metamorphic processes
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MetamorphismMetamorphism
The change of shape or form of a rock due to
temperature and/or pressure
SOLID STATE re-crystallisation.
i.e. no melting takes place
CHANGE FORM
There are no additions to or removal of
elements (apart from some volatile removal)There is no change in bulk chemistry
(isochemical)
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What forms of Metamorphism areWhat forms of Metamorphism are
there?there?
Essentially there are two main ways in whichrocks can be metamorphosed:
1. Firstly by the increase in temperatureContact Metamorphism
2. Secondly by the application of pressure
(this could result in an increase in temp)
Regional Metamorphism
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REGIONAL METAMORPHISMREGIONAL METAMORPHISM
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There are two types of pressure involved in
metamorphism:
confining pressure and directed pressure.
The pressure is uniform
in all directions;
Increases with depth
Associated with
mountain building
processes
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Confining PressureConfining Pressure
Confining pressure can beuniform, pressure can be
applied in equal amounts
from all directions. This
usually happens at depth.
It can also be differential.This means greater pressure
can be applied in one
direction than the others
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Differential StressDifferential Stress
If differential stress is present duringmetamorphism, it can have a profound effect on
the texture of the rock.
Use the Blu Tak to experiment with different
effects of pressure on rocks.
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Rounded grains can become
flattened in the direction of
maximum compressional stress.
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Minerals that crystallize or grow in the
differential stress field may develop a
preferred orientation
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There are 4 main sources of temperature
changes in metamorphic rocks:
Increasing pressure/temperature withdepth
Radioactive decay
Deformation Migrating magmas
Locally, heating may occur as a result of
frictional shear heating in fault zones.
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Some points to remember:
Heat flows from regions of high to lowtemperature.
Substances have different heat capacities.
Temperature almost invariably increaseswith depth, but the rate at which the
temperature changes (geothermalgradient) depends on the tectonic settingand rock composition.
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An over simplified view of the twoAn over simplified view of the two
main sources of heat and pressuremain sources of heat and pressure
Regional
Metamorphism;
Associated with
destructive plate margins
Temperature AND
Pressure (directed)
Contact (Thermal)
Metamorphism;
Associated with
igneous intrusions
Mainly Temperature
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Contact (Thermal)Contact (Thermal)
MetamorphismMetamorphism
Contact
(thermal) heating of country rocks
during igneous intrusion or beneath
thick flows
Growth of new metamorphic mineralsin random orientations
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Metamorphic AureolesMetamorphic Aureoles
TypicalPluton
Andulu
cite/C
ordie
rite
hornfels
Sp
otted
Rock
When big enough to plot on a map the area affected by thermal
metamorphism is called anAUREOLE
The original rock into which the igneous rock intrudes is called the
COUNTRY ROCK
COUNTRY ROCK
AUREOLE
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Baked MarginsBaked Margins
COUNTRY ROCK
(ORIGINAL ROCK)
BAKED MARGINS
DYKE
How would you expect the grain size
to change near the margin?
What would you expect the grain
size to be?
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The Contact metamorphic rock willThe Contact metamorphic rock will
depend upon the original country rockdepend upon the original country rock
INTRUSION
SHALE
SANDSTONE
LIMESTONE
SPOTTED ROCK HORNFELS
META QUARZITE
MARBLE
COARSE
METAQUARZITE
COARSE MARBLE
(ORTHOQUARZITE)
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Contact Metamorphism of aContact Metamorphism of a
LimestoneLimestone
Limestone Marble
Granoblastic textureBioclastic texture
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Triassic Sandstone Metaquarzite
Granoblastic textureWell sorted, sub-rounded
texture
Contact Metamorphism of aContact Metamorphism of a
SandstoneSandstone
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Regional MetamorphismRegional Metamorphism
Large scale
metamorphismcharacteristic of
mountain belts and
shield areas as a result
of tectonism
Involves burial toproduce elevated
pressures and
temperatures, and
deformation to
produce tectonic
fabrics
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Regional MetamorphismRegional MetamorphismTEMPERATURE
PRESSURE/D
EPTH
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TexturesTextures
There are a range of metamorphic textures
that can be displayed. To an extent these
depend on the level of metamorphism, but
the original rock type is also important.
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Alignment of minerals in regionalAlignment of minerals in regional
metamorphismmetamorphism
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Development of slaty cleavageDevelopment of slaty cleavage
Slaty Cleavage resultsfrom the recrystallisation
of clay minerals into tiny
mica flakes. Because
each mica flake is a flat
tabular crystal it grows
to align itself at right
angles to the direction ofpressure, this forms
parallel layers which can
be easily cleaved.
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Cleavage and beddingCleavage and bedding
Bedding (relict)
Cleavage
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Progressive regional metamorphismProgressive regional metamorphism
Shale Slate
Schist Gneiss
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Progressive regional metamorphismProgressive regional metamorphism
Shale
Main textural differences
Slate Schist Gneiss
Increasing Grain size
Increasing segregation of minerals
Loss of slaty cleavage
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REGIONAL METAMORPHISMREGIONAL METAMORPHISM
Several different grades of metamorphism
can be produced when mudstones and
shales are affected by regional
metamorphism.
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LOW-GRADE REGIONALLOW-GRADE REGIONAL
METAMORPHISMMETAMORPHISM
High pressure
Low temperature
Mudstones and shales are turned to slate,slates as a result of slaty cleavage.
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MEDIUM-GRADE REGIONALMEDIUM-GRADE REGIONAL
METAMORPHISMMETAMORPHISM
High pressure
Moderately high temperature
Occurs during mountain building All traces of bedding lost. Clay minerals all
recrystallised to mica, rock reforms into a
schist showing parallel foliation.
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HIGH-GRADE REGIONALHIGH-GRADE REGIONAL
METAMORPHISMMETAMORPHISM
Intense pressure
Intense heat
Takes place at depth during formation of
mountain chains. Fine sediments completely transformed
into Gneiss (pronounced nice).
Minerals separate into light and darkbands, rock is coarse grained and it ispossible to identify minerals.
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