agent of metamorphism.docx

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Agents controllingmetamorphism


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Bahria UniversityIslamabad Campus) Department of Earth and Environmental Sciences

TABLE OF CONTENTS

INTRODUCTION

02

1.1 metamorphic rocks

1.2metamorphism

2. AGENTS OF METAMORPHISM.

02-04

2.1 heat as a metamorphic agent

2.1.1changes caused by heat

2.1.2 Source of heat?

2.2 CONFINING PRESSURE AND DIFFERENTIAL STRESS..

04-05

2.3 CHEMICALLY ACTIVE

FLUIDS............................................................................................

...06-07

2.3. Source of chemically active uids

2.4 THE IMPORTANCE OF PARENT ROCK..

.07-0

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1.1METAMORP!C ROC"

Roc#s that ha$e% in a soli& state% 'n&ergone changes in minerolog(% te)t're% orchemical composition as a res'lt of heat or press're.

TERM!NOLO*+ ,

The original rock that altered to make a metamorphic rock is called the protolith.

1.- METAMORP!SM

When rocks are subjected to deep burial, tectonic forces such as folding, and high pressures and

temperatures, the textures and mineral compositions begin to change. This process, called

metamorphism. Itis the solid-state transformation (no melting) of a rock mass into a rock of

generall the same chemistr but !ith different texture s and minerals. "suall the

metamorphic rock looks #uite different from the original rock, called the parent rock or

protolith. Metamorphicrocksoften sho! contorted patterns of folding that indicate the !ere

soft enough to bend (plastic deformation). $olding is achie%ed b the application of great

pressure o%er long periods. The intensit of the metamorphism increases !ith increasing

temperature and&o r pressure, and the highest 'grade' of metamorphism approaches partial

melting of the rock, almost completing the rock ccle.

- .A*ENTS OF METAMORP!SM

The agents of metamorphism include heat, pressure (stress), and chemically active fluids. uring

metamorphism, rocks are usuall subjected to all three metamorphic agents simultaneousl.

o!e%er, the degree of metamorphism and the contribution of each agent %ar greatl from one

en%ironment to another.

-.1 EAT AS A METAMORP!C A*ENT

The most important factor dri%ing metamorphism is heatbecause it pro%ides the energ to dri%e

chemical reactions that result in the recrstalli*ation of existing minerals and&or the formation of

ne! minerals. +ecall from the discussion of igneous rocks that an increase in temperature causes

the ions !ithin a mineral to %ibrate more rapidl. %en in a crstalline solid, !here ions

arestrongl bonded, this ele%ated le%el of acti%it allo!s indi%idual atoms to migrate more freel

bet!een sites in the crstalline structure.

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-.1.1CAN*ES CASE/ B+ EAT

eat affects arth materials, especiall those that form in lo!-temperature en%ironments, in t!o

!as. $irst, it promotes recrstalli*ation of indi%idual mineral grains. This is particularl true of

clas, fine grained sediments, and some chemical precipitates. igher temperatures promote

recrstalli*ation !here fine particles tend to coalesce into larger grains of the same mineralog.

econd, heat ma raise the temperature of a rock to the point !here one, or more, of the minerals

are no longer chemicall stable. In such cases, the constituent ions tend to arrange themsel%es

into crstalline structures that are more stable in the ne! high-energ en%ironment. uch

chemical reactions result in the creation of ne! minerals !ith stable configurations that ha%e an

o%erall composition roughl e#ui%alent to that of the original material. (In some en%ironments

ions ma migrate into or out of a rock unit, thereb changing its o%erall chemical composition.)

In summar, if !e !ere to tra%erse a region of metamorphic rocks (no! uplifted and exposed)

!hile tra%eling in the direction of increasing intensit of metamorphism, !e !ould expect to

obser%e t!o changes largel attributable to increased temperature. The grain si*e of the rocks

!ould increase and the mineralog !ould graduall change.

-.1.-0AT !S TE SORCE OF EAT

arths internal heat comes mainl from energ that is continuall being released b radioacti%e

deca and thermal energ generated during the formation of our planet. +ecall that temperaturesincrease !ith depth at arate kno!n as thegeothermal gradient.

In the upper crust, this increase in temperature a%erages bet!een /01C and 201C per kilometer

($igure 3.3). Thus, rocks that formed at arths surface !ill experience a gradual increase in

temperature as the are taken to greater depths. When buried to a depth of about 4 kilometers (5

miles), !here temperatures are about /001C, cla minerals tend to become unstable and begin to

recrstalli*e into ne! minerals, such as chlorite and musco%ite that are stable in this

en%ironment. (Chlorite is mica like mineral formed b the metamorphism of dark 6mafic7 silicate

minerals.) o!e%er, man silicate minerals, particularl those found in crstalline igneous rocks

#uart* and feldspar for example remain stable at these temperatures. Thus, metamorphic changesin these minerals generall occur at much greater depths. n%ironments !here rocks ma be

carried to great depths and heated include con%ergent plate boundaries !here slabs of sediment-

laden oceanic crust are being subducted. In addition, rocks ma become deepl buried in large

basins !here gradual subsidence results in %er thick accumulations of sediment($igure /.3).

uch locations, exemplified b the 8ulf of 9exico, are kno!n to de%elop metamorphic

conditions near the base of the pile. $urthermore, continental collisions, !hich result in crustal

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thickening b folding and faulting, cause rocks to become deepl buried !here ele%ated

temperatures ma trigger partial melting. eat ma also be transported from the mantle into e%enthe shallo!est laers of the crust b igneous intrusions. +ising mantle plumes, up!elling at mid-

ocean ridges, and magma generated b partial melting of mantle rock at subduction*ones are

three examples ($igure /.3). :ntime magma forms and buoantl rises to!ard the surface,

metamorphism occurs. When magma intrudes relati%el cool rocks at shallo! depths, the host

rock is ;baked. contrast, thermal heating is e%ident !here magma intrudes the upper crust.

-.- CONF!N!N* PRESSRE AN/ /!FFERENT!AL STRESS

?ressure, like temperature, also increases !ith depth as the thickness of the o%erling rock

increases. >uried rocks are subjected to confining pressure, !hich is analogous to !ater pressure,

!here the forces are applied e#uall in all directions. The deeper ou go in the ocean, the greater

the confining pressure. The same is true for rock that is buried. Confining pressure causes the

spaces bet!een mineral grains to close, producing a more compact rock ha%ing a greater densit.

$urther, at great depths, confining pressure ma cause minerals to recrstalli*e into ne! minerals

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that displa a more compact crstalline form. Confining pressure does not, ho!e%er, fold and

deform rocks like those sho!n in. In addition to confining pressure, rocks ma be subjected todirected pressure. This occurs, for example, at con%ergent plate boundaries !here slabs of

lithosphere collide. ere the forces that deform rock are une#ual in different directions and are

referred to as differential stress. "nlike confining pressure, !hich ;s#uee*es< rock e#uall in all

directions, differential stresses are greater in one direction than in others. :s sho!n in $igure

/./>, rocks subjected to differential stress are shortened in the direction of greatest stress and

elongated, or lengthened, in the direction perpendicular to that stress. :s a result, the rocks

in%ol%ed are often folded or flattened(similar to stepping on a rubber ball). :long con%ergent

plate boundaries the greatest differential stress is directed roughl hori*ontal in the direction of

plate motion, and the least pressure is in the %ertical direction. Conse#uentl, in these settings the

crust is greatl shortened (hori*ontall) and thickened (%erticall). :lthough, differential stresses

are generall small !hen compared to confining pressure, the are important in creating the

%arious textures exhibited b metamorphic rocks. In surface en%ironments !here temperatures

are comparati%el lo!, rocks are brittle and tend to fracture !hen subjected to differential stress.

Continued deformation grinds and pul%eri*es the mineral grains into small fragments. >

contrast, in high-temperature en%ironments rocks areductile. When rocks exhibit ductile

beha%ior, their mineral grains tend to flatten and elongate !hen subjected to differential stress

($igure /./). This accounts for their abilit to deform b flo!ing (rather than fracturing) to

generate intricate folds.

FIG

URE 2.2 Confining pressure and differential stress as metamorphic agents. A. In a depositional

en%ironment, as confining pressure increases, rocks deform b decreasing in %olume. . uring

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mountain building, rocks subjected to differential stress are shortened in the direction that

pressure is applied, and lengthened in the direction perpendicular to that force.

-.2 CEM!CALL+ ACT!3E FL!/S

$luids composed mainl of !ater and other%olatile components, including carbon

dioxide,are belie%ed to pla an important role in some tpes of metamorphism. $luids

that surround mineral grains act as catalsts topromote recrstalli*ation b enhancing

ion migration. In progressi%el hotter en%ironments these ion-rich fluids become

correspondingl more reacti%e. When t!o mineral grains are s#uee*ed together, the

parts of their crstalline structures that touch are the most highl stressed. Ions

located at these sites are readil dissol%ed b the hotfluids and migrate along the

surface of the grain to the pore spaces located bet!een indi%idual grains. Thus,

hdrothermal fluids aid in the recrstalli*ation of mineral grains b dissol%ing

material from regions of highstress and then precipitating (depositing)this material in

areas of lo! stress. :s a result,minerals tend to recrystallize and growlonger in a

direction perpendicular to compressional stresses.

Where hot fluids circulate freel throughrocks, ionic exchange ma occur

bet!eent!o adjacent rock laers, or ions ma migrate great distances before the are

finall deposited.The latter situation is particularlcommon !hen !e consider hot

fluids that escape during the crstalli*ation of an igneous pluton. If the rocks that

surround the pluton differ markedl in composition from that of the in%ading fluids,

there ma be a substantial exchange of ions bet!een the fluids and host rocks. When

this occurs, a change in the o%erall composition of the surrounding rock results. The

change in composition b interaction !ith fluids is called metasomatism.

-.2.1 0AT !S TE SORCE OF CEM!CALL+ ACT!3E FL!/S

Water is plentiful in the pore spaces of most sedimentar rocks, as!ell as in fractures

in igneous rocks. In addition, man minerals,such as clas, micas, and amphiboles,

are hydratedand thus contain !ater in their crstalline structures. le%ated

temperatures associated !ith lo! to moderate-grade metamorphism cause the

dehdration of these minerals. @nce expelled, the !ater mo%es along the surfaces of

indi%idual grains and is a%ailable to facilitate iontransport. o!e%er, in high-grade

metamorphic en%ironments,!here temperatures are extreme, these fluids ma be

dri%en from the rocks. +ecall that !hen oceanic crust is subducted to depths of about

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300 kilometers, !ater expelledfrom these slabs migrates into the mantle !edge

abo%e,!here it triggers melting ($igure /.2).

FIGURE 2.!

-.4 TE !MPORTANCE OF PARENT ROC"

9ost metamorphic rocks ha%e the same o%erall chemicalcomposition as the parent

rock from !hich the formed, exceptfor the possible loss or ac#uisition of %olatiles

such as!ater (/@) and carbon dioxide (C@/). $or example, themetamorphism of

shale results in slate, !here cla mineralsrecrstalli*e to form micas. (The minute

crstals of #uart*and feldspar found in shale are not altered in the transformationof

shale to slate and thus remain intermixed !ith themicas.) :lthough the mineralogchanges in the transformationof shale to slate, the o%erall chemical composition

ofslate is comparable to that of the rock from !hich it !as deri%ed.$urther, !hen the parent rock has a mafic composition,such as basalt, the

metamorphic product !ill be rich inminerals containing iron and magnesium unless,

of course,there has been a substantial loss of these atoms.

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In addition, the mineral makeup of the parent rock determines,to a large extent, the

degree to !hich each metamorphicagent !ill cause change. $or example, !henmagmaforces its !a into existing rock, high temperatures and associatedhot ion-rich

fluids tend to alter the host rock. Whenthe host rock is composed of minerals that are

comparati%elunreacti%e, such as the #uart* grains found in clean#uart* sandstone,

%er little alteration ma take place.o!e%er, if the host rock is a ;dirt< limestone

that containsabundant silica-rich cla, the calcite (CaC@2) in the limestonema react

!ith silica (i@/) in the clas to form !ollastonite(Cai@2) plus carbon dioxide

(C@/). In thissituation the *one of metamorphism ma extend for se%eralkilometers

from the magma bod.

FIGURE 2."metamorphic rocks, originall from laers of sandstone, shale, and

assorted mineral laers tpicall present in ocean beds. ome rust, so means at least

one iron substance is present in it as !ell.

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2. REFERENCES

9. :. T+I8A B . D. W:EE,Department ofEarth Sciences,

Monash niversity, !layton, "ictoria #$%&, 'ustralia

+ . . += 0 =, School ofEarth Sciences, Macuarie niversity,

orth *yde, .S. +. $ $#, 'ustralia

9. :. T+I8A B . D. W:EE,Department ofEarth Sciences,Monash niversity, !layton, "ictoria #$%&, 'ustralia

+ . . += 0 =, School ofEarth Sciences, Macuarie niversity,

orth *yde, .S. +. $ $#, 'ustralia

Introduction to ?hsical 8eolog - Thompson and Turk WW

arthF:nFIntroductionFtoF?hsicalF8eolog

ngineeringF8eologFFecondFdition

Introduction to ?hsical 8eolog - Thompson and Turk WW

arthF:nFIntroductionFtoF?hsicalF8eolog

ngineeringF8eologFFecondFdition

httpG&&!2.salemstate.edu&Hlhanson&gls300&arthF$/030&FarthF9etamorphism.html

httpsG&&!!!.google.com&searchJ#KfluidsLinLmetamorphismBclient

httpG&&elements.geoscience!orld.org&content&M&2&352.short

httpG&&!!!.geol.umd.edu&Hjmerck&geol300&lectures&3M.html

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http://w3.salemstate.edu/~lhanson/gls100/Earth_F2010/9_Earth_Metamorphism.htmlhttps://www.google.com/search?q=fluids+in+metamorphism&clienthttp://elements.geoscienceworld.org/content/6/3/153.shorthttp://www.geol.umd.edu/~jmerck/geol100/lectures/16.htmlhttps://www.google.com/search?q=fluids+in+metamorphism&clienthttp://elements.geoscienceworld.org/content/6/3/153.shorthttp://www.geol.umd.edu/~jmerck/geol100/lectures/16.htmlhttp://w3.salemstate.edu/~lhanson/gls100/Earth_F2010/9_Earth_Metamorphism.html

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