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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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METAMORPHIC TEXTURES
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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METAMORPHIC TEXTURES
1oliated te0tures
$ock or slaty clea!ageClosely spaced planar surfaces along which
rocks splitDe!elops in a number of ways
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METAMORPHIC TEXTURES
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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METAMORPHIC TEXTURES
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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METAMORPHIC TEXTURES
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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