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$


8/14

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!


9/14

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

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About Igneous Rock.docx