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