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Metamorphic Rock
Rock Cycle
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.
HEAT
1.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 with
depth causes Regional Metamorphism
*Heat may come from large bodies of molten rock
rising under a wide geographic area.
HEAT
1.Intrusions of hot
magma can bake
rocks as it intrudes
them. Lava flows
can also bake rocks
on the ground
surface.
2. Lava or magma in contact with other
rock causes Contact Metamorphism.
Hornfels is a common
contact metamorphic rock.
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. Host
rocks are red siltstones.
Pressure
1. Burial Pressure. Pressure increases with
depth due to the weight of the overlying
rocks.
A cubic foot of granite weighs 167.9 pounds.
Increase of pressure and temperature with depth
causes Regional Metamorphism. Regional
metamorphism occurs at depths of 5 - 40 km.
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 pressure
where the force is applied equally in all directions.
Pressure
2. Tectonic pressures associated with
convergent plate boundaries and
continental collision also cause Regional
Metamorphism.
Pressure along fault zones causes Dynamic
Metamorphism, due to the crushing and
ductile flow of rock.
Convergent plate boundary
Chemical Fluids
In some metamorphic settings, new materials
are introduced by the action of
hydrothermal solutions (hot water with
dissolved ions). Many metallic ore
deposits form in this way.
• Hydrothermal solutions associated with
magma bodies
Chemical Fluids
Black smokers - Sea water percolates through
newly formed oceanic crust, dissolving
out metallic sulfide minerals.
The hot sea water
rises along
fractures and
pours from vents
in the seafloor as
black clouds of
dark mineral-rich
water pour from
the vent.
Black smokers
Sulfide minerals (such as pyrite, sphalerite, and
galena) and copper precipitate when the hot
water comes in contact with cold sea water.
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.
Classification of Metamorphic Rocks
Compaction
• The grains move closer together.
• The rock becomes more dense.
• Porosity is reduced.
• Example: clay to shale to slate
clay
to
shale
to
slate
(sediment) (sedimentary rock) (metamorphic)
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-like
minerals such as muscovite and biotite become
oriented perpendicular to the direction of force. This
preferred orientation is called foliation.
The diagram illustrates the effect of foliation.
A texture of this
sort in a
metamorphic
rock is called
FOLIATION
and the rocks are
said to be
FOLIATED.
Metamorphic Textures
• Foliation is a broad term referring to the
alignment of sheet-like minerals. Types of
foliation:
Metamorphic Textures
• Schistosity - alignment of large mica flakes, as in
a mica schist derived from the metamorphism of
shale.
• Slaty cleavage - alignment of very fine-grained
micas, as in a slate derived from the
metamorphism of shale.
• Phyllitic structure - alignment of fine-grained
micas, as in a phyllite.
• Gneissic banding - segregation of light and dark
minerals into distinct layers in the rock, as in a
gneiss.
Slaty cleavage
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.
Phyllitic structures
Phyllite - fine-grained metamorphic
rock. Has a frosted sheen, resembling
frosted eye shadow. This is no
coincidence.
Cosmetics commonly contain ground up
muscovite (ground to a size similar to that
occurring naturally in phyllite.)
Gneissic Banding
Gneiss - (pronounced "nice") - a
banded or striped rock with
alternating layers of dark and light
minerals. The dark layers
commonly contain biotite, and the
light layers commonly contain
quartz and feldspar.
Schistosity
Schist - metamorphic rock containing
abundant obvious micas, several
millimeters 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
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.
Non-foliated
Limestone to Marble
Sandstone to Quartzite
Marble - fizzes in acid because its dominant
minerals is calcite (or dolomite). The parent
rock is limestone (or dolostone).
Quartzite - interlocking grains of quartz.
Scratches glass. The rock fractures through the
grains (rather than between the grains as it
does in sandstone). The parent rock is quartz
sandstone.
Mineral changes in
metamorphic rocks
1. Recrystallization rearrangement of
crystal structure of
existing minerals.
Commonly many small crystals merge to form larger
crystals, such as the clay in shale becoming micas in
slate, phyllite, and schist.
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". State
mineral of Georgia.
Metamorphic index minerals
1. In regional metamorphic terranes, the
temperature and pressure regime is
indicated by the distribution of
metamorphic minerals across a large area.
Low metamorphic grade (low temperatures
and pressures) - about 200 degrees C
Slate and phyllite
• chlorite
• muscovite
• biotite
Metamorphism of Basalts and
Gabbros
• Greenschist - Olivine, pyroxene, and plagioclase in an
original basalt change to amphiboles and chlorite (both
commonly green) as water in the pore spaces reacts with
the original minerals at temperatures and pressures of low
grade metamorphism.
• Amphibolite - As pressure and temperature increase to
intermediate grades of metamorphism, only dark colored
amphiboles 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, the
foliation is lost and a granulite that has a granulitic texture
is produced.
Greenschist –
Olivine,
pyroxene, and
plagioclase in an
original basalt
change to
amphiboles and
chlorite (both
commonly green)
Amphibolite -
•As pressure and temperature increase to intermediate grades of
metamorphism, only dark colored amphiboles 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,
the foliation is lost and a
granulite that has a
granulitic texture is
produced.