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Chapter 04
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####### FOCUSonCONCEPTS
, arth’ssurfaceisconstantlychanging,movedtolower elevationsbygravity,andcarriedawaybywater,wind,orice’sphysical landscapeissculptured-endingprocess, weatheringandmasswasting,andwhydoesthetypeandrateof weatheringvaryfromplacetoplace?Whatmechanismsacttomoveweathereddebrisdownslope? Soil,animportantproductoftheweatheringprocessandavitalresource,isalsoexamined.
Toassistyouinlearningtheimportantconceptsinthischapter,focusonthefollowingquestions: <23 WhatareEarth'sexternalprocesses,andwhatrolesdotheyplayintherockcycle? -@;1;?1 Whatarethetwomaincategoriesofweathering?Inwhatwaysaretheydifferent? Whatfactorsdeterminetherateatwhichrockweathers? <55; Whatissoil?Whatarethefactorsthatcontrolsoilformation? <1:-‘: Whatfactorsinfluencenaturalratesofsoilerosion?Whatimpacthavehumanshad? (5511 Howisweatheringrelatedtotheformationoforedeposits? #1? Whatismasswastingandwhatroledoesitplayinthedevelopmentofvalleys? 1 Whatarethecontrolsandtriggersofmasswasting? l Whatcriteriaareusedtodivideanddescribethevarioustypesofmasswasting? Whatarethegeneralcharacteristicsofslump,rockslide,debrisflow,earthflow,andcreep?
Earth’s External Processes
0 SculpturingEarth'sSurface
fir"->'":nZ-afin:ITI 9 WeatheringandSoil
Weathering, masswasting,anderosionare calledexternal processesbecausetheyoccuratornearEarth’ssurfaceandare poweredbyenergyfromtheSun part of the rock cycle because they are responsible for transformingsolidrockintosediment. Tothecasualobserver,thefaceofEarthmayappeartobe withoutchange,unaffectedbytime, 200 yearsagomost peoplebelievedthatmountains,lakes,anddesertswereperma- nentfeaturesofanEarththatwasthoughttobenomorethana fewthousandyearsold yearsoldandthatmountainseventuallysuccumbtoweathering anderosion,lakesfillwithsedimentoraredrainedbystreams, anddesertscomeandgowithchangesinclimate. Earthisadynamicbody’ssurfaceare graduallyelevatedbymountainbuildingandvolcanicactivity. TheseinternalprocessesderivetheirenergyfromEarth’sinterior. Meanwhile,opposingexternalprocessesarecontinuallybreak- ingrock apart andmovingthe debris tolower elevations r;1; ~<-5. ).Thelatterprocessesinclude:
Weathering—thephysicalbreakdown(disintegration) andchemicalalteration(decomposition)ofrocksator nearEarth’ssurface.
Masswasting—thetransferofrockandsoildownslope undertheinfluenceofgravity.
Erosion—thephysicalremovalofmaterialbymobile agentssuchaswater,wind,orice. T Wefirstturnourattentiontotheprocessofweatheringandthe productsgeneratedbythisactivity,weatheringcannot beeasilyseparatedfromtheothertwoprocessesbecause,as weatheringbreaksrocksapart,itfacilitatesthemovementofrock debrisbymasswastinganderosion,thetransportof materialbymasswastinganderosionfurtherdisintegratesand decomposestherock.
####### CONCEPTcnscx 4.
QListexamplesofEarth'sexternalandinternalprocesses. QFromwheredotheseprocessesderivetheirenergy?
Weathering
SculpturingEarth'sSurface
‘gm"'23:1-121",nr./fH"! FWeatheringandSoil
Weatheringgoesonallaroundus,-butitseemslikesuchaslow andsubtleprocessthatitiseasytounderestimateitsimportance. Yet,itisworthrememberingthatweatheringisabasicpartofthe rockcycleandthusakeyprocessintheEarthsystem isalsoimportanttohumans—eventothoseofuswhoarenot studyinggeology, manyofthelife-sustaining mineralsandelementsfoundinsoil,andultimatelyinthefoodwe
86 CHAPTER 4 Weathering,Soil,andMassWasting
Weatheringoccurswhenrockismechanicallyfragmented (disintegrated) andfor chemicallyaltered (decomposed). Mechanicalweatheringisaccomplishedbyphysicalforcesthat breakrockintosmallerandsmallerpieceswithoutchanging therock"smineralcomposition achemicaltransformationofrockintooneormorenewcom- pounds paper andsmallerpieces,whereasdecompositionoccurswhenthe paperissetafireandburned. Inthefollowingsectionswediscussthevariousmodesof mechanicalandchemicalweathering thesetwocategoriesseparately,keepinmindthatmechanical andchemicalweatheringprocessesusuallyworksimultaneously innatureandreinforceeachother.
CONCEPTcnscx4.
QWhatarethetwobasiccategoriesofweathering? QHowdotheproductsofeachcategoryofweatheringdiffer?
Mechanical Weathering
' SculpturingEarth'sSurface
mmQ 223 /NIrn }WeatheringandSoil
Whenarockundergoesmechanicalweathering,itisbrokeninto smallerandsmallerpieces,eachretainingthecharacteristicsofthe originalmaterial largeone. showsthatbreakingarockintosmallerpieces increasesthesurfaceareaavailableforchemicalattack,by breakingrocksintosmallerpieces,mechanicalweatheringincreases theamountofsurfaceareaavailableforchemicalweathering.
Innature,fourphysicalprocessesareespeciallyimportantin breakingrocksintosmallerfragments:frostwedging,saltcrystal growth,expansionresultingfromunloading(sheeting),andbio- logicalactivity,althoughtheworkoferosionalagents suchaswaves,wind,glacialice,andrunningwaterisusuallycon- sideredseparatelyfrommechanicalweathering,itisneverthe- lessimportant,they relentlesslydisintegratethesematerials.
FrostWedging Ifyouleaveaglassbottleofwaterinthefreezerabittoolong, youwillfindthebottlefractured waterhastheuniquepropertyofexpandingabout 9 percent uponfreezing exposedwaterpipesruptureduringfrigidweather alsoexpectthissameprocesstofracturerocksinnature is,infact,thebasisforthetraditionalexplanationoffrost wedging,the freezingwaterenlargesthecracksandangularfragmentsare eventuallyproduced(‘.=.~‘-‘.i=;.nr:-arsea). Formanyyears,theconventionalwisdomwasthatmostfrost wedgingoccurredinthemannerjustdescribed,how- ever,researchhasshownthatfrostwedgingcanalsooccurinadif- ferentway, theyexpandorfrostheavedue tothegrowthoficelenses massesoficegrowlargerbecausetheyaresuppliedwithwater migratingfromunfrozenareasasthinliquidfilms accumulatesandfreezes,thesoilisheavedupward processoccurswithinthecracksandporespacesofrocks oficegrowlargerastheyattractliquidwaterfromsurrounding pores, causingittofracture.
5BernardHallet,“WhyDoFreezingRocksBreak?”Science 314 (November2006):1092-1099.
Chemicalweatheringcanoccuronlytothoseportionsofarockthatareexposedtothe
elements,therebyincreasing
thesurfaceareaavailableforchemicalattack.
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MechanicalWeathering 87
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i*';i‘.<xi:;-Traditionaldepictionoffrostwedging,itexpands,exertingaforcegreatenoughtobreakrock
frostwedgingoccursinasettingsuchasthis,thebrokenrockfragmentsfallto thebaseofthecliffandcreateacone-shaped
accumulationknownasatalusslope.(PhotobyTomBean/Corbis)
SaltCrystalGrowth
Anotherexpansiveforcethatcansplitrocksiscreatedbythe growthofsaltcrystals commonsettingsforthisprocess breakingwavesorsaltygroundwaterpenetratescrevicesandpore spacesinrock,saltcrystalsform crystalsgraduallygrowlarger,theyweakentherockbypushing apartthesurroundinggrainsorenlargingtinycracks. Thissameprocesscanalsocontributetocrumblingroadways wheresaltisspreadtomeltsnowandiceinwinter- solvesinwaterandseepsintocracksthatquitelikelyoriginated fromfrostaction,thegrowthofsalt crystalsfurtherbreaksthepavement.
Sheeting
Whenlargemassesofigneousrock,particularlythosecomposed
ofgranite,areexposedbyerosion,concentricslabsbegintobreak
loose-likelayersiscalled
sheetingandprobablytakesplaceinresponsetothegreat
reductioninpressurethatoccurswhentheoverlyingrockis
erodedaway,theouterlayers
expandmorethantherockbelowandthusseparatefromtherock
body(i‘i'.;j'-§§'.lY-=.‘:i‘-.- Ia).Continuedweatheringeventuallycauses
theslabstoseparateandspalloff,creatingexfoliationdomes
(ex=off,folium=leaf). Excellent examples of exfoliation domesincludeStoneMountaininGeorgiaandHalfDomein YosemiteNationalPark(Figure4). Deepundergroundminingprovidesuswithanotherexample ofhowrocksbehaveoncetheconfiningpressureisremoved. Largerockslabssometimesexplodeoffthewallsofnewlycut minetunnelsbecauseoftheabruptlyreducedpressure ofthistype,plusthefactthatfracturingoccursparalleltothefloor ofaquarrywhenlargeblocksofrockareremoved,stronglysup- portstheprocessofunloadingasthecauseofsheeting. Althoughmanyfracturesarecreatedbyexpansion,others areproducedbycontractionasigneousmaterialscool(see Figure9,p,andstillothersbytectonicforcesduring mountainbuilding formadefinitepatternandarecalledjoints(seeFigure4,p. 91 andFigure10,p).Iointsareimportantrockstructures thatallowwatertopenetratedeeplyandstarttheprocessof weatheringlongbeforetherockisexposed.
BiologicalActivity Weatheringisalsoaccomplishedbytheactivitiesoforganisms, includingplants,burrowinganimals,andhumans searchofmineralsandwatergrowintofractures,andastheroots grow,theywedgetherockapart its).Burrowinganimals furtherbreakdowntherockbymovingfreshmaterialtothe
ChemicalWeathering 89
attack contributestomechanicalweathering theouterportionsofsomerocks,which,inturn,makesthem moresusceptibletobeingbrokenbymechanicalweathering processes. Chemicalweatheringinvolvesthecomplexprocessesthat altertheinternalstructuresofmineralsbyremovingand/or addingelements,theoriginalrock decomposesintosubstancesthatarestableinthesurfaceenvi- ronment,theproductsofchemicalweatheringwill remainessentiallyunchangedaslongastheyremaininanenvi- ronmentsimilartotheoneinwhichtheyformed.
WaterandCarbonicAcid
Waterisbyfarthemostimportantagentofchemicalweathering. Althoughpurewaterisnonreactive,asmallamountofdissolved materialisgenerallyallthatisneededtoactivateit dissolvedinwaterwilloxidizesomematerials,when anironnailisfoundinmoistsoil,itwillhaveacoatingofrust (ironoxide),andifthetimeofexposurehasbeenlong,thenailwill besoweakthatitcanbebrokenaseasilyasatoothpick rockscontainingiron-richmineralsoxidize,ayellowtoreddish- brownrustwillappearonthesurface Carbondioxide(CO2)dissolvedinwater(H20)formscar- bonicacid(HZCO3),thesameweakacidproducedwhensoft drinksarecarbonated itfalls throughthe atmosphere, and additionalamounts releasedbydecayingorganicmatterareacquiredasthewater percolatesthroughthesoil veryreactive hydrogenion(H+) andthebicarbonate ion (HCOQ). Acidssuchascarbonicacidreadilydecomposemanyrocks andproducecertainproductsthatarewatersoluble, themineralcalciteCaCO3,whichcomposesthecommonbuild- ingstonesmarbleandlimestone,iseasilyattackedbyevena weaklyacidicsolution.
‘f Ironreactswithoxygentoformironoxide,asseen O11theserustedbarrels.(PhotobyStevenRobertson/istockphoto)
H
P” ._-.-and
HowGraniteWeathers Toillustratehowrockchemicallyweatherswhenattackedby carbonicacid,wenowconsidertheweatheringofgranite,an abundantcontinentalrock quartzandpotassiumfeldspar feldsparcomponentofgranitetakesplaceasfollows:
2KAlSi3O8+2(H++H003“)+H20—> potassiumfeldspar carbonicacid water
Ai,si,o5(oi-1),, +2i<++ 2 Hco;+ 4 sio, claymineral potassium bicarbonite silica ion ion insolution
Inthisreaction,thehydrogenions(H+)attackandreplacepotas- siumions(IC)inthefeldsparstructure,therebydisruptingthe crystallinenetwork,thepotassiumisavailableas anutrientforplantsorbecomesthesolublesaltpotassiumbicar- bonate(KHCO3),whichmaybeincorporatedintootherminerals orcarriedtotheoceanindissolvedformbystreams. Themostabundantproductsofthechemicalbreakdown offeldsparareresidualclayminerals productofweatheringandareverystableundersurfacecon- ditions,claymineralsmakeupahighpercent- ageoftheinorganicmaterialinsoils,themost abundantsedimentaryrock,shale,containsahighproportion ofclayminerals. Inadditiontotheformationofclaymineralsduringthisreac- tion,somesilicaisremovedfromthefeldsparstructureandis carriedawaybygroundwater(waterbeneathEarth’ssurface).This dissolvedsilicawilleventuallyprecipitatetoproducenodulesof chertorflint,fillintheporespacesbetweensedimentgrains,or becarriedtotheocean,wheremicroscopicanimalswillremove ittobuildhardsilicashells. Tosummarize,theweatheringofpotassiumfeldspargener- atesaresidualclaymineral,asolublesalt(potassiumbicarbon- ate),andsomesilicathatentersintosolution. Quartz,theothermaincomponentofgranite,isveifyresistant tochemicalweathering;itremainssubstantiallyunalteredwhen attackedbyweaklyacidicsolutions,whengranite weathers,thefeldsparcrystalsdullandslowlyturntoclay,releas- ingtheonceinterlockedquartzgrains,whichstillretaintheir fresh,glassyappearance soil,muchistransportedtotheseaortoothersitesofdeposition, whereitbecomesthemainconstituentofsuchfeaturesassandy beachesandsanddunes thesedimentaryrocksandstone.
WeatheringofSilicateMinerals Table4 commonsilicateminerals makeupmostofEarth’scrustandthatthesemineralsare composedessentiallyofonlyeightelements weathered,silicatemineralsyieldsodium,calcium,potassium, andmagnesiumions,whichformsolubleproductsthatmaybe
__90 CHAPTER 4 Weathering,Soil,andMassWasting
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####### TheOldMan
####### oftheMountain
TheOldManoftheMountain,alsoknown asTheGreatStoneFaceorsimplyThePro- file,wasoneofNewHampshire's(the GraniteState)best-knownandmost enduringsymbols(Figure4).Begin- ningin1945,itappearedatthecenterof theofficialstateemblem(seeinset betweenphotos).Itwasanaturalrockfor- mationsculptedfromConwayRedGranite that,whenviewedfromtheproperloca- tion,gavetheappearanceofanoldman. Eachyearhundredsofthousandsofpeople traveledtoviewtheOldMan,whichpro- trudedfromhighonCannonMountain, 360 meters(1,200feet)aboveProfileLake innorthernNewHampshire'sFranconia NotchStatePark. OnSaturdaymorning,May3,2003,the peopleofNewHampshirelearnedthatthe famouslandmarkhadsuccumbedtonature
TABLE4 ProductsofWeathering ResidualProducts MaterialinSolution Silica Silica,1<+jN;.,{oii+‘ C I siiioe,ca“,ha? C
Mineral Quartz Quartzgrains Feldspars I Clayminerals Amphibole Clayminerals Ironoxides Olivine Ironoxides Silica,Mg“
removedbygroundwater- gen,producingrelativelyinsolubleironoxides,whichgivesoila reddish-brownoryellowishcolor remainingelements—aluminum,silicon,andoxygen—joinwith watertoproduceresidualclayminerals,eventhehighly insolubleclaymineralsareveryslowlyremovedbysubsurfacewater.
SpheroidalWeathering Inadditiontoalteringtheinternalstructureofminerals,chemical weatheringcausesphysicalchangesaswell,when angularrockmassesarechemicallyweatheredaswaterenters alongjoints,theytendtotakeonasphericalshape cornersandedgesoftheangularblocksbecomemorerounded. Thecornersareattackedmostreadilybecauseoftheirgreater surfacearea,ascomparedtotheedgesandfaces, calledspheroidalweathering,givestheweatheredrockamore roundedorsphericalshape
gtl
STATEtr aaiile-
####### 0,
FIGURE4,highabove FranconiaNotchinNewHampshire'sWhiteMountains,asitappearedpriortoMay3, 2003 thefamousgraniteoutcropafteritcollapsedonMay3,2003 sculptedtheOldManultimatelydestroyedit.(AssociatedPressphotosbyJimCole)
andcollapsed(Figure4).Thecol- processesthatcreateditinthefirstplace. lapseendeddecadesofeffortstoprotectthe Ultimately,frostwedgingandotherweath- statesymbolfromthesamenatural eringprocessesprevailed.
####### CONCEPTcnrzcx4.
QHowiscarbonicacidformedinnature? QWhatproductsresultwhencarbonicacidreactswithpotassium feldspar? QExplainhowtheroundedbouldersinFigure4 formed.
Rates of Weathering
SculpturingEarth'sSurface
fin->"'11Z-iHI! PWeatheringandSoil Severalfactorsinfluencethetypeandrateofrockweathering havealreadyseenhowmechanicalweatheringaffectstherateof weathering,theamountof surfaceareaexposedtochemicalweatheringisincreased importantfactorsexaminedhereincluderockcharacteristicsand climate.
RockCharacteristics Rockcharacteristicsencompassallofthechemicaltraitsofrocks, includingmineralcompositionandsolubility,any physicalfeatures,suchasjoints(cracks), canbeimportant becausetheyinfluencetheabilityofwatertopenetraterock.
92 CHAPTER 4 Weathering,Soil,andMassWasting
iii;(iii 0 Asaconsequenceofburninglargequantitiesofcoal andpetroleum,morethan 27 milliontonsofsulfurandnitrogen oxidesarereleasedintotheatmosphereeachyearintheUnited States,someofthese pollutantsareconvertedintoacidsthatthenfalltoEarth'ssurfaceas rainorsnow,acidrain acceleratesthechemicalweatheringofstonemonumentsand structures,includingthisbuildingfacadeinLeipzig,Germany. (PhotobyDougPlumrner/PhotoResearchers,Inc.)
Differentialweatheringandsubsequenterosionareresponsible forcreatingmanyunusualandsometimesspectacularrockfor- mationsandlandforms.
####### CONCEPTcnncx4. 5
QWhatrockcharacteristiccausedtheheadstonesinFigure4. weathersodifferently? QHowdoesclimateinfluenceweathering?
i~"iۤt-i.';i';tDifferentialweatheringisillustratedbythesesculpted rockpinnaclesinArizona'sMonumentValley.(Photobyjovannigl Shutterstock)
Soil
Soilcoversmostlandsurfaces,itisone ofourmostindispensableresources,likeairandwater,soil istakenforgrantedbymanyofus thisvitallayerinperspective. Science,inrecentyears,hasfocusedmoreandmoreon the Earthasaplanet,onethatforallweknowisunique—where athinblanketofair,athinnerfilmofwater,andthethinnest veneerofsoilcombinetosupportaweboflifeofwondrous diversityincontinuouschange/ Soilhasaccuratelybeencalled“thebridgebetweenlifeand theinanimateworld.”Alllife—theentirebiosphere—owesitsexis- tencetoadozenorsoelementsthatmustultimatelycomefrom Earth’scrust, plantscarryouttheintermediaryroleofassimilatingtheneces- saryelementsandmakingthemavailabletoanimals,including humans.
AnInterfaceintheEarthSystem WhenEarthisviewedasasystem,soilisreferredtoasan interface—acommonboundarywheredifferentpartsofasystem interact wherethesolidEarth,theatmosphere,thehydrosphere,andthe biospheremeet complexenvironmentalinteractionsamongdifferentpartsofthe Earthsystem,soilgraduallyevolvestoastateof equilibrium,orbalance,withtheenvironment andsensitivetoalmosteveryaspectofitssurroundings, whenenvironmentalchangesoccur—inclimate,vegetativecover, oranimal(includinghuman)activity—thesoilresponds suchchangeproducesagradualalterationofsoilcharacteristics untilanewbalanceisreached thelandsurface,soilfunctionsasafundamentalinterface, providinganexcellentexampleoftheintegrationamongmany partsoftheEarthsystem.
WhatIsSoil? Withfewexceptions,Earth'slandsurfaceiscoveredbyregolith (rhegos=blanket,lithos=stone),thelayerofrockandmineral fragmentsproducedbyweathering materialsoil,butsoilismorethananaccumulationofweath- ereddebris matter,water,andair—thatportionoftheregoliththatsup- portsthegrowthofplants majorcomponentsinsoilvary,thesamefourcomponents alwaysarepresenttosomeextent(Figure4).Aboutonehalf ofthetotalvolumeofagood-qualitysurfacesoilisamixture ofdisintegratedanddecomposedrock(mineralmatter)and humus,thedecayedremainsofanimalandplantlife(organic matter).Theremaininghalfconsistsofporespacesamongthe solidparticleswhereairandwatercirculate.
BlackEddy,"AFragileSeamofDarkBlueLight,"inProceedingsoftheGlobalChange ResearchForum.U,1993,p.
ControlsofSoilFormation 93
25%air
p 45% ; '-W§f§iti'=-' matters‘. 3’_A'
5%organicmatter/
1%. granted,butitcontainsagreatdealoflife. Moreover,thiscomplexmediumsupportsnearlyallplantlife,which inturnsupportsanimallife (byvolume)ofasoilingoodconditionforplantgrowth percentagesvary,eachsoiliscomposedofmineralandorganic matter,water,andair.(PhotobyColinMolyneux/GettyImages)
Althoughthemineralportionofthesoflisusuallymuchgreater thantheorganicportion,humusisanessentialcomponent additiontobeinganimportantsourceofplantnutrients,humus enhancesthesoil’sabilitytoretainwater airandwatertoliveandgrow,theportionofthesoflconsistingof porespacesthatallowforthecirculationofthesefluidsisasvitalas thesolidsoflconstituents. Soilwaterisfarfrom“pure”water;instead,itisacomplex solutioncontainingmanysolublenutrients providesthenecessarymoistureforthechemicalreactionsthat sustainlife;italsosuppliesplantswithnutrientsinaformthey canuse isthesourceofnecessaryoxygenandcarbondioxideformost microorganismsandplantsthatliveinthesoil.
SoilTextureandStructure
Mostsoilsarefarfromuniformandcontainparticlesofdifferent sizes. Textureisaverybasicsoilpropertybecauseitstronglyinfluences thesoil’sabilitytoretainandtransmitwaterandair,bothofwhich areessentialtoplantgrowth dryoutquickly,theporespacesofclay-rich soilsmaybesosmallthattheyinhibitdrainage,andlong-lasting puddlesresult,whentheclayandsiltcontentisvery high,plantrootsmayhavedifficultypenetratingthesoil. Becausesoilsrarelyconsistofparticlesofonlyonesize, texturalcategorieshavebeenestablishedbasedon thevarying proportionsofclay,silt, andsand classesusedbytheU ii’i;;mi:i2:- Forexample,point/ilonthistriangulardiagram(left center)representsasoilcomposedof 10 percentsilt, 40 per- centclay,and 50 percentsand.
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j»l_1‘}€-’"L;T}'%l,l{i.'iZ}iThetextureofanysoilcanberepresentedbyapoint
onthissoil-texturediagram
factorsusedtoestimateagriculturalpotentialandengineering
characteristics.(AfterU)
Thesoilscalledloam,whichoccupythecentralportionofthe diagram,arethoseinwhichnosingleparticlesizepredomi- natesovertheothertwo plantlifebecausetheygenerallyhavebettermoisturecharac- teristicsandnutrientstorageabilitythandosoilscomposed predominantlyofclayorcoarsesand. Soilparticlesareseldomcompletelyindependentofone another,theyusuallyformclumpscalledpedsthatgive soilsaparticularstructure- nized:platy,prismatic,blocky,andspheroidal importantbecauseitinfluencestheeaseofasoil’scultivationas wellasthesusceptibilityofasofltoerosion,soflstruc- tureaffectstheporosityandpermeabilityofsoil(i.,theeasewith whichwatercanpenetrate).Thisinturninfluencesthemove- mentofnutrientstoplantroots- allyallowformoderatewaterinfiltration,whereasplatyand spheroidalstructuresarecharacterizedbyslowerinfiltrationrates.
####### concsprCHECK 4. 6
QWhyissoilconsideredaninterfaceintheEarthsystem? QHowisregolithdifferentfromsoil? QWhyistextureanimportantsoilproperty? QUsingthesoiltexturediagram(Figure4),namethesoil thatconsistsof 60 percentsand, 30 percentsilt,and 10 percentclay.
Controls ofSoil Formation
Soilistheproductofthecomplexinterplayofseveralfactors mostimportantoftheseareparentmaterial,time,climate,plants andanimals,andtopography interdependent,theirrolesareexaminedseparately.
ControlsofSoilFormation 95
rlili
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i‘~"iGiiPiE4.225Thenatureofthevegetationinanareacanhaveasignificantinfluenceonsoilformation. Theorganiclitterreceivedbythesoilfromtheconifersishighinacidresins,whichcontributestoanaccumulationofacidinthesoil result,intensiveacidleachingisanimportantsoil-formingprocess.(PhotobyBillBrooks/Alamy)B's SonoranDesertisverydifferentincharacterfromthenorthernconiferousforest.(Photoby RussBishop/agefootstook)
Plantsandanimalsfurnishorganicmattertothesoil bogsoilsarecomposedalmostentirelyoforganicmatter,whereas desertsoilsmaycontainonlyatinypercentage quantityoforganicmattervariessubstantiallyamongsoils,itis theraresoflthatcompletelylacksit. Theprimarysourceoforganicmatterisplants,althoughani- malsandtheuncotmtablemicroorganismsalsocontribute organicmatterdecomposes,importantnutrientsaresuppliedto plants,aswellastoanimalsandmicroorganismslivinginthesoil. Consequently,soilfertilitydependsinpartontheamountof organicmatterpresent,thedecayofplantandanimal remainscausestheformationofvariousorganicacids- plexacidshastentheweatheringprocess ahighwater-holdingabilityandthusaidswaterretentioninasoil. Microorganisms,includingfungi,bacteria,andsingle-celled protozoa,playanactiveroleinthedecayofplantandanimal
remains,amaterialthatnolonger resemblestheplantsandanimalsfromwhichitisformed- tion,certainmicroorganismsaidsoilfertilitybecausetheyhave theabilitytoconvertatmosphericnitrogengasintosoflnitrogen compounds. Earthwormsandotherburrowinganimalsacttomixthemin- eralandorganicportionsofasoil,forexample,feed onorganicmatterandthoroughlymixsoilsinwhichtheylive,often movingandenrichingmanytonsperacreeachyear holesalsoaidthepassageofwaterandairthroughthesoil.
Topography Thelayofthelandcanvarygreatlyovershortdistances variationsintopographycanleadtothedevelopmentofavariety oflocalizedsoiltypes
96 CHAPTER 4 Weathering,Soil,andMassWasting
lengthandsteepnessofslopeshaveasignificantimpactonthe amountoferosionandthewatercontentofsoil. Onsteepslopes,soilsareoftenpoorlydeveloped- uationslittlewatercansoakin,andasaresult,soilmoisturemay beinsufficientforvigorousplantgrowth,becauseof acceleratederosiononsteepslopes,thesoilsarethinornonex- istent(seeFigure4). Incontrast,waterloggedsoilsinpoorlydrainedbottomlands haveamuchdifferentcharacter dark matterthataccumulatesbecausesaturatedconditionsretardthe decayofvegetation aflat-to-undulatinguplandsurface, minimumerosion,andsufficientinfiltrationofwaterintothesofl. Slopeorientation,thedirectionaslopeisfacing,alsoissig- nificant-latitudesoftheNorthernHemisphere,a south-facingslopereceivesagreatdealmoresunlightthandoes anorth-facingslope,asteepnorth-facingslopemay receivenodirectsunlightatall solarradiationreceivedcausessubstantialdifferencesinsoiltem- peratureandmoisture,whichinturninfluencesthenatureofthe vegetationandthecharacterofthesoil. Althoughwehavedealtseparatelywitheachofthesoil-forming factors,rememberthatallofthemworktogethertoformsoil singlefactorisresponsibleforasoilbeingasitis,itisthe combinedinfluenceofparentmaterial,time,climate,plantsand animals,andtopographythatdeterminesasoil’scharacter.
comcsprcnscx4.
QListthefivebasiccontrolsofsoilformation. QWhichfactorismostinfluentialinsoilformation? QHowmightthedirectionaslopeisfacinginfluencesoil formation?
The Soil Profile
Itisimportanttorealizethatsoil-formingprocessesoperatefrom thesurfacedownward,variationsincomposition,texture, structure,andcolorgraduallyevolveatvaryingdepths verticaldifferences,whichusuallybecomemorepronouncedas timepasses,divide thesoil intozonesorlayersknownas horizons,youwouldseethatits wallsarelayered horizonsconstitutesthesoilprofile(i'.¥‘.ig i presentsanidealizedviewofawell-developedsoil profileinwhichfivehorizonsareidentified downward,theyaredesignatedasO,A,E,B,andC,respectively. Thesefivehorizonsarecommontosoilsintemperateregions. Thecharacteristicsandextentofdevelopmentofhorizonsvary indifferentenvironments,differentlocalitiesexhibitsoil profilesthatcancontrastgreatlywithoneanother. TheOhorizonconsistslargelyoforganicmaterial contrasttothelayersbeneathit,whichconsistmainlyofmineral matter- tersuchaslooseleavesandotherorganicdebristhatarestill recognizable,thelowerportionoftheOhorizonis
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B. <:;.‘.-itiWeallknowthatabookshouldnotbejudgedby itscover surface-sectionfromthesurface throughallofthesoil’shorizonsandintotheparentmaterial profileshowsawell-developedsoilinsoutheasternSouthDakota. (PhotobyE)B thissoil inPuertoRicoareindistinct,givingitarelativelyuniform appearance.(PhotocourtesyofSoilScienceSocietyofAmerica)
madeupofpartlydecomposedorganicmatter(humus)inwhich plantstructurescannolongerbeidentified, theOhorizonisteemingwithmicroscopiclife,includingbacte- ria,fungi,algae,andinsects oxygen,carbondioxide,andorganicacidstothedevelopingsoil.
98 CHAPTER 4 Weathering,Soil,andMassWasting
TABLE4
Alfisols Moderatelyweatheredsoilsthatformunderborealforestsorbroadleafdeciduousforests,richinironandaluminum accumulateinasubsurfacelayerinresponsetoleachinginmoistenvironments,productivesoils,becausetheyareneithertoo wetnortoodry. Andisols Aridosols orsaltinsubsoil;loworganiccontent(seeFigure4). Entisols
Youngsoilsinwhichtheparentmaterialisvolcanicashandcinders,depositedbyrecentvolcanicactivity. Soilsthatdevelopindryplaces;insufficientwatertoremovesolubleminerals;mayhaveanaccumulationofcalciumcarbonate,gypsum,
Youngsoilshavinglimiteddevelopmentandexhibitingpropertiesoftheparentmaterial formedonrecentriverdepositstoverylowforthoseformingonshiftingsandorrockyslopes. Gelisols yearslowsoil-formingprocesses. Histosols
Youngsoilswithlittleprofiledevelopmentthatoccurinregionswithpermafrost
Organicsoilswithlittleornoclimaticimplications. Dark,partiallydecomposedorganicmaterialconrmonlyreferredtoaspeat. Inceptisols Weaklydevelopedyoungsoilsinwhichthebeginning(inception)ofprofiledevelopmentisevident,they existfromtheArctictothetropics. Mollisols Dark,softsoilsthathavedevelopedundergrassvegetation,generallyfoundinprairieareas-richsurfacehorizonthatisrichin calciumandmagnesium.Soilfertilityisexcellent borealoralpinetotropical(seeFigure4). Oxisols Soilsthatoccuronoldlandsurfacesunlessparentmaterialswerestronglyweatheredbeforetheyweredeposited tropicsandsubtropicalregions,oxisolsareheavilyleached,hencearepoorsoilsforagriculturalactivity (seeFigure4). Spodosols Soilsfoundonlyinhumidregionsonsandymaterial(seeFigure4)andcoolhumidforests. Beneaththedarkupperhorizonofweatheredorganicmaterialliesalight-coloredhorizonofleachedmaterial,thedistinctivepropertyof thissoil. Ultisols Soilsthatrepresenttheproductsoflongperiodsofweatheringfivaterpercolatingthroughthelsoilconcentratesclayparticlesinthelower horizons(argillichorizons).Restrictedtohumidclimatesinthetemperateregionsandthetropics,wherethegrowingseasonislong. oexten' ' g,h ' ' Vertisols
Abundantwaterandalongfrost-freeperiodcontributet srveleachrn encepoorersorlquality. Soilscontaininglargeamountsofclay,whichshrinkupondryingandswellwiththeadditionofwater, providedthatadequatesuppliesofwaterareavailabletosaturatethesoilafterperiodsofdrought SHGSSGSOI1l1l'EIl.
patternoftheSoilTaxonomy’s 12 soilorders- tionsystems,theSoilTaxonomyisnotsuitableforeverypurpose. Itisespeciallyusefulforagriculturalandrelatedland-usepurposes, butitisnotausefulsystemforengineerswhoarepreparingeval- uationsofpotentialconstructionsites.
####### CONCEPTcnscx 4. 9
QVllhyaresoilsclassified?
Soil Erosion
SoilsarejustatinyfractionofallEarthmaterials,yettheyarea vitalresource plants,theyaretheveryfoundationofthehumanlife-support system productivityofsoilsthroughfertilizationandirrigation,soilscan bedamagedordestroyedbycarelessness inprovidingfood,fiber,andotherbasicmaterials,soilsareamong ourmostabusedresources. Perhapsthisneglectandindifferencehasoccurredbecause asubstantialamountofsoilseemstoremainevenwheresoil erosionisserious,althoughthelossoffertiletop- soilmaynotbeobvioustotheuntrainedeye,itisagrowingprob- lemashumanactivitiesexpandanddisturbmoreandmoreof Earth’ssurface.
HowSoilIsEroded Soilerosionisanaturalprocess;itispartoftheconstantrecycling ofEarthmaterialsthatwecalltheroclccycle, erosionalforces,especiallywaterandwind,movesoilcomponents fromoneplacetoanother,raindropsstrikethe landwithsurprisingforce(f".~“'?i*' Eachdropactslikeatiny bomb,blastingmovablesoilparticlesout oftheirpositionsinthe soilmass,waterflowingacrossthesurfacecarriesawaythe dislodgedsoilparticles ofwater,thisprocessistermedsheeterosion. Afterathin,unconfinedsheethasflowedforarelativelyshort distance, threadsofcurrenttypicallydevelopandtinychannels calledrillsbegintoform,knownas gullies,arecreatedasrillsenlarge(1? -s Whennormal farmcultivationcannoteliminatethechannels,weknowtherills havegrownlargeenough tobecalledgullies- lodgedsoilparticlesmoveonlyashortdistanceduringeachrain- fall,substantialquantitieseventuallyleavethefieldsandmake theirwaydownslopetoastream, thesesoilparticles,whichcannowbecalledsediment,aretrans- porteddownstreamandeventuallydeposited.
RatesofErosion Weknowthatsoilerosionistheultimatefateofpracticallyall soils,erosionoccurredatslowerratesthanitdoes todaybecausemoreofthelandsurfacewascoveredandprotected
SoilErosion
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i?"?ii3¥.iRi*r'15Globalsoilregions Taxonomy's 12 SO11orders.(AfterU,Natural ResourcesConservationService,WorldSoilResourcesStaff)
bytrees,shrubs,grasses,andotherplants,human activitiessuchasfarming,logging, andconstruction,which removeordisruptthenaturalvegetation,havegreatlyaccelerated therateofsoilerosion,the soilismoreeasilysweptawaybythewindorcarrieddownslope bysheetwash. Naturalratesofsoilerosionvarygreatlyfromoneplaceto anotheranddepend onsoilcharacteristicsaswellassuchfac- torsasclimate,slope,andtypeofvegetation, erosioncausedbysurfacerunoffmaybeestimatedbydeter- miningthesedimentloadsofthestreamsthatdraintheregion. Whenstudiesofthiskindweremadeonaglobalscale,theyindi- catedthatpriortotheappearanceofhumans,sedimenttrans- portbyriverstotheoceansamountedtojustover 9 billionmetric tonsperyear(1metricton=1,000kilograms).Bycontrast,the amountofmaterialcurrentlytransportedtotheseabyriversis
Fitiiiiir 4 inWhenrtrsraining,millionsofwaterdropsarefalling atvelocitiesapproaching 10 meterspersecond(35kilometersper hour) Whenwaterdropsstrikeanexposedsurface,soilparticlesmay splashashighas 1 metermtothearrandlandmorethanameter awayfromthepointofrarndropimpactSoildislodgedbysplash erosionISmoreeasilymovedbysheeterosion(Photocourtesyof USDAfNaturalResourcesConservationService)
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WeatheringCreatesOreDeposits 101
Fifiiiilfi4 Dakota, especiallywhenthefieldsarebare anddeflectitupward,whichdecreasesthelossoffinesoilparticles. B-round protectionfromwinderosionforthiscropland.(PhotosbyErwinC U.S./NaturalResourcesConservationService)
CONCEPTcnscx4
QHowhavehumanactivitiesaffectedtherateofsoilerosion? QWhataretwodetrimentaleffectsofsoilerosionasidefromthe lossoftopsoil?
Weathering Creates
OreDeposits
Weatheringcreatesmanyimportantmineraldepositsbycon- centratingminoramountsofmetalsthatarescatteredthrough unweatheredrockintoeconomicallyvaluableconcentrations. Suchatransformationisoftentermedsecondaryenrichment andtakesplaceinoneoftwoways,chemical weatheringcoupledwithdownward-percolatingwaterremoves undesiredmaterialsfromdecomposingrock,leavingthedesired elementsenrichedintheupperzonesofthesoil isbasicallythereverseofthefirst,thedesirableelements thatarefoundinlowconcentrationsnearthesurfaceareremoved andcarriedtolowerzones,wheretheyareredepositedand becomemoreconcentrated.
Bauxite Theformationofbauxite,the principaloreof aluminum,isoneimportantexampleofanore createdasaresultofenrichmentbyweathering processes(Fi~Jti-tis).Althoughaluminumisthe thirdmost abundantelement inEarth’scrust, economicallyvaluableconcentrationsofthisimpor- tantmetalarenotcommonbecausemostalu- minumistiedupinsilicatemineralsfromwhichit isextremelydifficulttoextract. Bauxiteformsinrainytropicalclimates aluminum-richsourcerocksaresubjectedtothe __p_m intenseandprolongedchemicalweathering . ofthetropics,mostoftl1ecommonelements, l includingcalcium,sodium,andpotassium, -3 areremovedbyleaching isextremelyinsoluble,itbecomesconcen- tratedinthesoil(asbauxite,ahydrated aluminumoxide).Thus,theformationof bauxitedependsonclimaticconditionsin whichchemicalweatheringandleachingare pronounced,plus,ofcourse,thepresenceof aluminum-richsourcerock manner,importantdepositsofnickeland cobaltdevelopfromigneousrocksrichinsil- icatemineralssuchasolivine. Thereissignificantconcernregardingtheminingofbauxiteand otherresidualdepositsbecausetheytendtooccurinenviromnen- tallysensitiveareasofthetropics oftropicalvegetation,thusdestroyingrainforestecosystems- over,thethinmoisture-retaininglayeroforganicmatterisalsodis- turbed,itbecomesbricldike andlosesitsmoisture-retainingqualities- ductivelyfarmednorcanitsupportsignificantforestgrowth long-termconsequencesofbauxiteminingareclearlyofconcernfor developingcountriesinthetropicswherethisimportantoreismined.
FiC§URE4 weatheringprocessesundertropicalconditions redorbrowntonearlywhite.(PhotobyE)
102 CHAPTER 4 Weathering,Soil,andMassWasting
OtherDeposits
Many copper andsilverdepositsresultwhenweathering processesconcentratemetalsthataredispersedthroughalow- gradeprimaryore containingpyrite(FeS2),themostcommonandwidespread sulfidemineral weathers,sulfuricacidforms,whichenablespercolatingwaters todissolvetheoremetals,themetalsgradually migratedownwardthroughtheprimaryorebodyuntiltheyare precipitated inthechemistryofthesolutionwhenitreachesthegroundwater zone(thezonebeneaththesurfacewhereallporespacesarefilled withwater).Inthismanner,thesmallpercentageofdispersedmetal canberemovedfromalargevolumeofrockandredepositedasa higher-gradeoreinasmallervolumeofrock.
####### CONCEPTcnscrc4 1
0 Howcanweatheringcreateanoredeposit?Whatimportant oreisanexample?
Mass Wasting:
The Work of Gravity
Landslidesarespectacularexamplesofabasicgeologicprocess calledmasswasting movementofrock,regolith,andsoilunderthedirectinfluence ofgravity examinedinsubsequentchaptersbecausemasswastingdoesnot requireatransportingmediumsuchaswater,wind,orglacialice. Earth’ssurfaceisneverperfectlyflatbutinsteadconsistsof slopes;othersaremoderateor
gentle;othersareshortandabrupt. Someslopesaremantledwithsoilandcoveredbyvegetation;oth- ersconsistofbarrenrockandrubble great, theyarenotstaticfeaturesbecausetheforceofgravitycauses materialtomove andpracticallyimperceptible- sistofaroaringdebrisfloworathunderingrockavalanche- slidesareaworldwidenaturalhazard(Figure4).Whenthese naturalprocessesleadtolossoflifeandproperty,theybecome naturaldisasters. Mostmasswasting,whetherspectacularorsubtle,isthe resultofcircumstancesthatarecompletelyindependentof humanactivities anticipated, stepscanoftenbetakentocontroldownslopemovementsor limitthedamagesthatsuchmovementscancause- tialformasswastinggoesunrecognizedorisignored,theresults canbecostlyanddangerous mostdownslopemovementsoccurwhetherpeoplearepresent ornot,manyoccurrencesareaggravatedoreventriggeredby humanactions.
TheRoleofMassWasting Intheevolutionofmostlandforms,masswastingisthestepthat followsweathering,weatheringdoesnotproduce significantlandforms,landformsdevelopastheproducts ofweatheringareremovedfromtheplaceswheretheyoriginate. Onceweatheringweakensrockandbreaksitapart,masswasting transfersthedebrisdownslope,whereastream,actingasa conveyorbelt,usuallycarriesitaway(seeFigure4,p). Althoughtheremaybemanyintermediatestopsalongtheway, thesedimentiseventuallytransportedtoitsultimatedestination: thesea.
itiil-itiiiii-’i;ti%OnAugust6,2006,photographerHerbDunnwitnessedthisrockfallalongtheMercedRiverinCalifornia’sYosemiteNational Park,theimpactoffallingrockproducesanexplosion ofdustanddebris,therockfalltriggersachainreaction treesalongtheway.(©DunnfiightPhotograph)
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Chapter 04
Course: Secondary education (bio sci)
University: Eastern Visayas State University
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