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


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


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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Metamorphic Processes

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