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