasolutionbyindicatingtheconcentrationof
protonsinit• H2O"!H++OH–
• [H+]=[OH–]=10-7
• pHistheinverselogarithmoftheconcentrationofH+ionsinwater
• When[H+]=[OH–]=10-7,thenthesolutionisneutralandthepHis7• Valuelowerthan7indicateacidicconditions• Valueshigherthan7indicatealkaline(basic)conditions
theend