04 metamorphic

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METAMORPHISM

AND METAMORPHICROCKS

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METAMORPHISM

Transition of one rock into another by temperatures

and/or pressures unlike those in which it formed

Metamorphic rocks are produced from

Igneous rocks

Sedimentary rocks

Other metamorphic rocks

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METAMORPHISM

Metamorphism progresses incrementally from low-grade

to high-grade

During metamorphism the rock must remain solid

Metamorphic settings

Contact orthermal metamorphism dri!en

by a rise in temperature within the host rock

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METAMORPHISM

Metamorphic settings

"ydrothermal metamorphism chemical

alterations from hot# ion-rich water$egional metamorphismOccurs during mountain building

%roduces the greatest !olume of metamorphic rock

$ocks usually display &ones of contact and/or

hydrothermal metamorphism

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METAMORPHISM(NOT IN

PAPER)

"eat

Most important agent

$ecrystalli&ation results in new# stableminerals

Two sources of heatContact metamorphism heat from magma

'n increase in temperature with depth -

geothermal gradient ()* to +* degrees Celsius per

kilometer of depth,5


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AGENTS OF

METAMORPHISM

%ressure and differential stress

Increases with depth

Confining pressure applies forces eually inall directions

$ocks may also be sub.ected to differential

stress which is uneual in different

directions

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METAMORPHISM

7Figure 8.4


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AGENTS OF

METAMORPHISM

Chemically acti!e fluidsMainly water

nhances migration of ions'ids in recrystalli&ation of e0isting

mineralsSources of fluids

%ore spaces of sedimentary rocks1ractures in igneous rocks"ydrated minerals such as clays and micas

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AGENTS OF

METAMORPHISM

The importance of parent rock

Most metamorphic rocks ha!e the same

o!erall chemical composition as the

original parent rockMineral makeup determines# to a large

e0tent# the degree to which each

metamorphic agent will cause change

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METAMORPHIC TEXTURES

Te0ture - si&e# shape# and arrangement of mineral

grains

1oliation any planar arrangement of mineral grains

or structural features within a rock

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1oliation

0amples of foliation%arallel alignment of platy and/or elongated

minerals%arallel alignment of flattened mineral grains

and pebbles

Compositional banding

Slaty clea!age where rocks can be easily splitinto thin# tabular sheets

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METAMORPHIC

TEXTURES

1oliation

1oliation can form in !arious ways

including

$otation of platy and/or elongated minerals$ecrystalli&ation of minerals in the direction

of preferred orientation

Changing the shape of euidimensional grains

into elongated shapes that are aligned

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FOLIATION RESULTING

FROM DIRECTED STRESS

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1oliated te0tures

$ock or slaty clea!ageClosely spaced planar surfaces along which

rocks splitDe!elops in a number of ways

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1oliated te0tures

Schistosity%laty minerals are discernible with the

unaided eye0hibit a planar or layered structure

$ocks ha!ing this te0ture are referred to as

schist

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1oliated te0tures

2neissicDuring higher grades of metamorphism# ion

migration results in the segregation ofminerals

2neissic rocks e0hibit a distincti!e banded

appearance

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Other metamorphic te0tures

Those metamorphic rocks that lackfoliation are referred to as nonfoliated

De!elop in en!ironments where deformation isminimalTypically composed of minerals that e0hibit

euidimensional crystals

%orphyroblasticte0tures3arge grains# called porphyroblasts#

surrounded by a fine-grained matri0 of otherminerals

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COMMON METAMORPHIC

ROCKS

1oliated rocks

Slate4ery fine-grained

0cellent rock clea!ageMost often generated from low-grade

metamorphism of shale# mudstone# or siltstone

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COMMON METAMORPHIC

ROCKS

1oliated rocks

%hyllite2radational between slate and schist

%laty minerals not large enough to beidentified with the unaided eye

2lossy sheen and wa!y surfaces

0hibits rock clea!age

Composed mainly of fine crystals of musco!ite

and/or chlorite

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SLATE (LEFT) AND

PHYLLITE (RIGHT)

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COMMON METAMORPHIC

ROCKS

1oliated rocks

SchistMedium- to coarse-grained

%laty minerals (mainly micas, predominateThe term schistdescribes the te0ture

To indicate composition# mineral names are

used (such as mica schist,

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GARNET-MICA SCHIST

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COMMON METAMORPHIC

ROCKS 1oliated rocks

2neissMedium- to coarse-grained

5anded appearance"igh-grade metamorphism

Often composed of light-colored feldspar-rich

layers with bands of dark ferromagnesian

minerals

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CLASSIFYING METAMORPHIC

ROCKS

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COMMON METAMORPHIC

ROCKS 6onfoliated rocks

MarbleCoarse# crystalline

%arent rock was limestone or dolostoneComposed essentially of calcite or dolomite

crystals

7sed as a decorati!e and monument stone

0hibits a !ariety of colors

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MARBLE

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COMMON METAMORPHIC

ROCKS

6onfoliated rocks

8uart&ite1ormed from a parent rock of uart&-rich

sandstone8uart& grains are fused together

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QUARTZITE

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METAMORPHIC

ENVIRONMENTS

Contact or thermal metamorphism

$esult from a rise in temperature when

magma in!ades a host rock

9one of alteration (aureole,forms in therock surrounding the magma

Most easily recogni&ed when it occurs at

or near arth:s surface

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METAMORPHISM

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METAMORPHIC

ENVIRONMENTS

"ydrothermal metamorphism

Chemical alteration caused when hot#

ion-rich fluids circulate through fissures

and cracks that de!elop in rockMost widespread along the a0is of the

mid-ocean ridge system

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METAMORPHISM

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METAMORPHIC

ENVIRONMENTS $egional metamorphism

%roduces the greatest uantity of

metamorphic rock

'ssociated with mountain building

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METAMORPHIC

ENVIRONMENTS Other metamorphic en!ironments

5urial metamorphism'ssociated with !ery thick sedimentary strata

$euired depth !aries depending on thepre!ailing geothermal gradient

Metamorphism along fault &onesOccurs at depth and high temperatures

%re-e0isting minerals deform by ductile flow

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METAMORPHIC

ENVIRONMENTS Other metamorphic en!ironments

Impact metamorphismOccurs when high speed pro.ectiles called

meteorites strike arth:s surface$ocks are called impactites

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METAMORPHIC ZONES

Systematic !ariations in the mineralogy and te0tures

of metamorphic rocks are related to the !ariations in

the degree of metamorphism

Inde0 minerals and metamorphic grade

Changes in mineralogy occur from

regions of low-grade metamorphism to

regions of high-grade metamorphism

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METAMORPHIC ZONES

Inde0 minerals and metamorphic grade

Certain minerals# called inde0 minerals#

are good indicators of the metamorphic

conditions in which they formMigmatites"ighest grades of metamorphism that are

transitional to igneous rocks

Contain light bands of igneous components

along with areas of unmelted metamorphic

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METAMORPHISM AND

PLATE TECTONICS Most metamorphism occurs along con!ergent plate

boundaries

Compressional stresses deform the edges

of the plate1ormation of arth:s ma.or mountain

belts including the 'lps# "imalayas# and

'ppalachians

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METAMORPHISM AND

PLATE TECTONICS 3arge-scale metamorphism also occurs along

subduction &ones at con!ergent boundaries

Se!eral metamorphic en!ironments e0ist

hereImportant site of magma generation

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METAMORPHISM AND

PLATE TECTONICS Metamorphism at subduction &ones

Mountainous terrains along subduction

&ones e0hibit distinct linear belts of

metamorphic rocks"igh-pressure# low-temperature &ones nearest

the trench

"igh-temperature# low-pressure &ones further

inland in the region of igneous acti!ity

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METAMORPHIC ENVIRONMENTS

AND PLATE TECTONICS

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04 metamorphic

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