Session 1 - Igneous Rocks I

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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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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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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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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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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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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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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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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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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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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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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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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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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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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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

Session 1 - Igneous Rocks I

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Transcript of Session 1 - Igneous Rocks I