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PlutonismandIntrusiveIgneousRocksnotesfromthetextbook,integratedwithoriginalcontributions
AlessandroGrippo,Ph.D.
Granite,anintrusiveigneousrock,intheSierraNevadaTuolumneMeadows,YosemiteNationalPark,California
©AlessandroGrippo
Magma
• Anyrockcanbemeltedundertherightconditionofpressureandtemperature
• Thatmeltiscalledmagma
• Magmacancontainsolid,liquid,andgaseousmaterials
Magmavs.Lava
• Whenmagmasolidifies,itformsigneousrocks
– IfmagmasolidifiesslowlywithinEarth’ssurfaceitgeneratesintrusiveigneousrocks(e.g.,granite)
– IfmagmasolidifiesquicklyatEarth’ssurfaceitiscalledlava,anditgeneratesextrusiveigneousrocks(e.g.,basalt)
Howdoweclassifyigneousrocks?
• Weusecompositionandtextureoftherocks
• ThechemicalCompositionrefersmainlytotheirironandsilicacontent
• Texturereferstothesize,shape,andarrangementofmineralgrains(crystals,inthecaseofigneousrocks)
• Texturalclassification– Plutonicorintrusiverocks(gabbro-diorite-granite)arecoarse-
grainedandcooledslowlyatdepth– Grainsarevisibletothenakedeye– Twotextures:pegmatitic,phaneritic
– Volcanicorextrusiverocks(basalt-andesite-rhyolite)aretypicallyfine-grainedandcooledrapidlyattheEarth’ssurface
– Grainsarenotvisibletothenakedeye,orabsent(e.g.obsidian,avolcanicglass)– Sixtextures:porphyritic,aphanitic,glassy,vesicular,frothy,pyroclastic
• Compositionalclassification– Maficrocks(gabbro-basalt)containabundantdark-colored
ferromagnesianminerals– Intermediaterocks(diorite-andesite)containroughlyequalamounts
ofdark-andlight-coloredminerals– Felsicrocks(granite-rhyolite)containabundantlight-colored
minerals
Composition• Rockchemistry,particularlysilica(SiO2)content,determines
mineralcontentandgeneralcolorofigneousrocks– Ultramaficrockshave<45%silica,byweight,andarecomposed
almost entirelyofdark-coloredferromagnesianminerals• Mostcommonultramaficrockisperidotite(intrusive)
– Maficrockshave~50%silica,byweight,andcontaindark-coloredmineralsthatareabundantiniron,magnesiumandcalcium• Intrusive/extrusivemaficrocks-gabbro/basalt
– Intermediaterockshavesilicacontentsbetweenthoseofmaficandfelsicrocks• Intrusive/extrusiveintermediaterocks-diorite/andesite
– Felsic(silicic)rockshave>65%silica,byweight,andcontainlight-coloredmineralsthatareabundantinsilica,aluminum,sodiumandpotassium• Intrusive/extrusivefelsicrocks-granite/rhyolite
EightTextures• Pegmatitic
– Visiblecrystalsbiggerthan1cminsize• Phaneritic(Coarse-grained)
– Visiblecrystalssmallerthan1cminsize(mostcommon)• Porphyritic
– Twocrystalsizestogetherinthesamerock• Aphanitic(Fine-grained)
– Invisiblecrystals(mostcommon)• Glassy
– Nocrystals(e.g.,obsidian)
• Vesicular– vesiclesleftintherockbyescapinggases;e.g.,vesicularbasalt
• Frothy– avarietyofvesicularthatappliestofelsic,viscousmagmas;e.g.,pumice
• Pyroclastic,orFragmental– aftervolcanicexplosions;e.g.,volcanicbrecciaandvolcanictuff
HowMagmaForms• Heatfrombelow
– Heatupward(byconductionandconvection)fromtheveryhot(>5000°C)corethroughthemantleandcrust
• Heatvs.pressure– Meltingpointofmineralsincreaseswithincreasingpressure– Inthehottestregionswithintheuppermantleandcrust,pressurecan
belowenoughformeltingtooccur• Hotwaterunderpressure
– Waterbecomesincreasinglyreactiveathighertemperatures– Atsufficientpressuresandtemperatures,highlyreactivewatervapor
canreducethemeltingpointofrocksbyover200°C• Mineralmixtures
– Mixturesofminerals,suchasquartzandpotassiumfeldspar,canresultinthemeltingofbothattemperatureshundredsofdegreeslowerthaneithermineralwouldmeltonitsown
• Discontinuousseriesofcrystallization– Maficminerals(olivine,pyroxene,amphibole,biotite)crystallizein
sequencewithdecreasingtemperature
– Asonemineralbecomeschemicallyunstableintheremainingmagma,anotherbeginstoform
• Continuousseriesofcrystallization– Plagioclasefeldsparformswithachemicalcompositionthatevolves– (fromCa-richtoNa-rich)withdecreasingtemperature
• Finalstagesofcrystallization– Atlowertemperatures,ifthereisstillmagmaleft,coolingwillnot
affectthetwoendtermsoftheseries(biotiteandNa-plagioclase)
– ItwillformquartzandK-feldspardirectlyoutofmagma
• Alargevarietyofigneousrocksisproducedbyalargevarietyofmagmacompositions
• Maficmagmaswillcrystallizeintobasaltorgabbroifearly-formedmineralsarenotremovedfromthemagma
• Intermediatemagmaswillsimilarlycrystallizeintodioriteorandesiteifmineralsarenotremoved
• Separationofearly-formedferromagnesianmineralsfromamagmabodyincreasesthesilicacontentoftheremainingmagma
• Mineralsmeltinthereverseorderofthatinwhichtheycrystallizefromamagma
MagmaEvolution
• Achangeinthecompositionofamagmabodyisknownasmagmaevolution
• Magmaevolutioncanoccurbydifferentiation,partialmelting,assimilation,ormagmamixing
Differentiation
• Differentiationinvolvesthechangingofmagmacompositionbytheremovalofdenserearly-formedferromagnesianmineralsbycrystalsettling
• Partialmeltingproducesmagmaslessmaficthantheirsourcerocks,becauselowermeltingpointmineralsaremorefelsicincomposition
• Assimilationoccurswhenahotmagmameltsandincorporatesmorefelsicsurroundingcountryrock
• Magmamixinginvolvesthemixingofmoreandlessmaficmagmastoproduceoneofintermediatecomposition
IntrusiveRockBodies
• Intrusiverocksexistinbodiesorstructuresthatpenetrateorcutthroughpre-existingcountryrock
• Intrusivebodiesaregivennamesbasedontheirsize,shapeandrelationshiptocountryrock
– Shallowintrusions• Form<2kmbeneathEarth’ssurface• Chillandsolidifyfairlyquicklyincool• countryrock• Generallycomposedoffine-grainedrocks
– Deepintrusions:Plutons• FormatconsiderabledepthbeneathEarth’ssurfacewhenrisingblobs
ofmagma(diapirs)gettrappedwithinthecrust• Crystallizeslowlyinwarmcountryrock• Generallycomposedofcoarse-grainedrocks
IgneousActivityandPlateTectonics
• Igneousactivityoccursprimarilyatorneartectonicplateboundaries
• Maficigneousrocksarecommonlyformedatdivergentboundaries
– Increasedheatflowanddecreasedoverburdenpressureproducemaficmagmasfrompartialmeltingoftheasthenosphere
• Intermediateigneousrocksarecommonlyformedatconvergentboundaries
– Partialmeltingofbasalticoceaniccrustproducesintermediatemagmas
• Felsicigneousrocksarecommonlyformedadjacenttoconvergentboundaries
– Hotrisingmagmacausespartialmeltingofthegraniticcontinentalcrust
• Intraplatevolcanism– Risingmantleplumescanproducelocalizedhotspotsandvolcanoes
whentheyproducemagmasthatrisethroughoceanicorcontinentalcrust
– Hawaiiisanexample