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RocksandtheRockCyclenotesfromthetextbook,integratedwithoriginalcontributions

AlessandroGrippo,Ph.D.

Gneiss(ametamorphicrock)fromCatalinaIsland,California©AlessandroGrippo

review•  Rocks

–  naturallyformedaggregatesofminerals–  canbecomposedofmanyindividualmineralsofthesamekind(e.g.,

limestone)ordifferentminerals(e.g.,granite)–  somerocksarecomposedofnon-mineralsubstances(e.g.,obsidian,

coal)

•  Minerals–  solids–  thatarenaturallyoccurring–  areinorganic–  arecrystalline–  haveaspecificchemicalcomposition–  havedistinctivephysicalproperties

Rocks

•  Rocksareclassifiedintothreegroups,basedontheprocessesbywhichtheywereformed–  igneousrocks– sedimentaryrocks

– metamorphicrocks

IgneousRocks•  Solidrocksofanykindcanmeltinthe

uppermantleandinthecrust,cratingpocketsofmagma(orlava,ifmagmacomestothesurface)

•  Anyrockthatformsfromcoolingofmagmaorlavaiscalledanigneousrock

•  CoolingcanoccurslowlywithinEarth’ssurface(intrusiveigneousrocks)orquicklyatEarth’ssurface–forinstancebecauseofavolcaniceruption(extrusiveigneousrocks)

•  Mostigneousrocksshowinterlockingarrangementsofmineralcrystalsthatdevelopsasmagma(orlava)coolsandsolidifies

Interlockingcrystalsinapegmatiticgranite(intrusiveigneousrock)

white:Na-Plagioclase pink:K-feldspargray:Quartz black:Biotite

SedimentaryRocks•  AnyrockatornearEarth’ssurface

canbeweatheredintosediment–  brokendowninsmallerfragments,

calledgravel,sand,silt,clay–  dissolvedintoions,suchasNa+,

Ca2+,K+,Mg2+,etc.

•  Sedimentistransportedbyrunningwater,wind,ice;itisthendepositedandturnedintoasedimentaryrock

•  Sedimentaryrockscanbeclastic(detrital),chemical,organic

•  Theycharacteristicallydevelophorizontallayers

Roundedgrainsofgravelarecementedinthisclasticsedimentaryrockcalled

conglomerate

MetamorphicRocks•  Anyrocksubjecttoconditionsthatare

differentfromthoseunderwhichtheyoriginallyformed,arechanged,ormetamorphosedasasolid(withoutmelting,withoutneedingwater,likewhenyoubakeanapple)

•  metamorphismoccurswithinEarthanditismainlybycontact(withasourceofheat)orregional(inregionsofintense,non-uniformpressure)

Freshapplevs.bakedapple

(metamorphicapple)

Above:

contactmetamorphicrock(marble)

Below:

regionalmetamorphicrock(schist)

TheRockCycle

•  Wehavejustdescribedhowanykindofrockcanbeturned,giventherightconditions,intoanyotherkindofrock

•  ThesechangescanbesummarizedintowhatwecalltheRockCycle

TheRockCycle

RockTexturesandCompositions

•  Recognizingtextureandcompositionofrocksallowsustodistinguishamongthethreegroupsofigneous,sedimentary,andmetamorphic

•  Inafewwords:–  textureishowtherocklookslike– compositioniswhattherockismadeof

Texture

•  Texturereferstothesize,shape,and/orarrangementofitsmineralgrains–  Igneousrocks

•  crystals(sometimesvisible,sometimesmicroscopic,notalwaysuniform)

– Sedimentaryrocks•  crystals,butalsofragmentofpreviousrocks(gravel,sand,silt,clay)thatarecementedtogether

– Metamorphicrocks•  crystals(usuallylargeanduniforminsize,visible)

Texture

Anexampleofclastictexture:grainsofsanddepositedovertimeashorizontallayers,thencementedtogethertoformasandstone(aclasticsedimentaryrock)

ZionNationalPark,Utah©AlessandroGrippo

Anexampleofcrystallinetexture:bandsoflight-coloredminerals(feldspars,quartz)anddark-coloredminerals(biotite,amphiboles)inagneiss(aregionalmetamorphicrock)

SanGabrielMountains,California©AlessandroGrippo

Composition

•  Compositionreferstothetypeandabundanceofmineralscontainedinarock

–  Largemineralscanbeidentifiedbysight–  Smallmineralscanbeidentifiedthroughahandlens–  Extremelysmallmineralscanbeidentifiedwithamicroscope–  Allmineralshavecharacteristicphysicalproperties–  Somemineralshavedistinctdiagnosticproperties

–  Ultimately,acompositionisrenderedthroughachemicalformulaandaname:

•  QuartzSiO2 CalciteCaCO3 HematiteFe2O3 Olivine(Fe,Mg)2SiO4

CompositionBlackbandsofchert(SiO2)alternatingwithredbands

ofhematite(Fe2O3)

BandedIronFormations,Minnesota

Tubularstructuresofhalite(NaCl),akatablesalt

SearlesLake,Trona,California

Yellowbandsandcrystalsofsulfur(S)

DeathValley,California

Twosizesofcrystalsingranite:quartz(SiO2),

Na-plagioclase(NaAlSi3O8),andhornblende

(Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH)2

SierraNevada,Bishop,California

Allpictures:©AlessandroGrippo

RocksandtheRockCycle

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