Metamorphic Rock2015

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Metamorphic Rock


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Rock Cycle


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Metamorphism means

"changed form".

Agents of Metamorphism

Changes occur because of:

• Heat

• Pressure

•

Chemical fluids*Rocks adjust to become more stable under new,

higher temperatures and pressures.


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HEAT1.There are several sources of heat for

metamorphism.

a. Geothermal gradient

Temperature increases with depth at a rate of 20 -

30 degrees C per km in the crust.Ultimate source of the heat? Radioactive decay.

b. Increase of temperature and pressure withdepth causes Regional Metamorphism

*Heat may come from large bodies of molten rockrising under a wide geographic area.


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1.Intrusions of hot

magma can bake

rocks as it intrudesthem. Lava flows

can also bake rocks

on the ground

surface.

HEAT


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Hornfels is a common

contact metamorphic rock.

2. Lava or magma in contact with other

rock causes Contact Metamorphism.


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Example of :

Contact metamorphism

along a narrow (approx.

1 meter wide) diabase

dike in the Deep River

Basin of North Carolina.Diabase weathers tan.

Contact metamorphic

hornfels are gray. Hostrocks are red siltstones.


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Pressure

1. Burial Pressure. Pressure increases withdepth due to the weight of the overlying

rocks.

A cubic foot of granite weighs 167.9 pounds.

Increase of pressure and temperature with depthcauses Regional Metamorphism. Regional

metamorphism occurs at depths of 5 - 40 km.


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Buried rocks are subjected to the force, or stress, Pressure

(stress)as metamorphic agent exerted by the load above.

This confining pressure is analogous to water pressurewhere the force is applied equally in all directions.


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Pressure

2. Tectonic pressures associated with

convergent plate boundaries and

continental collision also cause RegionalMetamorphism.

Pressure along fault zones causes Dynamic

Metamorphism, due to the crushing andductile flow of rock.


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Convergent plate boundary


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Chemical Fluids

In some metamorphic settings, new materials

are introduced by the action of

hydrothermal solutions (hot water withdissolved ions). Many metallic ore

deposits form in this way.

• Hydrothermal solutions associated withmagma bodies


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Chemical Fluids

Black smokers - Sea water percolates through

newly formed oceanic crust, dissolving

out metallic sulfide minerals.


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The hot sea waterrises alongfractures and

pours from ventsin the seafloor as black clouds ofdark mineral-rich

water pour fromthe vent.


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Black smokers

Sulfide minerals (such as pyrite, sphalerite, andgalena) and copper precipitate when the hotwater comes in contact with cold sea water.


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How do rocks change?

1. Metamorphism causes changes in:

2. Texture

3. Mineralogy

Texture

• The processes of compaction and

recrystallization change the texture of

rocks during metamorphism.


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Classification of Metamorphic Rocks


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Compaction

• The grains move closer together.

• The rock becomes more dense.

• Porosity is reduced.

• Example: clay to shale to slate


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clay to shale to slate

(sediment) (sedimentary rock) (metamorphic)


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Recrystallization

Growth of new crystals. No changes in overall chemistry.

New crystals grow from the minerals already

present.

A preferred orientation of minerals commonly

develops under applied pressure. Platy or sheet-likeminerals such as muscovite and biotite become

oriented perpendicular to the direction of force. This

preferred orientation is called foliation.


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The diagram illustrates the effect of foliation.

A texture of this

sort in a

metamorphicrock is called

FOLIATION

and the rocks are

said to be

FOLIATED .


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Metamorphic Textures

• Foliation is a broad term referring to the

alignment of sheet-like minerals. Types of

foliation:

http://www.beyondbooks.com/bbx/tr/pv.asp?i=ear82/7c/00030855&u=http://www.union.edu/PUBLIC/GEODEPT/COURSES/geo-10/metamorphic.htmhttp://www.beyondbooks.com/bbx/tr/pv.asp?i=ear82/7c/00030855&u=http://www.union.edu/PUBLIC/GEODEPT/COURSES/geo-10/metamorphic.htm


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Metamorphic Textures

• Schistosity - alignment of large mica flakes, as ina mica schist derived from the metamorphism ofshale.

• Slaty cleavage - alignment of very fine-grainedmicas, as in a slate derived from themetamorphism of shale.

•

Phyllitic structure - alignment of fine-grainedmicas, as in a phyllite.

• Gneissic banding - segregation of light and darkminerals into distinct layers in the rock, as in a

gneiss.


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Slaty cleavage


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Slate - very fine grained rock.

Resembles shale.Has slaty cleavage which may be at an

angle to the original bedding. Relict

bedding may be seen on cleavage planes.Often dark gray in color. "Rings" when you

strike it. (Unlike shale, which makes a dull

sound. Temperature about 200 degrees C;Depth of burial about 10 km.


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Phyllitic structures


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Phyllite - fine-grained metamorphic

rock. Has a frosted sheen, resemblingfrosted eye shadow. This is no

coincidence.

Cosmetics commonly contain ground up

muscovite (ground to a size similar to that

occurring naturally in phyllite.)


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Gneissic Banding


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Gneiss - (pronounced "nice") - a

banded or striped rock withalternating layers of dark and light

minerals. The dark layers

commonly contain biotite, and the

light layers commonly contain

quartz and feldspar.


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Schistosity


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Schist - metamorphic rock containing

abundant obvious micas, severalmillimeters across. Several types of schist

may be recognized, based on minerals

which may be present:•mica schist

•garnet schist

•chlorite schist•kyanite schist

•talc schist


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Non-foliated

• Non-foliated or granular

metamorphic rocks are those which

are composed of equidimensional

grains such as quartz or calcite.

There is no preferred orientation.The grains form a mosaic.


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Non-foliated

Limestone to Marble Sandstone to Quartzite


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Marble - fizzes in acid because its dominant

minerals is calcite (or dolomite). The parent

rock is limestone (or dolostone).


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Quartzite - interlocking grains of quartz.

Scratches glass. The rock fractures through the

grains (rather than between the grains as itdoes in sandstone). The parent rock is quartz

sandstone.

Mi l h i


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Mineral changes in

metamorphic rocks

1. Recrystallization -

rearrangement of

crystal structure of

existing minerals.

Commonly many small crystals merge to form largercrystals, such as the clay in shale becoming micas inslate, phyllite, and schist.


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Mineral changes in

metamorphic rocks

1. Formation of new minerals - there are a

number of metamorphic minerals which form

during metamorphism and are found exclusively(or almost exclusively) in metamorphic rocks:

• Garnet - dark red dodecahedrons (12 sides)

• Staurolite - brown lozenge-shaped minerals,

commonly twinned to form "fairy crosses". Statemineral of Georgia.


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Metamorphic index minerals

1. In regional metamorphic terranes, thetemperature and pressure regime isindicated by the distribution ofmetamorphic minerals across a large area.

Low metamorphic grade (low temperaturesand pressures) - about 200 degrees CSlate and phyllite

•

chlorite• muscovite

• biotite

i f


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Metamorphism of Basalts and

Gabbros

• Greenschist - Olivine, pyroxene, and plagioclase in anoriginal basalt change to amphiboles and chlorite (bothcommonly green) as water in the pore spaces reacts withthe original minerals at temperatures and pressures of low

grade metamorphism.

• Amphibolite - As pressure and temperature increase tointermediate grades of metamorphism, only dark coloredamphiboles and plagioclase survive and the resulting rock

is called an amphibolite.• Granulite - At the highest grade of metamorphism the

amphiboles are replaced by pyroxenes and garnets, thefoliation is lost and a granulite that has a granulitic textureis produced.


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Greenschist –

Olivine,

pyroxene, and

plagioclase in an

original basaltchange to

amphiboles and

chlorite (both

commonly green)


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•As pressure and temperature increase to intermediate grades ofmetamorphism, only dark colored amphiboles and plagioclasesurvive and the resulting rock is called an amphibolite.

Amphibolite -


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At the highest grade ofmetamorphism the

amphiboles are replaced

by pyroxenes and garnets,

the foliation is lost and a

granulite that has agranulitic texture is

produced.

Granulite -


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Metamorphic Rock2015

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