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FOCUSomCONCEPTS

Howdoesmagmadifferfromlava?

ii;Whataretheagentsofmetamorphism?

0 Whatistherockcycleandwhyisitimportant?

't"t" hystudyrocks?Youhavealreadylearnedthatsomerocksandmineralshavegreateco- nomicvalue,allEarthprocessesdependinsomewayonthepropertiesofthese basicEarthmaterials,mountainbuilding,weathering, erosion,andevenearthquakesinvolverocksandminerals,abasicknowledgeofEarth p materialsisessentialtounderstandingmostgeologicphenomena. l Everyrockcontainscluesabouttheenvironmentinwhichitformed,somerocksare composedentirelyofsmallshellfragments shallowmarineenvironment eruptionordeepintheEarthduringmountainbuilding(;".1=‘ ’v).Thus,rockscontainawealthof . informationabouteventsthathaveoccurredover'Earth’slonghistory.

Toassistyouinlearningtheimportantconceptsinthischapter,focusonthefollowingquestions:

reWhatarethethreegroupsofrocksandthegeologicprocessesinvolvedintheformationofeach?

Whattwocriteriaareusedtoclassifyigneousrocks? Howdoestherateofcoolinginfluencethecrystalsizeofmineralsinigneousrocks? ifsWhatarethenamesandenvironmentsofformationforsomecommondetritalandchemicalsedimentaryrocks? ti;Whatprocesseschangesedimentintosedimentaryrock? , Whataretwocharacteristicsofsedimentaryrocks? Whataremetamorphicrocks,andhowdotheyform?

(T)Whatarethenames,textures,andenvironmentsofformationforsomecommonmetamorphicrocks?

Earth as a System:

TheRock Cycle

EarthMaterials

U’!95-?"":uZ-1OI'l"'| B»TheRockCycle

Earthasasystemisillustratedmostvividlywhenweexamine therockcycle interactionsamongthemanycomponentsandprocessesofthe Earthsystem Ithelpsusunderstandtheoriginof igneous,sedimentary,andmetamorphicrocksandhowtheyare connected,therockcycledemonstratesthatanyrock type,undertherightcircumstances,canbetransformedintoany othertype.

TheBasicCycle

Webeginourdiscussionoftherockcyclewithmoltenrock,called magma,whichformsbymeltingthatoccursprimarilywithin Earth’scrustanduppermantle,amagmabody oftenrisestowardthesurfacebecauseitislessdensethanthe surroundingrock’ssurface

whereiterupts as lava. Eventually,moltenrockcoolsand solidifies,aprocesscalledcrystallizationorsolidification rockmaysolidifyeitherbeneaththesurfaceor,followinga volcaniceruption,atthesurface,theresulting rocksarecalledigneousrocks. Ifigneousrocksareexposedatthesurface,theyundergo weathering,inwhichthedailyinfluencesoftheatmosphere slowlydisintegrateanddecomposerocks thatresultareoftenmoveddownslopebygravity,andthen pickedup andtransportedbyoneormoreerosionalagents- runningwater,glaciers,wind,orwaves,theseparti- clesanddissolvedsubstances,calledsediment,aredeposited. Althoughmostsedimentultimatelycomestorestintheocean, othersitesofdepositionincluderiverfloodplains,desertbasins, swamps,andsanddunes. Next,thesedimentsundergolitliifzTca.,atermmeaning “conversion into rock.” Sediment is usually lithifiedinto seclirnentmyrockwhencompactedbytheweightofoverlying materialsorwhencementedaspercolatinggroundwaterfillsthe poreswithmineralmatter. Iftheresultingsedimentaryrockbecomesdeeplyburiedoris involvedinthedynamicsofmountainbuilding,itwillbesub- jectedtogreatpressuresandintenseheat

54 CHAPTER 3 Rocks:MaterialsoftheSolidEarth pg

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Magma

Magmaformswhen rockmeltsdeep beneathEarth's surface.

Metamorphic Rock

Lava

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pressure(stress)causeit tobecomemetamorphicrock.

Whensedimentary rockisburieddeep inthecrust,heatand

ROCK CYCLE

Whenmagmaorlava coolsandsolidifies, 6 igneousrockforms.

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Igneous Rock

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Sedimentis compactedandcemented toformsedimentaryrock.

FIGURE3,rocksareconstantlyforming,changing,andreforming usunderstandtheoriginof thethreebasicrockgroups.

IgneousRocks:“FormedbyFire" 55

Igneous Rocks:

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Inthediscussionoftherockcycle,wepointedoutthat igneousrocksformasmagmacoolsandcrystallizes. Butwhatismagmaandwhatisitssource?Magmais moltenrockgeneratedbypartialmeltingofrocksin Earth’smantle andinthelowercrustin smaller amounts elements foundinsilicate minerals. Siliconand oxygenarethemainconstituentsinmagma,with lesseramountsofaluminum,iron,calcium,sodium, potassium,magnesium, andothers also containssomegases,particularlywatervapor,which areconfinedwithinthemagmabodybytheweight (pressure)oftheoverlyingrocks. Onceformed’emagmabodyeueyeneyfleestowardtheemfeee atomsaresystematicallyaddedtothesecentersofcrystalgrowth. becauseitislessdensethanthesurroundingrocks whentheCrystalsgrowlergeenoughfortheiredoestomeet,their moltenreekFeeeheetheSuffeeerWhereitiseeuedlave growthceasesforlackofspace,alloftheliquidis Sometimes,lavaisemittedasfountainsproducedwhenescaping transformedintoeSolidmessofinteflooloogolystele gasespropelmoltenrockskyward,magmais TheroteofCoolingStronglyinfluencesorystelSizeIfemagma explosivelyejectedfromvents,producingaspectaculareruption ooolsVerySlowly’itallowsatomstomiemteovergreetdisteooeeCoo_ Suchasthe 1980 empeenofMountSt‘Helene‘However’meeterup' sequently,slowcoolingresultsintheformationoflargecrystals tionsarenotviolent;rather,volcanoesmoreoftenemitquietout- otherhood,ifooojl-nooooursrapidly’theatomslosetheirmotionend peuringeefleve quicklycombine Igneousreeksthatformwhenmoltenreeksolidifiesatthe allcompetefortheavailableatoms,rapidcoolingresultsin surfaceareclassifiedasextrusiveorvolcanic(aftertheRoman fl.,efo,.moo§on_ofosolidmossofsmo”mte,.g,.owneoomlo firegodVulcan).Extrusiveigneousrocksareabundantinwestern IfegeologistencountersigneousrookContainingCrystalslarge portionsoftheAmericas,includingthevolcanicconesoftheCas- enoughtobeSeenwiththeunaidedeye)itmeansitformedfrom cadeRangeandtheextensivelavaflowsoftheColumbiaPlateau. moltenrookwhichCooledSlowlyatdepthButiftheCrystalscan Inaddition, manyoceanicislands,includingtheHawaiian beSeenoleero,onlywithemicroscope’thegeologistknowsthat Islands,arecomposedalmostentirelyofvolcanicigneousrocks. themagmaCooledquickly,atornee].EerthrsSurfeee_ Mostmagma,however,losesitsmobilitybefore reachingthesurfaceandeventuallycrystallizesdeep MountRushmoreNationalMemorial,locatedintheBlackHillsofSouth belowtheSurfaceIgneousrocksthatformatdepth Dakota,iscarvedfromtheintrusiveigneousrockgranite cooledveryslowlyatdepthandhassincebeenupliftedandtheoverlyingrocks strippedawaybyerosion.(PhotobyBarbaraA/Shutterstock)

ijiiiiieti FluidbasalticlavaemittedfromHawaii'sKilaueaVolcano.(PhotobyJoe Carini/Photolibrary)

aretermedintrusiveorplutonic(afterPluto,thegod ofthelowerworldinclassicalmythology).Intrusive igneousrocksremainatdepthunlessportionsofthe crustareupliftedandtheoverlyingrocksstripped ' - awaybyerosion rocksoccurinmanyplaces,includingMountWash- ington,NewHampshire;StoneMountain,Georgia; MountRushmoreintheBlackHillsofSouthDakota; andYosemiteNationalPark,California(i.=‘.igtrr‘e3%).

FromMagmatoCrystallineRock - -

Magmaisaveryhot,thickfluidthatcontainssolids -r (mineralcrystals)andgases magmabodyiscomposedofatomsthatmoveabout freely,randommovementsslow, andatomsbegintoarrangethemselvesintoorderly patterns—aprocesscalledcrjystallization continues,numeroussmallcrystalsdevelopand

IgneousRocks:“FormedbyFire" 57

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Scoriaisavolcanicrockthatisvesicular asgasbubblesescapenearthe topofalavaflow.(Photocourtesyof E)

individualmineralscanbeidentifiedwiththeunaidedeye- iteisaclassicexample(Figure3). Alargemassofmagmalocatedatdepthmayrequiretensof thousands,evenmillions,ofyearstosolidify crystallizeunderdifferentenvironmentalconditions(tempera- ture,pressure),itispossibleforcrystalsofonemineraltobecome quitelargebeforeothersevenstarttoform containingsomelargecrystalsmovetoadifferentenvironment- forexample,byeruptingatthesurface-—theremainingmolten portionofthelavawouldcoolmorequickly, whichwillhavelargecrystalsembeddedinamatrixofsmaller crystals,issaidtohaveaporphyritictexture(Figure3). Duringsomevolcaniceruptions,moltenrockisejectedinto theatmosphere,whereitisquenchedquickly generaterockhavingaglassytexture(Figure3).Glassresults whenunorderedatomsare“frozeninplace”beforetheyareable touniteintoanorderlycrystallinestructure,mag- mascontaininglargeamountsofsilica(SiO2)aremorelikelyto formrocksthatexhibitaglassytexturethanthosewithalowsil- icacontent. Obsidian,acommontypeofnaturalglass, issimilarin appearancetoadarkchunkofmanufacturedglass Anothervolcanicrockthatusuallyexhibitsaglassytextureis pumice,pumiceformswhenlarge amountsofgasescapefrommoltenrocktogenerateagray,frothy mass Insomesamples,thevesiclesarequitenotice- able,whereasinothers,thepumiceresemblesfineshardsof intertwinedglass-filledvoids, manysamplesofpumicewillfloatinwater(Figure3).

IgneousCompositions

Igneousrocksarecomposedmainlyofsilicateminerals analysisshowsthatsiliconandoxygen—-usuallyexpressedasthe silica(SiO2)contentofamagma—arebyfarthemostabundant

Students SometimesAsk...

YoumentionedthatNativeAmericansusedobsidianfor makingarrowheadsandcuttingtools materialtheyused? depositsmadeofsilicacalled whatevermaterialswerelocally jasper,chert,opal,flint,andeven availabletomaketools,includingjade anyhard,compactrockmaterial havealimitedgeographic thatcouldbeshaped distributionandcannowhelp includesmaterialssuchasthe anthropologistsreconstructtrade metamorphicrocksslateand routesbetweendifferentgroups quartziteandsedimentary ofIndians.

constituentsofigneousrocks,plusionsof aluminum(Al),calcium(Ca),sodium(Na), potassium(K), magnesium(Mg),andiron(Fe),makeuproughly 98 percentby weightofmostmagmas,magmacontainssmall amounts ofmanyotherelements, includingtitanium and manganese,andtraceamountsofmuchrarerelementssuchas gold,silver,anduranium. i Asmagmacoolsandsolidifies,theseelementscombineto formtwomajorgroupsofsilicateminerals richinironand/ormagnesiumandarerelativelylowinsilica (S102).Olivine,pyroxene,amphibole,andbiotitemicaarethe commondarksilicatemineralsofEarth’scrust,the lightsilicatescontaingreateramountsofpotassium,sodium,and calciumandarericherinsilicathandarksilicates includequartz,muscovitemica,andthemostabundantmineral group,thefeldspars 40 percentofmost igneousrocks,inadditiontofeldspar,igneousrockscontain somecombinationoftheotherlightand/ordarksilicateslisted earlier.

Obsidian,anaturalglass,wasusedbyNative Americansformakingarrowheadsandcuttingtools.(Photoby E;insetphotobyJeffreyScovil)

58 CHAPTER 3 Rocks:MaterialsoftheSolidEarth

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iiifiifit‘:Pumice,aglassyrock,isverylightweightbecauseit containsnumerousvesicles.(PhotobyE;insetphotobyChipClark)

ClassifyingIgneousRocks Igneous rocks are classifiedby theirtexture and mineral composition itscoolinghistory,whereasitsmineralcompositionislargelya consequenceofthechemicalmakeupoftheparentmagmaand theenvironmentofcrystallization. Despitetheirgreatcompositionaldiversity,igneousrockscan bedividedintobroadgroupsaccordingtotheirproportionsof lightanddarkminerals textureandmineralcompositionisprovidedini§‘it;;iit*e3.§i.

Granitic(Felsic)Rocks Nearoneendofthecontinuumare rockscomposedalmostentirelyoflight-coloredsilicates—quartz

andpotassiumfeldspar dominantmineralshaveagraniticcomposition refertograniticrocksasbeingfelsic,atermderivedfromfeldspar andsilica(quartz).Inadditiontoquartzandfeldspar,mostgranitic rockscontainabout 10 percentdarksilicateminerals,usuallybiotite micaandamphibole(about 70 percent)andaremajorconstituentsofthecontinentalcrust. Graniteisacoarse-grainedigneousrockthatformswhere largemassesofmagmaslowlysolidifyatdepth ofmountainbuilding,graniteandrelatedcrystallinerocksmay beuplifted,wheretheprocessesofweatheringanderosionstrip awaytheoverlyingcrust areexposedatthesurfaceincludePikesPeakintheRockies, MountRushmoreintheBlackHills,StoneMountaininGeorgia, andYosemiteNationalParkintheSierraNevada(i.="i‘5:§ili"'";€- iiii). Graniteisperhapsthebest-knownigneousrock inpartbecauseofitsnaturalbeauty,whichisenhancedwhen polished,andpartlybecauseofitsabundance granitearecommonlyusedfortombstones,monuments,and countertops. Rliyoliteistheextrusiveequivalentofgraniteand,likewise,is composedessentiallyoflight-coloredsilicates(Figure3).This factaccountsforitscolor,whichisusuallybufftopinkorlightgray. Rhyoliteisfine-grainedandfrequentlycontainsglassfragmentsand voids,indicatingrapidcoolinginasurfaceenvironment- trasttogranite,whichiswidelydistributedaslargeintrusivemasses, rhyolitedepositsarelesscommonandgenerallylessvoluminous. YellowstoneParkisonewell-known flowsandthickashdepositsofrhyoliticcompositiona.

i?jiiI§‘iiii.%i3 Classificationofthemajorgroupsofigneousrocksbasedontheirmineralcompositionandtexture-grainedrocksare plutonic,solidifyingdeepunderground-grainedrocksarevolcanic,orsolidifyasshallow,thinplutons,dense rocks,composedalmostentirelyofmineralscontainingironandmagnesium’ssurface,theserocksare believedtobemajorconstituentsoftheuppermantle.

Chemical ii Granitic Composition (Folsio)

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60 CHAPTER 3 Rocks:MaterialsoftheSolidEarth p

Basaltic(Mafic)Rocks Rocksthatcontainsubstantialdark silicatemineralsandcalcium-richplagioclasefeldspar(butno quartz)aresaidtohaveabasalticcomposition(Figure3). Basalticrockscontainahighpercentageofdarksilicateminerals, sogeologistsalsorefertothemasmafic(frommagnesiumand ferrum,theLatinnameforiron).Becauseoftheirironcontent, basalticrocksaretypicallydarkeranddenserthangraniticrocks. Basalt,themostcommonextrusiveigneousrock,isaverydark greentoblack,fine-grainedvolcanicrockcomposedprimarilyof pyroxene,olivine,andplagioclasefeldspar, suchastheHawaiianIslandsandIceland,arecomposedmainly

FIGURE 3312 BasalticlavaflowingfromKilaueavolcano,Hawaii. (PhotobyRogerRessmeyer/CORBIS)

_ .... ...-,:_ _ ._..- _ -3.,( ,. _-__, _:J 1 I-2s;“ :4.'. -'

ofbasalt(Fiijilifté Ftuthermore,theupperlayersoftheoceanic crustconsistofbasalt,largeportionsofcen- tralOregonandWashingtonwerethesitesofextensivebasaltic outpourings. Thecoarse-grained,intrusiveequivalentofbasaltisgabbro (Figure3).Althoughgabbroisnotcommonlyexposedatthe surface,itmakesupasignificantpercentageoftheoceaniccrust.

Andesitic(Intermediate)Rocks AsyoucanseeinFigure3, rockswithacompositionbetweengraniticandbasalticrocksare saidtohaveanandesiticcomposition,afterthecommonvol- canicrockandesite,orintermediatecomposition rockscontainamixtureofbothlight-anddarl<-coloredminerals, mainlyamphiboleandplagioclasefeldspar- egoryofigneousrocksisassociated withvolcanicactivitytypicallycon- finedtocontinentalmargins magmaofintermediatecomposi- tioncrystallizesatdepth,itforms the coarse-grained rock called diorite(Figure3).

UltramaficRocks Anotherimpor- tantigneousrock,peridotite,contains mostlythedark-coloredminerals olivineandpyroxeneandthus,falls ontheoppositesideofthecomposi- A tionalspectrumfromgraniticrocks (Figure3).Becauseperidotiteis composedalmostentirelyofdark silicateminerals,itschemicalcom- positionisreferredtoasultramafic. Althoughultramaficrocksarerareat

Earth’ssurface,peridotiteisbelieved -- tobethemainconstituentofthe uppermantle.

; HowDifferent Q. r IgneousRocksForm Becausealargevarietyofigneous rocksexist,itislogicaltoassumethat , anequallylargevarietyofmagmas alsoexist,geologistshave observedthatasinglevolcanomay extrudelavasexhibitingquitediffer- '5 entcompositions ledthemtoexaminethepossibility thatmagmamightchange(evolve) andthusbecometheparenttoa varietyofigneousrocks

B ._‘|s 9.. thisidea,apioneeringinvestigation -- intothecrystallizationofmagma wascarriedoutbyN firstquarterofthe20thcentury.

. Bowen'sReactionSeries Ina i laboratorysetting,Bowendemon- stratedthatmagma,withitsdiverse

IgneousRocks:“FormedbyFire" 61

Temperature Regimes

ii Bowen'sReaction- Series- c,,%';",ff§-$2?r

..............,......-w-.. Hightemperature I if (~1200°C)

Coongmagma

D49/O

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Bowen'sreactionseriesshowsthesequenceinwhichmineralscrystallizefromamagma mineralcompositionoftherockgroupsinFigure3 temperaturerange.

chemistry,crystallizesoveratemperaturerangeofatleast200°C, unlikesimplecompounds(suchaswater),whichsolidifyatspe- cifictemperatures,certainmineralscrystallize firstatrelativelyhightemperatures(topof :?:.*.i:3).Atsucces- sivelylowertemperatures,othermineralsbegintocrystallize arrangementofminerals,showninFigure3,becameknownas Bowen'sreactionseries. Bowendiscoveredthatthefirstmineraltocrystallizefroma bodyofmagmaisolivine ofpyroxene,aswellasplagiocla-sefeldspai:Atintermediatetem- peraturesthemineralsamphiboleandbiotitebegintocrystallize. Duringthelaststageofcrystallization,aftermostofthe magmahassolidified,themineralsmascot/iteandpotassium feldsparmayform(Figure3).Finally,quartzcrystallizesfrom anyremainingliquid sameigneousrock,becausequartzcrystallizesatmuchlower temperaturesthanolivine. Analysisofigneousrocksprovidesevidencethatthiscrystal- lizationmodelapproximateswhatcanhappeninnature- ticular,wefindthatmineralsthatforminthesamegeneral temperaturerangeonBowen’sreactionseriesarefoundtogether inthesameigneousrocks,noticeinFigure3 themineralsquartz,potassiumfeldspar,andmuscovite,located inthesameregionofBowen’sdiagram,aretypicallyfound togetherasmajorconstituentsoftheigneousrockgranite.

MagmaticDifferentiation Bowendemonstratedthatdifferent minerals crystallizefrommagmasystematically Bowen’sfindingsaccormtforthegreatdiversityofigneousrocks? Duringthecrystallizationprocess,thecompositionofmagma continuallychanges,they selectivelyremovecertainelementsfromthemagma,which

leavestheremainingliquidportiondepletedintheseelements. Occasionally,separationofthesolidandliquidcomponentsof magmaoccursduringcrystallization,whichcreatesdifferentmineral assemblages,calledcrystalsettling,occurs whentheearlierformedmineralsaremoredense(heavier)than theliquidportionandsinktowardthebottomofthemagma chamber,asshownin Whentheremainingmolten materialsolidifies—eitherinplaceorinanotherlocationifit migratesintofracturesinthesurroundingrocks-—itwlllformarock withachemicalcompositionmuchdifferentfromtheparentmagma (Figure3).Theformationofoneormoresecondarymagmas fi'omasingleparentmagmaiscalledmagmaticdifferentiation. Atanystageintheevolutionofamagma,thesolidandliquid componentscanseparateintotwochemicallydistinctunits- thermore,magmaticdifferentiationwithinthesecondarymagma cangenerateotherchemicallydistinctmassesofmoltenrock. Consequently,magmaticdifferentiationandseparationofthe solidandliquidcomponentsatvariousstagesofcrystallization canproduceseveralchemicallydiversemagmasand,ultimately, avarietyofigneousrocks.

AssimilationandMagmaMixing Strongevidencesuggeststhatthechemicalcompositionofamagma canchangebyprocessesotherthanmagmaticdifferentiation example,asmagmamigratesupwardthroughthecrust,itmay incorporatesomeofthesurroundinghostrock,aprocesscalled assimilation bodycanbealterediscalledmagmamzlxing wheneveronemagmabodyintrudesanotherthathasadifferent composition,convectiveflowmaystirthetwo magmastogenerateafluidwithadifferentcomposition.

SedimentaryRocks:CompactedandCementedSediment 63

evenhouseholddustareexamplesofsedimentproducedbythis never-endingprocess. Theweatheringofbedrockandthetransportanddeposition oftheweatheringproductsarecontinuous,sediment isfoundalmosteverywhere,the materialsnearthebottomarecompactedbytheweightofthe overlyinglayers,thesesedimentsarecemented togetherbymineralmatterdepositedfromwaterinthespaces betweenparticles. Geologistsestimatethatsedimentaryrocksaccountforonly about 5 percent(byvolume) ofEarth’souter 16 kilometers (10miles).However,theimportanceofthisgroupofrocksisfar greaterthanthispercentageimplies exposedatEarth’ssurface,youwouldfindthatthegreatmajorityare sedimentary(1ii*itgi"ii't;;<=..:~‘=;115;).Indeed,about'7 5 percentofallrockout- cropsonthecontinentsaresedimentary,wecanthink ofsedimentaryrocksascomprisingarelativelythinandsomewhat discontinuouslayerintheuppermostportionofthecrust makessensebecausesedimentaccumulatesatthesurface. Itisfromsedimentaryrocksthatgeologistsreconstructmany detailsofEarth’shistory varietyofdifferentsettingsatthesurface,therocklayersthatthey eventuallyformholdmanycluestopastsurfaceenvironments. Theymayalsoexhibitcharacteristicsthatallowgeologiststodeci- pherinformationaboutthemethodanddistanceofsediment

StudentsSometimesAsk...

WhyarethesedimentaryrocksinFigure3? Inthewesternand southwesternUnitedStates, steepcliffsandcanyonwalls madeofsedimentaryrocks oftenexhibitabrilliantdisplay ofdifferentcolors's CanyonlandsNationalParkis onegoodexample(Figure3). AnotherisArizona’sGrand Canyonwherewecansee layersthatmaybered,orange, purple,gray,brown,andbuff (seeFigure11,p). Sedimentaryrocksinmore

humidplacesarealsocolorful, buttheyareusuallycovered bysoilandvegetation. Themostimportant “pigments"areironoxides, andonlyverysmallamounts areneededtocolorarock. Hematitetintsrocksredorpink, whereaslimoniteproduces shadesofyellowandbrown. Whensedimentaryrocks containorganicmatter,it oftencolorsthemblackorgray.

transport,itissedimentaryrocksthatcontainfos- sils,whicharevitalevidenceinthestudyofthegeologicpast. Finally,manysedimentaryrocksareimportanteconomically. Coal,whichisburnedtoprovideasignificantportionofU-

_Fftft;';3.'1'sCanyonlandsNationalPark'75percent ofallrockexposuresoncontinentsaresedimentaryrocks.(PhotobyJeffGnass)

64 CHAPTER 3 Rocks:MaterialsoftheSolidEarth

DetritalSedimentaryRocks

ClasticTexture(artidesize) Sedlment. Name D l

""-i.'.-‘=7-'°a'.-17'7"It‘l"‘-3':o\b‘_C.‘t“u.~~l?g:'b*".'-1-:-''?c.';.Q0-0u'1:.,;,.-ia.".:n@‘laulilfi '7}l‘C1-i.'.;,-;"..‘I‘.on",‘Ql‘¢'-0'!‘-"'0atQ.'' 1 '-,:;-Iw’v:‘.'OJr-lat"..-~~..','E

,5 Q‘- Gravel C Coarse 1 - ____________ (over 2 mm) J Gravel $4-5‘ (Angularparticles) Breccia .5ll|‘;."'1§\‘,)1:Q|j*:t*0ue'‘U__.:;‘-0- IgaitQ-:. l‘Il>IJ"§0°‘:;,'.:‘‘QC’;0°! 03 fig!-5-",1» s d ll‘¢-‘'0'»-luv'-1‘»I|"-'Fg-'9-#.,°',aas-Lilif‘' r~ 7 .0‘;-Q-' mas‘.‘J1"‘{Q (1/1|\g(‘j;g';nI1_|m)- "-in9! -‘e_E;ltr? (Ifabundantfeldspar *" gig? ispresenttherock I 3,3’ iscalledArkose) :.. Un 1 *-1:’r. 9 = Ei 1 5‘-‘§.'_=8Ur=:.;a..i=.>F.-:.ra 0 ‘vi'90:ll,»5‘3: 0 l tF.'1?—f>T:'=31'-5-Y‘-‘‘-3"’- l Fine

####### 1/256mm) .. lI

Siltstone

RockName

i§.§§Q' (Roundedparticles) l-congwmerate a ~ -~ ~o ~ Finetocoarse h h

.__.-, _m-

Sandstone

ChemicalandOrganicSedimentaryRocks K: lll l‘ :> .:E:I Composition :~.itr Texture RockName ‘ l . Crystalline ti ‘ Nonclastic: , Limestone ii

§oQ3m:oo~mlI-I-m(D§—-I"'

crystalline _~ Travertine |I| : 1 i,1, __ 21 , Clastic:Visible shellsandshell '. l? Camite,Caco fragmentsloosely 5 oquma _:1’ = 3 cemented ; 1 it Clasticzvarioussizel - ll. ShellsandShell i§tFossiliferous. ‘ fragmentscementedi Limestone t ;. t withcalcitecement i ., Clastic:Microscopic DO' Veryfine l , (lessthan Mud § 0?; 1/256mm) Z

t ‘ii shellsandclay i. 3 Chalk _ ___ _ __ E ____ _ ___ill

CD ll .= Nonclastic:Very ,Chert(Iightcolored)il Q“a"Z'Slog I Flint(darkcolored)

_.=; Identificationofsedimentaryrocks rocksaredividedintotwomajorgroups,detritalandchemical, basedontheirsourceofsediment detritalrocksisparticlesize,whereastheprimarybasisfor distinguishingamongchemicalrocksistheirmineralcomposition.

tricalenergy,isclassifiedasasedimentaryrock energyresources(petroleumandnaturalgas)occurinpores withinsedimentaryrocks sourcesofiron,aluminum,manganese,andfertilizer,plusnrurrer- ousmaterialsessentialtotheconstructionindustry.

ClassifyingSedimentaryRocks Materialsaccumulatingassedimenthavetwoprincipalsources. First,sedimentsmayoriginateassolidparticlesfromweathered rocks,suchastheigneousrocksearlierdescribed arecalleddetritus,andthesedimentaryrocksthattheyformare calleddetritalsedimentaryrocks ._%i~‘). Thesecondmajorsourceofsedimentissolublematerial pro- ducedlargelybychemicalweathering- stancesareprecipitatedbackassolids,theyarecalledchemical sediment,andtheyformchemicalsedimentaryrocks lookatdetritalandchemicalsedimentaryrocks(Figure3). DetritalSedimentaryRocks Thoughawidevarietyofminerals androckfiagmentsmaybefoundindetritalrocks,clayminerals andquartzdominate,claymineralsarethe mostabundantproductofthechemicalweatheringofsilicate minerals,especiallythefeldspars,ontheotherhand,is abundantbecauseitisextremelydurableandveryresistantto chemicalweathering,whenigneousrockssuchasgranite areweathered,individualquartzgrainsaresetfree. Geologistsuseparticlesizetodistinguishamongdetritalsed- imentaryrocks particlesmakingupdetritalrocks/Vhengravel-sizeparticlespre- dominate,therockiscalledconglomerateifthesedimentisrounded Zi..’5.‘,-i."'.)andbrecciaifthepiecesareangular(Figure3).

l finecrystalline

-ww-a-wt--t._..

__ ____ _ .., - _ . . ‘llll Gypsum Nonclastic:Fineto CaSO,,2H2O coarsecrystalline * RockGypsum ~ - i Halite,NaCl i N°"°'aS“°‘Fl"?l°coarsec|y_s_talllne , RcckSalt .l;r. .-__.m I, _ ..-itll l t » Nonclastic: I Alteredragmentsplant Fine-grained. l 1 BituminousCoal l 1 organlcmatter 1 li - 1 llAll\A.‘IAI.-l..»_l»

Angularfragmentsindicatethattheparticleswerenottransported veryfarfromtheirsourcepriortodepositionandsohavenothad cornersandroughedgesabraded rockswhensand-sizegrainsprevail Shale,themost commonsedimentaryrock,ismadeofveryfine-grainedsediment ii).Siltstone,anotherratherfine-grainedrock,issome- timesdifficulttodifferentiatefromrockssuchasshale,whichare composedofevensmallerclay-sizesediment. Particlesizeisnotonlyaconvenientmethodofdividingdetrital rocks;thesizesofthecomponentgrainsalsoprovideusefulinfor- mationabouttheenvironmentinwhichthesedimentwasdeposited. Currentsofwaterorairsorttheparticlesbysize current,thelargertheparticlesizecarried,forexample,are movedbyswiftlyflowingrivers,rockslides,andglaciers isrequiredtotransportsand;thus,itiscommoninwindblowndunes, riverdeposits,andbeaches, accumulationsofthesematerialsaregenerallyassociatedwiththe quietwatersofalake,lagoon,swamp,ormarineenviromnent. Althoughdetritalsedimentaryrocksareclassifiedbyparticle size,incertaincasesthemineralcompositionisalsopartofnaming arock,mostsandstonesarepredominantlyquartz- rich,andtheyareoftenreferredtoasquartzsandstone, rocksconsistingofdetritalsedimentsarerarelycomposedofgrains ofjustonesize,arockcontainingquantitiesofboth sandandsiltcanbecorrectlyclassifiedassandysiltstoneorsilty sandstone,dependingonwhichparticlesizedominates.

66 CHAPTER 3 Rocks:MaterialsoftheSolidEarth

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A. FIGURE Chalkisasoft,porouslimestoneconsistingmainlyof hardpartsofmicroscopicorganisms,EastSussex, England ofwesternEurope,andweredepositedinamarineenvironmentduring theCretacousperiod.(PhotobyPrisms/Superstock)B debriscomprisingchalkdepositsconsistsofmicroscopicplates,called coccoliths,thataremainlydispersedfollowingthedeath ofsingle- celledalgae.(SteveGschmeissner/PhotoResearchers,Inc.)

chalk,asoft,porousrockmadeupalmostentirelyofthehard partsofmicroscopicorganismsthatarenolargerthanthehead ofapin ChalkCliffs exposedalongthesoutheastcoastofEngland (Figtrre Inorganiclimestoneformswhenchemicalchangesorhigh watertemperaturesincreasetheconcentrationofcalciumcar- bonatetothepointthatitprecipitates,thetypeof limestonethatdecoratescaverns,isoneexample- teristhesourceoftravertinethatisdepositedincaves drops reachtheairinacavern,someofthecarbondioxide dissolvedinthewaterescapes,causingcalciumcarbonateto precipitate. Dissolvedsilica(SiO2)precipitatestoformvarietiesofmicro- crystallinequartz(Figure3).Sedimentaryrockscomposedof microcrystallinequartzincludechert(lightcolor),flint(dark),

jasper(red),andagate(banded).Thesechemi- hi calsedimentaryrocksmayhaveeitheran inorganicorbiochemicalorigin,butthe , modeoforiginisusuallydifficultto 1' determine. Veryoften,evaporationcauses . mineralstoprecipitatefromwater. Suchmineralsincludehalite,the chiefcomponentofrocksalt,and -a§§.§,',, gypsum, the main ingredient of it rockgypsum r significantcommercialimportance. Haliteisfamiliartoeveryoneasthe commonsaltusedincookingandsea- B soningfoods,ithasmanyother usesandhasbeenconsideredimportant enoughthatpeoplehavesought,traded,and foughtoveritformuchofhumanhistory. GypsumisthebasicingredientofplasterofParis materialisusedmostextensivelyintheconstructionindustry for“drywall”andplaster. Inthegeologicpast,manyareasthatarenowdrylandwere coveredbyshallowarmsoftheseathathadonlynarrow

zl-W’

FIGURE3 Chertisanameusedforanumberofdense,hardrocks madeofmicrocrystallinequartz. A.(PhotobyJeffreyA)B colorofflintresultsfromorganicmatter.(PhotobyE)C redvariety,calledjasper,getsitscolorfromironoxide.(PhotobyE. Tarbuck)D toolsfromchert.(PhotobyLAVENTA/CORBIS/SYGMA)

‘N 3Q;

,_J,_~?,. g ff? .1. . ___ -5r:.‘;>Z-‘i -:;____ ftg ‘;E_' I-'.?j."""-'~.$5‘.l 1 '2'f‘ A

B C D

SedimentaryRocks:CompactedandCementedSediment 67

connectionsto theopenocean,water continuallymovedintothebaytoreplacewaterlostbyevapo- r s_s. ration,thewatersofthebaybecamesaturatedand saltdepositionbegan,thesearmsoftheseaaregone, andtheremainingdepositsarecalledevaporitedeposits. Onasmallerscale,evaporitedepositscanbeseeninsuch placesasDeathValley,California,followingrainsorperi- odsofsnowmeltinthemountains,streamsflowfromsur- i roundingmountainsintoanenclosedbasin evaporates,saltflatsformfromdissolvedmaterialsleftbehind asawhitecruston theground(Figuire3). Coalisquitedifferentfromotherchemicalsedimentary A rocks,whicharecalciteor

.=-,.=-I.-;:,';,-1=, -1-.--<;..-.,,_=<|.I-»‘.». .1 '' > .'¢- ' 1|-.‘

(T.'|allEll‘ ,

..-~~..."~~|~~--‘-....-_:---gr-.,-,,-g _-__-H--;—-_--;.. nationofapieceofcoalunderamicroscopeormagnifyingglass. _~~.-. - 1 '-_..-III oftenrevealsplantstructuressuchasleaves,bark,andwood .. thathavebeenchemicallyalteredbutarestillidentifiable p supportstheconclusionthatcoalistheendproductofthebur- ialoflargeamountsofplantmaterialoverextendedperiods.

..=,r-_ I

Theinitialstageincoalformationistheaccumulationof 5

(Figrire2). ,

largequantitiesofplantremains,specialconditions arerequiredforsuchaccumulations,becausedeadplantsnor- mallydecomposewhenexposedto theatmosphere environmentthatallowsforthebuildupofplantmaterialisa ; p swamp-deficient, if completedecay(oxidation)oftheplantmaterialisnotpossi- ble,suchenvironments -' AET"?-__< -

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r,,,.....-

.'-"7.'.§ | "‘j havebeencommon- = .r=_-‘1-<--""5 '.5"-‘..,.*.,.‘-,. mation,highertemperaturesand pressuresdrive offimpuritiesandvolatiles, asshown in Figure3. Lignite and bituminous coals aresedimentaryrocks, butanthraciteisametamorphicrock

f. ;:1 §..

‘ii;»"-I...*~~..'rn.-=»‘~~“""’ lfill.‘‘lfllt.‘.@..~.-i>-»-

Q .Ji-:___ 1 ':-—*--;~.,_

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LIGNITE (Soft,browncoal) -l

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BITUMINOUS (Soft,blackcoal)

‘- Stress ANTHRACITE (Hard,blackcoal) FIGURE3%Successivestagesintheformationofcoal.

FIGURE3,Utah.(PhotobyNashPhotos/Photolibraiy)

E'l':I_\r-'I7-i

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sedimentarylayersaresubjectedto thefoldinganddeformationassoci- atedwithmountainbuilding. Insummary,wedividesedimen- taryroclcsintotwomajorgroups: detritalandchemical- terionforclassifyingdetritalrocksis particlesize,whereaschemicalrocks aredistinguishedbytheirmineral composition categories pre- sentedherearemorerigidthanisthe actualstateofnature sedimentaryrocksareamixtureof morethanoneparticlesize- more,manysedimentaiyrocksclas- sifiedaschemicalalsocontainatleast smallquantitiesofdetritalsediment, andpracticallyalldetritalrocksare cementedwith materialthatwas originallydissolvedinwater.

MetamorphicRocks:NewRockfromOld 69

Box3.

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TheCarbonCycleand

SedimentaryRocks

Toillustratethemovementofmaterialand energyintheEarthsystem,letustakea brieflookatthecarboncycle(Figure3). Innature,mostcarbonatomsarebonded chemicallytootherelementstoformcom- poundssuchascarbondioxide,calciumcar- bonate, andthehydrocarbonsfoundincoal andpetroleum buildingblockoflifeasitreadilycombines withhydrogenandoxygentoformthefun- damentalorganiccompoundsthatcompose livingthings. Intheatmosphere,carbonisfound mainlyascarbondioxide(CO2).Atmospheric carbondioxideissignificantbecauseitisa greenhousegas,whichmeansitisaneffi- cientabsorberofenergyemittedbyEarth andthusinfluencestheheatingofthe atmosphere thatoperateonEarthinvolvecarbondiox- ide,thisgasisconstantlymovingintoand outof theatmosphere,through theprocessofphotosynthesis,plantsabsorb carbondioxidefromtheatmospheretopro- ducetheessentialorganiccompounds neededforgrowth theseplants(orconsumeotheranimalsthat eatplants)usetheseorganiccompoundsas asourceofenergyand,throughtheprocess ofrespiration,returncarbondioxidetothe atmosphere,whenplantsdie anddecayorareburned,thisbiomassisoxi- dized,andcarbondioxideisreturnedtothe atmosphere. Notalldeadplantmaterialdecays result,overlongspansofgeologictime,con- siderablebiomassisburiedwithsediment. Undertherightconditions,someofthesecar- bon-richdepositsareconvertedtofossil fuels—coa1,petroleum,ornaturalgas- ally,someofthefuelsarerecovered(minedor

pumpedfromawell)andburnedtorunfacto- riesandfuelourtransportationsystem resultoffossil-fuelcombustionisthereleaseof hugequantitiesofCO2intotheatmosphere. Certainlyoneofthemostactivepartsofthe carboncycleisthemovementofCO2fromthe atmospheretothebiosphereandbackagain. Carbonalsomovesfromthegeosphere andhydrospheretotheatmosphereand backagain,volcanicactivity earlyinEarth’shistoryisthoughttobethe sourceofmuchofthecarbondioxidefound intheatmosphere dioxidemakesitswaybacktothehydro- sphereandthen tothesolidEarthisbyfirst combiningwithwatertoformcarbonicacid (HZCO3),whichthenattackstherocksthat composethegeosphere chemicalweatheringofsolidrockisthesol- ublebicarbonateion(2HCO3'),whichiscar- riedbygroundwaterandstreamstothe ocean-dwellingorganisms extractthisdissolvedmaterialtoproduce

hardpartsofcalciumcarbonate(CaCO3). Whentheorganismsdie,theseskeletal remainssettletotheoceanfloorasbiochem- icalsedimentandbecomesedimentaryrock. Infact,thegeosphereisbyfarEarth's largestdepositoryofcarbon,whereitisa constituentofavarietyofrocks,themost abundantbeinglimestone,the limestonemaybeexposedatEarth’ssur- face,wherechemicalweatheringwillcause thecarbonstoredintherocktobereleased totheatmosphereasCO2. Insummary,carbonmovesamongall fourofEarth'smajorspheres toeverylivingthinginthebiosphere theatmospherecarbondioxideisanimpor- tantgreenhousegas, carbondioxideisdissolvedinlakes,rivers, andtheocean thegeosphere,carbonis containedincarbonatesedimentsandsedi- mentaryrocksandisstoredasorganicmat- terdispersedthroughsedimentaryrocks andasdepositsofcoalandpetroleum.

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g / ' CO2enteringthe i I,»"F' % atmosphere Z ,-I .COleavingthe “-""4 atzmosphere

FIGURE3,withemphasisontheflowofcarbon betweentheatmosphereandthehydrosphere,geosphere,andbiosphere arrowsshowwhethertheflowofcarbonisintooroutoftheatmosphere

Metamorphic Rocks:

New Rock from Old

EarthMaterials ‘gm->""2:Z-lOI PMetamorphicRocks I Recallfromthediscussionoftherockcyclethatmetamorphism isthetransformationofonerocktypeintoanother rocksareproducedfrompreexistingigneous,sedimentary,or

evenothermetamorphicrocks,everymetamorphicrock hasaparentroclc—therockfromwhichitwasformed. Metamorphism,whichmeans“tochangeform,”isaprocess thatleadstochangesinthemineralogy,texture(e.,grainsize), andsometimesthechemicalcompositionofrocks- phismtakesplacewhenpreexistingrockissubjectedtoaphysi- calorchemicalenvironmentthatissignificantlydifferentfrom thatinwhichitinitiallyformed- tionstherockgraduallychangesuntilastateofequilibriumwith thenewenvironmentisreached

70 CHAPTER 3 _Rocks:MaterialsoftheSolidEarth

occurattheelevatedtemperaturesandpressuresthatexistinthe zonebeginningafewkilometersbelowEarth’ssurfaceand extendingintotheuppermantle. Metamorphismoftenprogressesincrementally,fromslight changes(low-grademetarnorphisrn)tosubstantialchanges(high- gradernetamorphism).Forexample,underlow-grademetamor- phism,thecommonsedimentaryrockshalebecomesthemore compactmetamorphicrockcalledslate rocksaresometimesdifficulttodistinguish,illustratingthatthe transitionfromsedimentarytometamorphicrockisoftengrad- ualandthechangescanbesubtle. Inmoreextremeenvironments,metamorphismcausesa transformationsocompletethattheidentityoftheparentrock cannotbedetermined-grademetamorphism,suchfea- turesasbeddingplanes,fossils,andvesiclesthatmayhaveexisted intheparentrockareobliterated,whenrocksdeep inthecrust(wheretemperaturesarehigh)aresubjectedto directedpressure,theentiremassmaydeform,producinglarge- scalestructuressuchasfolds(:i‘.igr:@_*af¢r.:l?), Inthe most extreme metamorphicenvironments, the temperaturesapproachthoseatwhichrocksmelt, daringmetamorphismtherockmustremainessentiallysolid, forifcompletemeltingoccurs,wehaveenteredtherealmof igneousactivity. Mostmetamorphismoccursinoneoftwosettings:

Whenrockisintrudedbymagma,contactorthermal metamorphismmaytakeplace,

changeiscausedbytheriseintemperaturewithinthe hostrocksurroundingthemassofmoltenmaterial. 2. Duringmountainbuilding,greatquantitiesofrockare subjectedtodirectedpressuresandhightemperatures associatedwithlarge-scaledeformationcalledregional metamorphism.

Extensiveareasofmetamorphicrocksareexposedonevery continent manymountainbelts,wheretheymakeupalargeportionofa mountain’scrystallinecore, whicharegenerallycoveredbysedimentaryrocks,areunderlainby metamorphicbasementrocks,themeta- morphicrocksareusuallyhighlydeformedandintrudedbyigneous masses,significantpartsofEarth’scontinentalcrust arecomposedofmetamorphicandassociatedigneousrocks.

WhatDrivesMetamorphism? Theagentsofmetamorphismincludeheat,pressure(stress),and chemicallyactivefluids,rocksareusually subjectedtoallthreemetamorphicagentssimultaneously. However,thedegreeofmetamorphismandthecontributionof eachagentvarygreatlyfromoneenvironmenttoanother.

HeatasaMetamorphicAgent Thermalenergy(heat)isthe most importantfactor driving metamorphism. It triggers chemicalreactionsthatresultintherecrystallizationofexisting

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FOCUSomCONCEPTS

Earth as a System:

TheRock Cycle

ROCK CYCLE

Igneous Rocks:

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Metamorphic Rocks:

New Rock from Old

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