MagmaandIgneousRocks
MagmaComposition- I•Magmasvarywidelyincomposition•Mostlymadeof8elements;inorderof[]–•O,Si,Al,Fe,Ca,Na,Mg,K(mostlyOandSi-ca50%&25%)
•Magmasmadefromthecrustaremostly•O,Si,Al,Na,andK (lightcolour!)
•Thecompositionofmagmadependsupontherockfromwhichitwasformed(bymelting),andthemeltingconditions
MagmaComposition- II
•MagmasfromthemantlehavehigherlevelsofFe,Mg&Ca(darkcolour!)•AllmagmashavevaryingamountsofH,C&S,whichareconvertedintothegases(volatiles)H2O,CO2,andH2Sasthemagmacools.•Almostallrocksareformedbypartial,notcomplete,melting
Meltingofrocks
•UsuallyRocksdonotmeltbyheatingthemup
• Therearetwomainmechanismsbywhichrocksmelt
• decompressionmelting and
• fluxmelting
Decompressionmelting
•DecompressionmeltingtakesplacewithinEarth• Thebodyofrock(solid)isatagiventemperatureand• thepressureisreduced
• Thishappensastherockmovestowardthesurfaceeither• atamantleplume(ahotspote.g.Hawaii),or• inupwellingpartofamantleconvectioncell
• Then- partialmeltingstartstotakeplace.
Fluxmelting
• Twoconditions• arockisclosetoitsmeltingpointand• somewater(afluxthatpromotesmelting)isadded/injected
• Thenthemeltingtemperatureisreduced•And….partialmeltingstartstooccur
Figure3.9Commonsitesofmagmaformationintheuppermantle.Theblackcirclesareregionsofpartialmelting.Thebluearrowsrepresentwaterbeingtransferredfromthesubductingplatesintotheoverlyingmantle.[SE,afterUSGS(http://pubs.usgs.gov/gip/dynamic/Vigil.html)]
MagmaFormation– it’smostlyduetotectonics
Magmainteractionwithcountry(local)rocks
•Ashotmagmamovestowardsthesurfaceitinteractswiththesurroundingrock(esp.fromthemantleà lowercrust)
•Usuallycausespartialmeltingofthecountryrocksincethemagmaishotterthanthemeltingtempofsurroundingrock• Themoresilica-richpartsofthecountryrockarepreferentiallymelted whichcausesanincreaseinthesilicacontentofthemagma(lightercolour).
Thebehaviour ofcoolingmagma•Atveryhightemps(>1300°C)mostmagmatotallyliquid• Thetempdropsasmagmaslowlymovesupward• SiandOcombinetoformsilicatetrahedra•AscoolingcontinuestheSiO4 starttopolymerize• Thesechainscausemagmatobecomemoreviscous•Magmaviscosityhassignificantimplicationsforvolcaniceruptions(later)•Asthemagmacontinuestocool,crystalsstarttoform.
Crystallizationofmagma– mineralformation
•Mineralsinigneousrockscrystallizeoverarangeoftemps•Crystalsforminarelativelyfixedsequence• TheBowenreactionseries(Figure3.10inthetext)
•Coolingmagmacanhavecrystalsandyetremainmostlyliquid•Nocrystals/minerals– glassy(terminclassification!!)
Bowen’sreactionseries
Bowen’sreactionseriesI–Discontinuousbranch
•Olivinenormallycrystallizesfirst,at1300-1200°C.• Astempdrops,andassumingthatsomesilicaremains,theolivinecrystalsreact(combine)withsomeofthesilicainthemagmatoformpyroxene.• Aslongassilicaremainsandtherateofcoolingisslow,thisprocesscontinuesdownthediscontinuousbranch• olivinetopyroxene,Mg2SiO4 +SiO2 à2MgSiO3• pyroxenetoamphibole,and• amphiboletobiotite.
Bowen’sreactionseries
Bowen’sreactionseriesII– Continuousbranch
•Atthepointwherepyroxenestartstocrystallize-plagioclasefeldsparalsostartstocrystallize.•AtthattemptheplagioclaseisCa-rich(anorthite)•AsthetempfallstheplagioclasethatformsbecomesmoreNa-richvariety(aslongasthereissodiumleftinthemagma)•Canseechangeincompositionofplagioclasecrystalsunderamicroscope
Bowen’sreactionseries
Bowen’sreactionseriesIII
•Astempcontinuestodecreaseca.750-800oCthefollowingmineralsform:• K-feldspar,•Muscovite,• AndfinallyQuartz
Bowen’sreactionseries
Bowen’sreactionseriesIV• Howfarthereactionprocesscancontinuebeforeallofthesilicaisusedupisdeterminedbytheoriginalmagma• Thecompositionsoftypicalmaficandfelsicmagmasare•Maficmagmashave• 45%to55%SiO2,• about25%totalofFeO andMgO plusCaO• about5%Na2O+K2O
• Felsicmagmashave• about65%to75%SiO2 – muchmore• about5%FeO andMgO plusCaO – muchless• about10%Na2O+K2O
Bowen’sreactionseries
Formationofbasalt(gabbro)
• asaMAFICMAGMAstartstocool,someofthesilicacombineswithironandmagnesiumtomakeolivine.• furthercoolingcausesmuchoftheremainingsilicatogointocalcium-richplagioclase• anysilicaleftcausesconversionofsomeoftheolivinetopyroxene• mineralspresentwillbeolivine,pyroxene,andCa-richplagioclase.
• ifmagmacoolsslowlyunderground- productisgabbro(coarse)• ifitcoolsquicklyatthesurface- productisbasalt(fine)
Formationofgranite(rhyolite)
• FELSICMAGMAtendstobecoolerthanmaficmagmawhencrystallizationbeginssostartbycrystallizingpyroxene(notolivine)andplagioclase.• Ascoolingcontinues,thereactionsonthediscontinuousbranchwillproceed(silicaissoabundantinthismagma)• theplagioclasewillbecomeincreasinglyNa-rich,andeventuallyK- feldsparandthenpurequartzwillform.• Commonlyevenveryfelsicrockswillnothavebiotite/muscoviteduetosparsityofAland/orenoughHtoformOH– bothbeingnecessaryformicaminerals.• Typicalfelsicrocksaregranite(coarse)andrhyolite(fine)
FormationofDiorite(andesite)
• intermediatemagmasliesomewherebetweenthoseofmaficandfelsicmagmas.• typicalintermediaterocks• diorite(coarse)and• Andesite(fine)
Fractionalcrystallization
• ifthemagmahasalowviscosity– mostlikelyifmafic• thecrystalsthatformearly,suchasolivine,mayslowlysettletowardthebottomofthemagmachamber(zonedmagmachamber– nextslide)• thusthecompositionofthemagmaneartopofmagmachamberwillbecomemorefelsic(it’slosingsomeFeandMg-richcomponents)• Processisknownasfractionalcrystallization.• Thecrystalsthatsettlemay
• formanolivine-richlayernearthebottomofchamberor• theymayre-meltaslowerpartislikelytobehotterthantheupperpart
• Ifanymeltingtakesplace,crystalsettlingwillmakethemagmaatthebottomofthechambermoremaficthanitwasatbeginning
Zonedmagmachamber
Porphyritictexture• Ifcrystalsettlingdoesnottakeplace(magmaistooviscous)thentheprocessofcoolingwillcontinueaspredictedbytheBowenreactionseries• Insomesituationspartiallycooledbutstillliquidmagma,withcrystalsinit,willeithermovefartherupintoacoolerpartofthecrust(orallthewaytothesurfaceduringavolcaniceruption)• Ineitherofthesesituations,themagmathathasmovedtowardthesurfaceislikelytocoolmuchfasterthanitdidwithinthemagmachamber,andtherestoftherockwillhaveafinercrystallinetexture• Anigneousrockwithlargecrystalsembeddedinamatrixoffinercrystalsisindicativeofatwo-stagecoolingprocess,andthetextureisporphyritic
Classificationofigneousrocks
• Igneousrocksareclassifiedaccordingtotheirtextures.• Eitherphaneritic (coarse,visible,>0.5mm)• Oraphanitic(fine,notreallyvisible)
•Mostintrusive igneousrocksarephaneritic•Mostextrusive (volcanic)rocksareaphanitic.•Andtheircomposition…• (felsic,intermediate,mafic,ultra-mafic)
Magmacompositions(reminder:)• Thecompositionsoftypicalmaficandfelsicmagmasare• Maficmagmashave• 45%to55%SiO2• about25%totalofFeO andMgO plusCaO• about5%Na2O+K2O
• Felsicmagmashave• about65%to75%SiO2 – (muchmorethanmafic)
• about5%FeO andMgO plusCaO – muchless• about10%Na2O+K2O
• Intermediate – inbetweenthesetwocompositions• Ultra-mafic– maficcompw/inc FeO,MgO,CaO,decreaseothers
Crystalsize- texture
•Crystalsizeisproportionaltotherateofcooling• Slowcoolingofmagmaà largerthecrystals• Intrusiveigneousrocksoftenhavecrystals1cmlong• IfmagmabecomesH2Orich(esp.towardsendofcooling).Thenthispromotestherelativelyeasymovementofions,andthisallowscrystalstogrowlarge,sometimesafewcm• two-stagecoolingprocess – bothlargeandsmallcrystals- porphyritic• Ifgasbubblestrappedinrock- vesicular
Mineralcomposition
Readingfornextclass
• Volcanismpp84-109
• ENDfornowJ