about igneous rock.docx
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About Igneous Rocks
There are three great categories of rocks, named igneous, sedimentary and metamorphic, and
most of the time they're simple to tell apart. Although the three rock categories are connected in
the endless rock cycle, igneous rocks are traditionally taught first.
How to Tell Igneous Rocks
The main thing about igneous rocks is that they were once hot enough to melt. The following
traits are all related to that.
Because their mineral grains grew together tightly as the melt cooled, theyare relatively strong rocks.
They're made of primary minerals that are mostly black, white or gray. Anyother colors they may have are pale.
Their textures generally look like something that was baked in an oven. Theeven texture of coarse-grained granite is familiar from building stones orkitchen counters. ine-grained lava may look like black bread !including gasbubbles" or dark peanut brittle !including larger crystals".
Origin of Igneous Rocks
The word "igneous" comes from the Latin for fire, and what igneous rocks have in common is
that they formed by the cooling and crystallization of a melt. This material may have been lava
erupted at the arth's surface, or magma!unerupted lava at depths of up to a few kilometers, or
magma in deeper bodies !plutons. Those three different settings create three main types of
igneous rocks. #ock formed of lava is called extrusive, rock from shallow magma is called
intrusiveand rock from deep magma is calledplutonic. The deeper the magma, the slower it
cools and the larger the mineral crystals that form in it.
$gneous rocks form in three main places% where the tectonic plates pull apart !at mid&ocean
ridges, where plates come together !at subduction zones and where continents are pushed
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together, making the arth's crust thicker and allowing it to heat to melting. To learn more about
how igneous rocks form, see About agmaand About (olcanism.
)eople commonly think of lava and magma as a li*uid, like molten metal, but geologists find that
magma is usually a mush+a partially melted fluid loaded with mineral crystals. As it cools,
magma crystallizes into a series of minerals, some of which crystallize sooner than others. ot
-ust that, but as the minerals crystallize, they leave the remaining magma with a changed
chemical composition. Thus a body of magma evolves as it cools, and it also evolves as it moves
through the crust, interacting with other rocks.
nce magma erupts as lava, it freezes *uickly and preserves a record of its history underground
that geologists can decipher. $gneous petrology is a very comple/ field, and this article is only
the barest outline.
Igneous Rock Textures
The three types of igneous rocks differ in their te/tures, starting with the size of the mineral
grains. /trusive rocks cool *uickly !over periods of seconds to months and have invisible or
microscopic grains, or an aphanitic te/ture. $ntrusive rocks cool more slowly !over thousands of
years and have visible grains of small to medium size, orphaneritic te/ture.)lutonic rockscool
over millions of years and can have grains as large as pebbles+even meters across.
0ecause they solidified from a fluid state, igneous rocks tend to have a uniform fabric without
layers, and the mineral grains are packed together tightly. Think of the te/ture of something you
would bake in the oven. 1owever, in many igneous rocks large mineral crystals "float" in a fine&
grained groundmass. The large grains are calledphenocrysts, and a rock with phenocrysts is
called aporphyry2 that is, it has aporphyritic te/ture. )henocrysts are minerals that solidified
earlier than the rest of the rock, and they are important clues to the rock's history.
3ome e/trusive rocks have distinctive te/tures. bsidian,formed when lava hardens *uickly, has
a glassy te/ture. )umiceand scoriaare volcanic froth, puffed up by millions of gas bubbles
giving them a vesicular te/ture. Tuffis a rock made entirely of volcanic ash, fallen from the air
or avalanched down a volcano's sides. $t has apyroclastic te/ture. Andpillow lavais a lumpy
formation created by e/truding lava underwater.
Igneous Rock Types: Basalt, Granite and More$gneous rocks are classified by the minerals they contain. The main minerals in igneous rocks are
hard, primary ones% feldspar,*uartz,amphibolesandpyro/enes!together called "dark minerals"
by geologists, and olivinealong with the softer mineral mica.
The two best&known igneous rock types are basaltandgranite, which have distinctly different
compositions and te/tures. 0asaltis the dark, fine&grained stuff of many lava flows and magma
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intrusions. $ts dark minerals are rich in magnesium !g and iron !4e, hence basalt is called a
maficrock. 3o basalt is mafic and either e/trusive or intrusive.5raniteis the light, coarse&
grained rock formed at depth and e/posed after deep erosion. $t is rich in feldspar and *uartz
!silica and hence is called afelsicrock. 3o granite is felsic and plutonic.
0asalt and granite account for the great ma-ority of igneous rocks. rdinary people, even
ordinary geologists, use the names freely. !3tone dealers call any plutonic rock at all "granite."
0ut igneous petrologists use many more names. They generally talk about basalticandgranitic
orgranitoidrocks among themselves and out in the field, because it takes laboratory work to
determine an e/act rock type according to the official classifications. True granite and true basalt
are narrow subsets of these categories. 5et deeper into classification in this article.
A few of the less common igneous rock types can be recognized by non&specialists. 4or instance
a dark&colored plutonic mafic rock, the deep version of basalt, is called gabbro.A light&colored
intrusive or e/trusive felsic rock, the shallow version of granite, is calledfelsiteor rhyolite.And
there is a suite of ultramafic rockswith even more dark minerals and even less silica than basalt.
)eridotiteis the foremost of those.
Were Igneous Rocks !re "ound
The deep sea floor !the oceanic crust is made almost entirely of basaltic rocks, with peridotite
underneath in the mantle. 0asalts are also erupted above the arth's great subduction zones,
either in volcanic island arcs or along the edges of continents. 1owever, continental magmas
tend to be less basaltic and more granitic !learn more about arc volcanism.
The continents are the e/clusive home of granitic rocks. early everywhere on the continents, no
matter what rocks are on the surface, you can drill down and reach granitoid eventually. $n
general, granitic rocks are less dense than basaltic rocks, and thus the continents actually float
higher than the oceanic crust on top of the ultramafic rocks of the arth's mantle. The behavior
and histories of granitic rock bodies are among geology's deepest and most intricate mysteries.
Identification of Igneous Rocks
Grain
Size
Usual
ColorOther Composition
Rock
Type
fine dark glassy appearance lava glass bsidian
fine light many small bubbles lava froth from sticky lava )umice
fine dark many large bubbles lava froth from fluid lava 3coria
fine ormi/ed
light contains *uartz high&silica lava 4elsite
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fine or
mi/edmedium
between felsite and
basaltmedium&silica lava Andesite
fine or
mi/eddark has no *uartz low&silica lava 0asalt
mi/ed any color large grains in fine&grained matri/
large grains of feldspar, *uartz,pyro/ene or olivine
)orphyry
coarse lightwide range of color
and grain size
feldspar and *uartz with minor mica,
amphibole or pyro/ene5ranite
coarse lightlike granite butwithout *uartz
feldspar with minor mica, amphiboleor pyro/ene
3yenite
coarselight to
medium
little or no alkali
feldspar
plagioclaseand *uartz with dark
mineralsTonalite
coarsemedium to
dark
little or no *uartzlow&calcium plagioclase and dark
minerals
6iorite
coarsemedium todark
no *uartz2 may haveolivine
high&calcium plagioclase and darkminerals
5abbro
coarse dark dense2 always has
olivine
olivine with amphibole and7or
pyro/ene)eridotite
coarse dark densemostly pyro/ene with olivine and
amphibole)yro/enite
coarse green dense at least 89: olivine 6unite
very
coarse
any colorusually in small
intrusive bodies
typically granitic )egmatite
INTRODUCTION TO IGNEOUS ROCKS
Formation of Igneous Rocks
Igneous rocks form when molten (hot, liui!" elements cool #n! soli!if$ into cr$st#ls%
The cr$st#ls th#t form #re rich in the elements silicon #n! o&$gen, '$ f#r the most
#'un!#nt elements comrising E#rth)s crust #n! m#ntle% *hen 'on!e! together,
silicon #n! o&$gen form the silic#te ion, the 'uil!ing 'lock for #lmost #ll igneousminer#ls% The igneous rocks icture! in this we' site #re comose! of silic#te
miner#ls%
The cl#ssific#tion of igneous rocks is '#se! on two imort#nt #sects of #ll rocks on
E#rth, their te&ture #n! comosition, which #re !iscusse! in !et#il 'elow% Use this
cl#ssific#tion ch#rt to hel $ou i!entif$ $our igneous rocks%
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Textures of Igneous Rocks
The te&ture of #n igneous rock is !etermine! '$ its r#te of cooling% +olten elements
tr#e! un!ergroun!, c#lle! m#gm#, reuires from seer#l !#$s to thous#n!s of $e#rs
to cr$st#lli-e% To le#rn more #'out m#gm# #n! its effects on rocks comrising E#rth)s
crust, see the intrusions.lutons #ge% +olten rock erute! #t E#rth)s surf#ce, referre!
to #s l##, c#n soli!if$ in few secon!s or oer the course of seer#l !#$s% /or more
inform#tion #'out l## #n! olc#noes, see the olc#noes #n! l## flows #ge% The
er$ !ifferent r#tes of cooling for m#gm# #n! l## le#! to the !eeloment of two '#sic
te&tures of igneous rocks, !escri'e! 'elow% /or conenience of resent#tion the
igneous rocks #re !ii!e! into intrusie rocks (#lso c#lle! lutonic rocks" which form
un!ergroun!, #n! olc#nic rocks (#lso c#lle! e&trusie rocks" which form #t E#rth)s
surf#ce%
co#rse0gr#ine! te&ture Intrusie rocks, forme! !ue to slow cooling of m#gm#
un!ergroun!, #re comose! of l#rge cr$st#ls 'ec#use cr$st#ls h#! # long time to grow
'efore the cr$st#lli-#tion rocess w#s finishe!% This forms # co#rse0gr#ine! or
h#neritic te&ture% The ictures 'elow illustr#te this co#rse0gr#ine! te&ture% 1lso, note
the interlocking n#ture of the silic#te miner#l cr$st#ls comrising this rock%
gr#nite h#n! s#mle comose! of l#rge, interlocking miner#l cr$st#ls
gr#nite outcro in S#n G#'riel +ount#ins, C#liforni#
fine0gr#ine! te&ture E&trusie rocks, forme! !ue to r#i! cooling of l## #t E#rth)s
surf#ce, #re comose! of tin$ cr$st#ls or een unor!ere! #toms 'ec#use cr$st#lli-#tion
w#s comlete! within # few secon!s, hours, or erh#s !#$s% The resulting fine0
gr#ine! or #h#nitic te&ture is ch#r#cteristic of olc#nic igneous rocks% The rocks
shown 'elow show this fine0gr#ine! te&ture%
'#s#lt h#n! s#mle comose! of tin$, interlocking miner#l cr$st#ls
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r#i!l$ cooling hot l##
special igneous textures
gl#ss$ te&ture 0 forms #s l## cools er$ r#i!l$ #t E#rth)s surf#ce, in # m#tter of
secon!s or # few minutes% The resulting rock is comose! of unor!ere! #toms forming
# non0cr$st#lline m#ss, such #s the olc#nic rock o'si!i#n shown 'elow% Since
o'si!i#n is not comose! of cr$st#ls, it is not comose! of miner#ls, technic#ll$
se#king% This m#kes it !ifficult to #ssign o'si!i#n to # #rticul#r comosition (felsic,
interme!i#te or m#fic", confoun!ing geologists who feel comelle! to c#tegori-e mother
n#ture2
o'si!i#n h#n!s#mle
olc#nic rock outcro with 'oth gl#ss$ #n! froth$ te&tures3 4eff 5renner
for sc#le
froth$ te&ture 0 forms #s g#s0ch#rge! l## cools er$ r#i!l$ #t E#rth)s surf#ce% 1n
emle of such # olc#nic rock is umice, which is gl#ss$ like o'si!i#n, 'ut full of holes
forme! when tr#e! g#sses en!e! #s the l## soli!ifie!% 6ike o'si!i#n, umice is
!ifficult to cl#ssif$ #ccor!ing to st#n!#r! comosition#l c#tegories%
umice h#n! s#mle3 so full of #ir th#t it flo#ts on w#terorh$ritic te&ture 0
forms !ue to two c$cles of cooling, initi#ll$ er$ slowl$ un!ergroun!, then r#i!l$ #t
E#rth)s surf#ce% The rocks shown 'elow e#ch cont#in # com'in#tion of l#rge #n!
sm#ll cr$st#ls% In the first, some l#rge white cr$st#ls forme! #s m#gm# 'eg#n to slowl$
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cr$st#lli-e un!ergroun!% The rest of this rock, the gr#$ m#tri&, is comose! of tin$
cr$st#ls which forme! #s the still0molten m#gm# w#s erute! #t E#rth)s surf#ce where it
coole! er$ r#i!l$%
#n!esite h#n! s#mle illustr#ting the com'in#tion of co#rse #n! fine
igneous0rock te&tures
#nother igneous0rock h#n! s#mle illustr#ting the orh$ritic te&ture
$rocl#stic te&ture 0 forms #s #n e&losie olc#nic erution mi&es fr#gments of the
olc#no with hot #sh in the #tmoshere% 1s this hot com'in#tion of tin$ #sh #rticles#n! l#rger, #ngul#r fr#gments settles to the groun!, the$ 'l#nket the E#rth)s surf#ce #n!
wel! together% Such sect#cul#r eents c#n wie out #ll life forms in the #ffecte!
region surroun!ing # olc#no2 The olc#nic tuff shown 'elow is m#inl$ comose! of
#sh, #n! the olc#nic 'recci# cont#ins # lot of l#rge, #ngul#r fr#gments%
Click on hotos to see enl#rge! iews%
h#n!s#mles of rh$olite 0 olc#nic tuff #n! 'recci#
olc#nic outcro in New +e&ico showing the rough, $rocl#stic te&ture
Composition of Igneous Rocks
The comosition of #n igneous rock is !etermine! '$ the miner#ls resent in # s#mle%
The miner#log$ of #n igneous rock is # ro!uct of the com'in#tion of silicon !io&i!e with
other elements origin#ll$ resent within # 'o!$ of m#gm# or l##% The #ri#tions in
comositions of igneous rocks is, in #rt, # result of the gr#!u#l cooling of # 'o!$ of
m#gm# un!ergroun!% This slow rocess #llows miner#ls th#t form #t # high
temer#ture, like oliine #n! c#lcium0rich l#giocl#se fel!s#r, to cr$st#lli-e #n!
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se#r#te from the rem#ining m#gm#% Oer time, this rocess #lters the m#gm#)s
comosition, forming rogressiel$ lower0temer#ture miner#ls like u#rt- #n!
ot#ssium fel!s#r% This series of chemic#l ch#nges w#s o'sere! '$ N%6% 5owen,
#n! it is referre! to #s 5owen)s Re#ction Series%
Igneous rocks #re !ii!e! into three '#sic comosition#l grous, with the r#re ultr#m#fic
rocks not inclu!e! in the !escritions 'elow% Refer '#ck to the 5owens Re#ction
Series ch#rt, #s $ou scroll through the !escritions #n! hotogr#hs of the !ifferent
igneous0rock comositions%
felsic comosition 0 comose! mostl$ of the miner#ls u#rt- #n! ot#ssium fel!s#r
which #re gener#ll$ white to ink in color% +inor miner#l comonents of gr#nite c#n 'e
#mhi'ole #n! 'iotite mic#, 'oth of which #re 'l#ck in color% The rocks 'elow #re felsic
in comosition%
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gr#nite h#n! s#mle with miner#ls i!entifie!
gr#nitic l#n!sc#e3 Di#ne K#w#h#t# rel#&ing in 4oshu# Tree N#tion#l 7#rk
interme!i#te comosition 0 comose! mostl$ of the miner#ls #mhi'ole #n! so!ium0
rich l#giocl#se fel!s#r, with some u#rt- #n! $ro&ene ossi'le% Interme!i#te
igneous rocks usu#ll$ cont#in # com'in#tion of light #n! !#rk0colore! miner#ls, so the$
h#e # s#lt #n! eer color#tion (!iorite", or the$ #re siml$ gr#$ in color (#n!esite"%
The rocks shown 'elow #re interme!i#te in comosition%
!iorite h#n! s#mle with miner#ls i!entifie!
#n!esite h#n! s#mle
rh$olite outcro3 5#n!elier N#tion#l +onument cliff !wellings3 5o' C#reless for sc#le
m#fic comosition 0 comose! mostl$ of the miner#ls $ro&ene, c#lcium0rich
l#giocl#se fel!s#r #n! oliine% +#fic rocks #re usu#ll$ 'l#ck to !#rk green in color%
The rocks 'elow #re m#fic in comosition%
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g#''ro h#n! s#mle with miner#ls i!entifie!
'#s#lt h#n! s#mle with its ch#r#cteristic 'l#ck color
#gneous $ocks
Wat are Igneous Rocks#
neous rocks are formed from the
olidi%cation of molten rock material. There are
wo basic types& " intrusi$e igneous rocks
uch as diorite, gabbro, granite and pegmatite
at solidify below (arth's surface) and *"
xtrusi$e igneous rockssuch as andesite,
asalt, obsidian, pumice, rhyolite and scoria
at solidify on or above (arth's surface.
ctures and brief descriptions of someommon igneous rock types are shown on this
age.
%eg&atiteis a light-colored, extremely coarse
grained intrusive igneous rock. #t forms near th
margins of a magma chamber during the %nal
phases of magma chamber crystalliation. #t of
contains rare minerals that are not found in oth
parts of the magma chamber. The specimen sh
above is about two inches !%ve centimeters"
across.
http://geology.com/rocks/pegmatite.shtmlhttp://geology.com/rocks/pegmatite.shtml -
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ndesiteis a %ne-grained, extrusive igneous
ock composed mainly of plagioclase with
ther minerals such as hornblende, pyroxene
nd biotite. The specimen shown is about twoches !%ve centimeters" across.
asaltis a %ne-grained, dark-colored
xtrusive igneous rock composed mainly of
agioclase and pyroxene. The specimen
hown is about two inches !%ve centimeters"cross.
%eridotiteis a coarse-grained intrusive igneou
rock that is composed almost entirely of olivine
may contain small amounts of amphibole, felds
uart or pyroxene. The specimen shown aboveabout two inches !%ve centimeters" across.
%u&iceis a light-colored vesicular igneous roc
forms through very rapid solidi%cation of a mel
The vesicular texture is a result of gas trapped
the melt at the time of solidi%cation. The speci
shown above is about two inches !%ve centimeacross.
http://geology.com/rocks/andesite.shtmlhttp://geology.com/rocks/basalt.shtmlhttp://geology.com/rocks/peridotite.shtmlhttp://geology.com/rocks/pumice.shtmlhttp://geology.com/rocks/andesite.shtmlhttp://geology.com/rocks/basalt.shtmlhttp://geology.com/rocks/peridotite.shtmlhttp://geology.com/rocks/pumice.shtml -
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ioriteis a coarse-grained, intrusive
neous rock that contains a mixture of
ldspar, pyroxene, hornblende and
ometimes uart. The specimen
hown above is about two inches !%ve
entimeters" across.
a((rois a coarse-grained, dark colored,
trusive igneous rock that contains feldspar,
ugite and sometimes olivine. The specimen
hown above is about two inches !%ve
entimeters" across.
Ryoliteis a light-colored, %ne-grained,
extrusive igneous rock that typically contains
uart and feldspar minerals. The specimen
shown above is about two inches !%ve
centimeters" across.
)coriais a dark-colored, vesicular, extrusive
igneous rock. The vesicles are a result of trappe
gas within the melt at the time of solidi%cation
often forms as a frothy crust on the top of a lav
ow or as material e/ected from a volcanic ven
and solidifying while airborne. The specimen sh
above is about
http://geology.com/rocks/diorite.shtmlhttp://geology.com/rocks/gabbro.shtmlhttp://geology.com/rocks/rhyolite.shtmlhttp://geology.com/rocks/scoria.shtmlhttp://geology.com/rocks/diorite.shtmlhttp://geology.com/rocks/gabbro.shtmlhttp://geology.com/rocks/rhyolite.shtmlhttp://geology.com/rocks/scoria.shtml -
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raniteis a coarse-grained, light colored,
trusive igneous rock that contains mainly
uart and feldspar minerals. The specimen
bove is about two inches !%ve centimeters"cross.
O(sidianis a dark-colored volcanic glass that
forms from the very rapid cooling of molten roc
material. #t cools so rapidly that crystals do not
form. The specimen shown above is about two
inches !%ve centimeters" across. two inches !%vcentimeters" across.
http://geology.com/rocks/granite.shtmlhttp://geology.com/rocks/obsidian.shtmlhttp://geology.com/rocks/granite.shtmlhttp://geology.com/rocks/obsidian.shtml