Definition of Metamorphism

8/17/2019 Definition of Metamorphism

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Definition of Metamorphism

The word " Metamorphism" comes from the Greek: Meta = change, Morph = form, so

metamorphism means to change form. In geology this refers to the changes in mineral

assemblage and textre that reslt from sb!ecting a rock to pressres and temperatres differentfrom those nder which the rock originally formed.

The original rock that has ndergone metamorphism is called the protolith. rotolith can be

any type of rock and sometimes the changes in textre and mineralogy are so dramatic that is

difficlt to distingish what the protolith was.

• #ote that diagenesis and weathering are also a changes in form that occr in rocks. In

geology, howe$er, we restrict diagenetic processes to those which occr at temperatres

below %&&o' and pressres below abot (&& Ma )Ma stands for Mega ascals*, this is

e+i$alent to abot (,&&& atmospheres of pressre.

• Metamorphism therefore occrs at temperatres and pressres higher than %&& o' and

(&& Ma. ocks can be sb!ected to these higher temperatres and pressres as they

become bried deeper in the -arth. ch brial sally takes place as a reslt of

tectonic processes sch as continental collisions or sbdction.

• The pper limit of metamorphism occrs at the pressre and temperatre of wet partial

melting of the rock in +estion. /nce melting begins, the process changes to an igneos

process rather than a metamorphic process.

0ring metamorphism the protolith ndergoes changes in textre of the rock and the mineralmake p of the rock. These changes take place mostly in the solid state and are cased by

changes in physical or chemical conditions, which in trn can be cased by sch things as

brial, tectonic stress, heating by magma or interactions with flids.

Factors that Control Metamorphism

Metamorphism occrs becase rocks ndergo changes in temperatre and pressre and may be

sb!ected to differential stress and hydrothermal flids. Metamorphism occrs becase some

minerals are stable only nder certain conditions of pressre and temperatre. 1hen pressre

and temperatre change, chemical reactions occr to case the minerals in the rock to change to

an assemblage that is stable at the new pressre and temperatre conditions. 2t, the process is

complicated by sch things as how the pressre is applied, the time o$er which the rock is

sb!ected to the higher pressre and temperatre, and whether or not there is a flid phase

present dring metamorphism.

• Temperatre

o Temperatre increases with depth in the -arth along the Geothermal Gradient.

Ths higher temperatre can occr by brial of rock.


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o Temperatre can also increase de to igneos intrsion.

• ressre increases with depth of brial, ths, both pressre and temperatre will $ary

with depth in the -arth. ressre is defined as a force acting e+ally from all directions.

It is a type of stress, called hydrostatic stress, or uniform stress.

If the stress is not e+al from all directions, then the stress is called a differential stress.

There are two kinds of differential stress. Normal stress cases ob!ects to be

compressed in the direction of maximm principal stress and extended in the direction

of minimal stress. If differential stress is present dring metamorphism, it can ha$e a

profond effect on the textre of the rock. Shear stress cases ob!ects to be smeared ot

in the direction of applied stress.

0ifferential stress if acting on a rocks can ha$e a profond affect on the appearance or

textre of the rock.

onded grains can become flattened in

the direction of maximm stress.


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Minerals that crystalli3e or grow in the

differential stress field can ha$e a

preferred orientation. This is especially

tre of the sheet silicate minerals )the

micas: biotite and msco$ite, chlorite, talc,

and serpentine*.

These sheet silicates will grow with their sheets orientated perpendiclar to the direction

of maximm stress. referred orientation of sheet silicates cases rocks to be easily

broken along approximately parallel sheets. ch a strctre is called a foliation.

• 4lid hase.5 6ny existing open space between mineral grains in a rock can potentially

contain a flid. This flid is mostly 7%/, bt contains dissol$ed ions. The flid phase is

important becase chemical reactions that in$ol$e changing a solid mineral into a new

solid mineral can be greatly speeded p by ha$ing dissol$ed ions transported by the

flid. If chemical alteration of the rock takes place as a reslt of these flids, the

process is called metasomatism.

• Time 5 2ecase metamorphism in$ol$es changing the rock while it is solid,

metamorphic change is a slow process. 0ring metamorphism, se$eral processes are at

work. ecrystalli3ation cases changes in minerals si3e and shape. 'hemical reactions

occr between the minerals to form new sets of minerals that are more stable at the

pressre and temperatre of the en$ironment, and new minerals form as a reslt of

polymorphic phase transformations )recall that polymorphs are componds with the

same chemical formla, bt different crystal strctres.

8aboratory experiments sggest that the the si3es of the mineral grains prodced dring

metamorphism increases with time. Ths coarse grained metamorphic rocks in$ol$e

long times of metamorphism. -xperiments sggest that the time in$ol$ed is tens of

millions of years.

Grade of Metamorphism

Metamorphic grade is a general term fordescribing the relati$e temperatre and

pressre conditions nder which metamorphic

rocks form. 6s the temperatre and9or

pressre increases on a body of rock we say

that the rock ndergoes prograde

metamorphism or that the grade of

metamorphism increases.


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• 8ow5grade metamorphism takes place at temperatres between abot %&& to (%&o', and

relati$ely low pressre. 8ow grade metamorphic rocks are characteri3ed by an

abndance of hydrous minerals )minerals that contain water, 7%/, in their crystal

strctre*.

o -xamples of hydros minerals that occr in low grade metamorphic rocks:

'lay Minerals

erpentine

'hlorite

• 7igh5grade metamorphism takes place at temperatres greater than (%&o' and relati$ely

high pressre. 6s grade of metamorphism increases, hydros minerals become less

hydros, by losing 7%/ and non5hydros minerals become more common.

o -xamples of less hydros minerals and non5hydros minerals that characteri3e

high grade metamorphic rocks:

Msco$ite 5 hydros mineral that e$entally disappears at the

highest grade of metamorphism

2iotite 5 a hydros mineral that is stable to $ery high grades of

metamorphism.

yroxene 5 a non hydros mineral.

Garnet 5 a non hydros mineral.

Retrograde Metamorphism

6s temperatre and pressre fall de to erosion of o$erlying rock or de to tectonic plift, one

might expect metamorphism to a follow a re$erse path and e$entally retrn the rocks to their

original nmetamorphosed state. ch a process is referred to as retrograde metamorphism. If

retrograde metamorphism were common, we wold not commonly see metamorphic rocks at

the srface of the -arth. ince we do see metamorphic rocks exposed at the -arths srface

retrograde metamorphism does not appear to be common. The reasons for this inclde:

• chemical reactions take place more slowly as temperatre is decreased


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• dring prograde metamorphism, flids sch as 7%/ and '/% are dri$en off, and these

flids are necessary to form the hydros minerals that are stable at the -arths srface.

• chemical reactions take place more rapidly in the presence of flids, bt if the flids are

dri$en off dring prograde metamorphism, they will not be a$ailable to speed p

reactions dring retrograde metamorphism.

Metamorphic Rock Types

There are two ma!or sbdi$isions of metamorphic rocks.

;. 4oliated < These ha$e a planar foliation cased by the preferred orientation )alignment*

of minerals and formed nder differential stress.

They ha$e a significant amont of sheet silicate )platy minerals and are classified by

composition, grain si3e, and foliation type.

%. #on5foliated < These ha$e no e$ident planar fabric or foliation, crystalli3ed nder

conditions where there was no differential stress, and are comprised of e+ant minerals

only. These are classified mainly by the minerals present or the chemical composition of

the protolith.

Foliated Metamorphic Rocks

-xample 5 metamorphism of

a shale, made p initially ofclay minerals and +art3 all

of clay or silt si3e.

• Slate - lates form at low metamorphic grade by the growth of fine grained chlorite and

clay minerals. The preferred orientation of these sheet silicates cases the rock to easily

break along the planes parallel to the sheet silicates, casing a slatey cleavage. #ote that

in the case shown here, the maximm stress is applied at an angle to the original bedding

planes, so that the slatey clea$age has de$eloped at an angle to the original bedding.


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2ecase of the nearly perfect breakage along planes, slates are sefl for blackboardsand shingles.

• Phyllite 5 4ine mica5rich rock, formed by low < medim grade metamorphism. In a

phyllite, the clay minerals ha$e recrystalli3ed into tiny micas )biotite and msco$ite

which reflect a satiny lster. hyllite is between slate and schist.

• Schist 5 The si3e of the mineral grains tends to enlarge with increasing grade of

metamorphism. -$entally the rock de$elops a near planar foliation cased by the

preferred orientation of sheet silicates )mainly biotite and msco$ite*. art3 and

4eldspar grains, howe$er show no preferred orientation. The irreglar planar foliation atthis stage is called schistosity.

chist often has other minerals besides micas. These inclde minerals like 5 art3, 4eldspars,


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>yanite, Garnet, tarolite, and illimanite.

1hen these non5mica minerals occr with a grain si3e greater than the rest of the rock, they are

called pophyroblasts.

• Gneiss 6s metamorphic grade increases, the sheet silicates become nstable and dark

colored minerals like hornblende and pyroxene start to grow. These dark colored

minerals tend to become segregated in distinct bands throgh the rock, gi$ing the rock

a gneissic banding . 2ecase the dark colored minerals tend to form elongated crystals,

rather than sheet5 like crystals, they still ha$e a preferred orientation with their long

directions perpendiclar to the maximm differential stress.

• Granulite 5 6t the highest grades of metamorphism all of the hydros minerals and sheet

silicates become nstable and ths there are few minerals present that wold show a

preferred orientation. The reslting rock will ha$e a granlitic textre that is similar to a

phaneritic textre in igneos rocks.

• Migmatites < If the temperatre reaches the solids temperatre )first melting

temperatre*, the rock may begin to melt and start to co5mingle with the solids. ?sally

these melts are felsic with the mafic material remaining metamorphic.

Non-foliated Metamorphic Rocks

#on5foliated rocks lack a planar fabric . 6bsence of foliation possible for se$eral reasons:

• ock not sb!ected to differential stress.


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• 0ominance of e+ant minerals )like +art3, feldspar, and garnet*.

• 6bsence of platy minerals )sheet silicates*.

#on5foliated rocks are gi$en specific names based on their mineralogy and composition:

Amphibolite 5 These rocks are dark colored rocks with amphibole )sally hornblende* as their

ma!or mineral. They are sally poorly foliated and form at intermediate to high grades of

metamorphism of basaltic or gabbroic protoliths.

Hornfels 5 These are $ery fine grained rocks that sally form as a reslt of magma intrding

into fined grained igneos rocks or shales. The magma cases a type of metamorphism called

contact metamorphism )to be discssed later*.

Quartite 5 6 rock made p almost entirely of +art3. They are formed by metamorphism of

+art3 arenites )sandstones*. ince +art3 is stable o$er a large range of temperatres and pressres, no new minerals are formed dring metamorphism, and the only metamorphic effect

that occrs is recrystalli3ation of the +art3 reslting in interlocking crystals that make p a $ery

hard rock.

Marble 5 6 limestone or dolostone made p only of calcite or dolomite will metamorphose to a

marble which is made mostly recrystalli3ed calcite or dolomite. The ecrystalli3ation sally

obliterates all fossils. Marbles ha$e a $ariety of colors and are often complexly banded. They

are commonly sed as a decorati$e stone.

Protolith Composition

6lthogh textres and strctres of the protolith are sally destroyed by metamorphism, wecan still get an idea abot the original rock from the minerals present in the metamorphic rock.

Minerals that form, do so becase the chemical elements necessary to form them are present in

the protolith.

General terms sed to describe the chemical composition of both the protolith and the reslting

metamorphic rock are:

Pelitic 6lmina rich rocks, sally shales or mdstones. These start ot originally with clay

minerals and as a reslt of metamorphism, 6lmina rich minerals like micas, chlorite, garnet,

kyanite, sillimanite and andalsite form. 2ecase of the abndance of sheet silicates, peliticrocks commonly form slates, phyllites, schists, and gneisses dring metamorphism.

Mafic 5 These are Mg and 4e rich rocks with low amonts of i. Minerals like biotite,

hornblende and plagioclase form dring metamorphism and commonly prodce amphibolites.

!alcareous 5 These are calcim5rich rocks sally deri$ed from limestones or dolostones, and

ths contain an abndance of 'alcite. Marbles are the type of metamorphic rock that reslts.

Quarto-"eldspathic 5 ocks that contain an abndance of +art3 and feldspar fall into this

category. rotoliths are sally granites, rhyolites, or arkose sandstones and metamorphism

reslts in gneisses containing an abndance of +art3, feldspar, and biotite.


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Types of Metamorphism

Metamorphism can take place in se$eral different en$ironments where special conditions exist

in terms of pressre, temperatre, stress, conditions, or chemical en$ironments. 1e here

describe se$eral diff rent types of metamorphism that are recogni3ed.

• !ontact Metamorphism #also called thermal metamorphism$ 5 /ccrs ad!acent to

igneos intrsions and reslts from high temperatres associated with the igneos

intrsion. ince only a small area srronding the intrsion is heated by the magma,

metamorphism is restricted to a 3one srronding the intrsion, called

a metamorphic aureole. /tside of the contact areole, the rocks are

nmetamorphosed. The grade of metamorphism increases in all directions toward the

intrsion. 2ecase temperatre differences between the srronding rock and the

intrded magma are larger at shallow le$els in the crst, contact metamorphism is

sally referred to as high temperatre, low pressre metamorphism. The rock prodced

is often a fine5grained rock that shows no foliation, called a hornfels.

• %urial Metamorphism - 1hen sedimentary rocks are bried to depths of se$eral

hndred meters, temperatres greater than (&&o' may de$elop in the absence of

differential stress. #ew minerals grow, bt the rock does not appear to be

metamorphosed. The main minerals prodced are the @eolites. 2rial metamorphism

o$erlaps, to some extent, with diagenesis, and grades into regional metamorphism as

temperatre and pressre increase.

• &ynamic Metamorphism 5 This type of metamorphism is de to mechanicaldeformation, like when two bodies of rock slide past one another along a falt 3one.


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7eat is generated by the friction of sliding along the 3one, and the rocks tend to crshed

and pl$eri3ed de to the sliding. 0ynamic metamorphism is not $ery common and is

restricted to a narrow 3one along which the sliding occrred. The rock that is prodced

is called a mylonite.

• 'egional Metamorphism 5 This type of

metamorphism occrs o$er large areas

that were sb!ected to high degrees of

deformation nder differential stress.

Ths, it sally reslts in forming

metamorphic rocks that are strongly

foliated, sch as slates, schists, and

gneisses. The differential stress sally

reslts from tectonic forces that prodce

a compression of the rocks, sch as

when two continental masses collide

with one another. Ths, regionally

metamorphosed rocks occr in the cores

of montain ranges or in eroded

montain ranges. 'ompressi$e stresses

reslt in folding of the rock, as shown

here, and reslts in thickening of the

crst which tends to psh rocks down to

deeper le$els where they are sb!ected tohigher temperatres and pressres )ee

4igre A.%& in yor text*.

6 map of a hypothetical regionally metamorphosed area is shown in the figre below. Most

regionally metamorphosed areas can be di$ided into 3ones where a particlar mineral, called

an inde( mineral) is characteristic of the 3one. The 3ones are separated by lines )srfaces in

three dimensions* that mark the first appearance of the index mineral. These lines are

called isograds )meaning e+al grade* and represent lines )really srfaces* where the grade of

metamorphism is e+al. 6 map of a regionally metamorphosed areas are can be seen in figre

A.;B of yor text.


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Hydrothermal Metamorphism 5 #ear oceanic ridges where the oceanic crst is broken p by

extensional falts, sea water can descend along the cracks. ince oceanic ridges are areas where

new oceanic crst is created by intrsion and erption of basaltic magmas, these water5rich

flids are heated by the hot crst or magma and become hydrothermal flids. The hydrothermal

flids alter the basaltic oceanic crst by prodcing hydros minerals like chlorite and

talc. 2ecase chlorite is a green colored mineral the rocks hydrothermal metamorphic rocks arealso green and often called greenstones.

Subduction 'elated Metamorphism 5 6t a sbdction 3one, the oceanic crst is pshed

downward reslting in the basaltic crst and ocean floor sediment being sb!ected to relati$ely

high pressre. 2t, becase the oceanic crst by the time it sbdcts is relati$ely cool, the

temperatres in the crst are relati$ely low. ?nder the conditions of low temperatre and high

pressre, metamorphism prodces an nsal ble mineral, glacophane. 'ompressional

stresses acting in the sbdction 3one create the differential stress necessary to form schists and

ths the reslting metamorphic rocks are called bleschist

Shoc* Metamorphism 5 1hen a large meteorite collides with the -arth, the kinetic energy is

con$erted to heat and a high pressre shock wa$e that propagates into the rock at the impact

site. The heat may be enogh to raise the temperatre to the melting temperatre of the earth

rock. The shock wa$e prodces high enogh pressre to case +art3 to change its crystal

strctre to more a dense polymorph like coesite or stisho$ite. 6ncient meteorite impact sites

ha$e been disco$ered on the basis of finding this e$idence of shock metamorphism.

Metamorphic Facies

In general, metamorphic rocks do not ndergo significant changes in chemical composition

dring metamorphism. The changes in mineral assemblages are de to changes in thetemperatre and pressre conditions of metamorphism. Ths, the mineral assemblages that are


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obser$ed mst be an indication of the temperatre and pressre en$ironment that the rock was

sb!ected to. This pressre and temperatre en$ironment is referred to as Metamorphic

"acies.

The se+ence of metamorphicfacies obser$ed in any

metamorphic terrain, depends

on the geothermal gradient

that was present dring

metamorphism. 6 high

geothermal gradient sch as

the one labeled "6" in the

figre shown here, might be

present arond an igneos

intrsion, and wold reslt in

metamorphic rocks belonging

to the hornfels facies. ?nder

a normal geothermal gradient,

sch as "2" in the figre,

rocks wold progress from

3eolite facies to greenschist,

amphibolite, and eclogite

facies as the grade of

metamorphism )or depth of

brial* increased.

If a low geothermal gradient was present, sch the one labeled "'" in the diagram, then rocks

wold progress from 3eolite facies to bleschist facies to eclogite facies. Ths, if we know the

facies of metamorphic rocks in the region, we can determine what the geothermal gradient mst

ha$e been like at the time the metamorphism occrred.


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The Rock Cycle

2efore mo$ing on to the rest of the corse, yo shold read Interlde ' in yor textbook )pages

%B;5%BA*. #ow that we ha$e discssed the three types of rocks, it is important to nderstand

how the atoms that make p these rocks cycle throgh the earth. This cycling in$ol$es process

that will be discssed in detail throghot the remainder of this corse. ince the rock cycle

links the rock forming processes to tectonic process and to srface process )most of which will

be discssed throghot the rest of the corse* , it is important to nderstand the concept of the

rock cycle and the $arios linkages in$ol$ed.

• The rock cycle in$ol$es cycling of

elements between $arios types of rocks,

and ths mostly in$ol$es the lithosphere.

• The rock cycle in$ol$es the three types of

rocks as reser$oirs );* igneos, )%*

sedimentary, and )(* metamorphic.

• 'hemical elements can reside in each typeof rock, and geologic processes mo$e these

elements into another type of rock.


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• -nergy for the parts of the crstal cycle near the -arths srface is solar and gra$itational

energy )which control erosion and weathering*, whereas

• energy that dri$es processes beneath the srface is geothermal and gra$itational energy

)which control plift, sbsidence, melting, and metamorphism*.

1e here start or discssion with Colcanoes and Colcanic erptions and processes that are

in$ol$ed in the prodction of igneos rocks at the earths srface.

Questions on this material that might e asked on an e!am

;. 0efine the following: )a* geothermal gradient, )b* metamorphism, )c* differentialstress, )d* prograde metamorphism, )e* metasomatism )f* protolith, )g* foliation, )i*

metamorphic areole, )!* isograd, )k* greenstone, )l* bleschist.

%. tarting with a shale, describe the textral changes that wold occr to the rock dring

prograde metamorphism with differential stress conditions present.

(. 1hy is retrograde metamorphism ncommonD

E. 0escribe the following non5foliated metamorphic rocks )a* amphibolite, )b* +art3ite,

)c* marble, )d* hornfels.

F. 1hat are the terms sed to describe the general chemical composition of metamorphic

rocksD. 0escribe the type of rocks and minerals for nd in each.

B. 1hat are the $arios types of metamorphismD 0escribe the rocks prodced by each.

. 1hat is the progression of metamorphic facies that wold occr along a high geothermal

gradient, a normal geothermal gradient, and a low geothermal gradient.

Documents

Definition of Metamorphism