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8/16/2019 Session 1 - Igneous Rocks I
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Igneous Rocks
Petrology - Session 1
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General characteristics
of magma
Igneous rocks form as molten rock cools and
solidifies
General characteristics of magma: Parent material of igneous rocks
Forms from partial melting of rocks
Magma at surface is called lava
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General characteristics
of magma
General characteristic of magma Rocks formed from lava = extrusive, or
volcanic rocks Rocks formed from magma at depth =
intrusive, or plutonic rocks
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General characteristics
of magma
The nature of magma onsists of three components:
!i"uid portion = melt
#olids, if an$, are silicate minerals
%olatiles = dissolved gases in the melt,
including &ater vapor '()*+, caron
dioxide '*)+, and sulfur dioxide '#*)+
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General characteristics
of magma
r$stalli-ation of magma ooling of magma results in the
s$stematic arrangement of ions intoorderl$ patterns The silicate minerals resulting from
cr$stalli-ation form in a predictale
order Texture . si-e and arrangement of
mineral grains
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Igneous textures
Texture is used to descrie the overallappearance of a rock ased on the si-e,shape, and arrangement of interlocking
minerals Factors affecting cr$stal si-e
Rate of cooling
#lo& rate = fe&er ut larger cr$stals
Fast rate = man$ small cr$stals %er$ fast rate forms glass
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Igneous textures
Factors affecting cr$stal si-e
/ of silica '#i*)+ present
0issolved gases
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Igneous textures
T$pes of igneous textures
1phanitic 'fine.grained+ texture
Rapid rate of cooling Microscopic cr$stals
Ma$ contain vesicles 'holes from gas ules+
Phaneritic 'coarse.grained+ texture
#lo& cooling !arge, visile cr$stals
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Aphanitic texture
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Phaneritic texture
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Igneous textures
nT$pes of igneous texturesnPorph$ritic texture
n
Minerals form at different temperaturesn !arge cr$stals 'phenocr$sts+ are emedded
in a matrix of smaller cr$stals 'groundmass+
nGlass$ texture
n %er$ rapid cooling of lavan Resulting rock is called osidian
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Igneous textures
nT$pes of igneous textures
nP$roclastic texturen Fragmental appearance produced $ violent
volcanic eruptions
n *ften appear more similar to sedimentar$
rocks
nPegmatitic texture
n 2xceptionall$ coarse grained
n Form in late stages of cr$stalli-ation of
granitic magmas
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Porphyritic texture
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Glassy texture
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Igneous compositions
nIgneous rocks are composed primaril$ of
silicate minerals
n0ark 'or ferromagnesian+ silicates
n *livine, p$roxene, amphiole, and iotitemica
n!ight 'or nonferromagnesian+ silicates
n 3uart-, muscovite mica, and feldspars
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Igneous compositions
nGranitic versus asaltic compositions
nGranitic compositionn !ight.colored silicates
n Termed felsic ' feldspar and si lica+ in
composition
n (igh amounts of silica '#i*)+
n
Ma4or constituent of continental crust
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Igneous compositions
nGranitic versus asaltic compositions
n5asaltic compositionn 0ark silicates and calcium.rich feldspar
n Termed mafic ' magnesium and ferrum, for
iron+ in composition
n (igher dense than granitic rocks
n omprise the ocean floor and man$ volcanic
islands
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Basaltic lava dropping into the ocean along Kilauea Volcano along
the southeastern coast of the big island of Hawaii
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Igneous compositions
n*ther compositional groups
nIntermediate 'or andesitic+ compositionn ontain )6/ or more dark silicate minerals
n 1ssociated &ith explosive volcanic activit$
n7ltramafic compositionn Rare composition that is high in magnesium
and ironn omposed entirel$ of ferromagnesian
silicates
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Igneous compositions
n#ilica content as an indicator of composition
n2xhiits a considerale range in the crustn 86/ to 9/
n#ilica content influences magma ehaviornGranitic magmas = high silica content and viscous
n5asaltic magmas = much lo&er silica contentand more fluid.like ehavior
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Igneous compositions
n;aming igneous rocks < granitic rocks
nGraniten Phaneritic
n *ver )6/ "uart-, aout 6/ or more feldsparn %er$ aundant . often associated &ith
mountain uilding
n The term granite includes a &ide range of
mineral compositions
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Granite
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Igneous compositions
n;aming igneous rocks < granitic rocks
nRh$oliten 2xtrusive e"uivalent of granite
n Ma$ contain glass fragments and vesicles
n 1phanitic texture
n !ess common and less voluminous than
granite
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Rhyolite
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Igneous compositions
n;aming igneous rocks < granitic rocks
n*sidiann 0ark colored
n Glass$ texture
nPumicen %olcanic
n
Glass$ texturen Froth$ appearance &ith numerous voids
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Pumice is very glassy andsharp with countless vesicles!
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Igneous compositions
n;aming igneous rocks < intermediate
rocks
n
1ndesiten %olcanic origin
n 1phanitic texture
n0ioriten Plutonic e"uivalent of andesite
n oarse grained
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Andesite
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Diorite
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Igneous compositions
n;aming igneous rocks < asaltic
rocks
n5asaltn %olcanic origin
n 1phanitic texture
n omposed mainl$ of p$roxene and calcium.
rich plagioclase feldsparn Most common extrusive igneous rock
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Basalt
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Igneous compositions
n;aming igneous rocks < mafic rocks
nGaron Intrusive e"uivalent of asalt
n Phaneritic texture consisting of p$roxene and
calcium.rich plagioclase
n #ignificant / of the oceanic crust
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Gabbro
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Igneous compositions
n;aming igneous rocks < p$roclastic
rocksnomposed of fragments e4ected during
a volcanic eruption
n %arieties
n Tuff = ash.si-ed fragments
n %olcanic reccia = particles larger than ash
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Origin of magma
n(ighl$ deated topic
nGenerating magma from solid rocknRole of heatn Temperature increases in the upper crust
'geothermal gradient+ average et&een )o
to >o per kilometer of depth
n Rocks in the lo&er crust and upper mantle are
near their melting points
n 1n$ additional heat ma$ induce melting
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Origin of magma
nRole of pressuren Increases in confining pressure cause an
increase in a rock?s melting temperature
n @hen confining pressures drop,decompression melting occurs
nRole of volatilesn %olatiles 'primaril$ &ater+ cause rocks to
melt at lo&er temperatures
n Important factor &here oceanic lithospheredescends into the mantle
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Decompression melting
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Evolution of magmas
n 1 single volcano ma$ extrude lavas
exhiiting ver$ different compositions
n2ach volcanic eruption tends to exhiit a
uni"ue geochemical fingerprint, defined $
trace element percentages
n5o&en?s reaction seriesn Minerals cr$stalli-e in a s$stematic fashion ased on their
melting points
n 0uring cr$stalli-ation, the composition of the li"uid portion of
the magma continuall$ changes
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Boen!s reaction series
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Evolution of magmas
nProcesses responsile for changing a
magma?s compositionnMagmatic differentiation
n #eparation of a melt from earlier formed
cr$stals
n 1ssimilationn
hanging a magma?s composition $ theincorporation of surrounding rock odies into
a magma
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"agma evolves as the
hotter minerals crystalli#e
and settle to the bottom of
the magma chamber
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Evolution of magmas
nProcesses responsile for changing a
magma?s compositionnMagma mixing
n T&o chemicall$ distinct magmas ma$ produce
a composition "uite different from either
original magma
A i il ti i i d
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Assimilation" magma mixing" and
magmatic differentiation
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Evolution of magmas
nPartial melting and magma formationnIncomplete melting of rocks is kno&n as
partial melting
nFormation of asaltic magmasn Most originate from partial melting of
ultramafic rock in the mantle at oceanic
ridges
n !arge outpourings of asaltic magma are
common at 2arth?s surface
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Evolution of magmas
nPartial melting and magma formationnFormation of andesitic magmas
n Produced $ interaction of asaltic magmas
and more silica.rich rocks in the crust
n Ma$ also evolve $ magmatic differentiation
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Evolution of magmas
nPartial melting and magma formationnFormation of granitic magmas
n Most likel$ form as the end product of
cr$stalli-ation of andesitic magma
n Granitic magmas are more viscous than other
magmas so the$ tend to lose their moilit$
efore reaching the surface
n Tend to produce large plutonic structures