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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
theEND