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metamorphism: The process by which one kind of rock transforms into a different
kind of rock.
metamorphic rocks: a rock that forms from the modification of another rock.
protolith: preexisting rock that was deformed and modified into a metamorphic
rock.
Metamorphic rocks are either foliated : Layering formed as a consequence of the
alignment of mineral grains, or of compositional banding in a metamorphic rock.
foliation
or nonfoliated : Rock containing minerals that recrystallized during metamorphism,
but which has no layering has appeared.
parent rock is the same thing as protolith
foliation
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garnet
Metamorphic minerals : are the new minerals that form in metamorphic rocks
due to increase in temperature and/or pressure.
Metamorphic textures: are the ways that metamorphic minerals grow in a
metamorphic rock.
Above this metamorphic rock has a strong foliation with garnet as its
distinctive metamorphic mineral. The garnet grew in a pyroclastic
metamorphic texture.
Formation of a metamorphic rock take a very long time that involves several
different processes:
•
Recrystallization: to change the shape and size of a mineral without changing
its original composition.
•
Phase change: to change one mineral into a different mineral
•
Metamorphic reaction: growth of new minerals out of the protolith due to
metamorphic processes. This is a type of diffuse mineral growth.
•
Pressure solution: process of dissolution at points of contact, between grains,
where compression is greatest, producing ions that then grow into new
minerals.
•
Plastic deformation: deformational process in which mineral grains behave like
plastic and, when compressed or sheared, become flattened or elongate
without cracking or breaking.
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Recrystallization
Metamorphic reaction
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Pressure solution
Plastic deformation
Phase change occurs at depth in the
earth when the mineral olivine converts
into a different mineral structure called
a spinel type structure.
Metamorphism is caused by heating of a rock and subjecting a rock to high
pressures . In the earth, as we have seen with igneous rocks, pressure and
temperature change together. A good example of this is the Al2SiO5 mineral
system. The 3 minerals below all have the same chemical formula (Al 2SiO5); but,
the molecular structure of each mineral is different. Temperature and pressure
are the controlling factors.
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Metamorphic rocks are commonly deformed as a result of their metamorphism.
This leads to a preferred orientation of mineral grains in the rocks. The rocks
become deformed by...
differential stress: a condition causing a material to experience a push or pull
in one direction of a greater magnitude than the push or pull in another
direction; in some cases, differential stress can result in shearing.
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There are two kinds of differential stress:
1) Normal stress: The push or pull that is perpendicular to a
surface.
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2) Shear stress: A stress that moves one part of a material sideways past another
part.
These diagrams show how differential
stress cause preferred mineral
orientations.
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Metamorphic minerals tend to be equant , elongate and platy after they form in a
metamorphic rock.
Metamorphic reactions usually take place in the presence of very hot fluids called
hydrothermal fluids. These fluids react chemically with the rocks during
metamorphism.
Supercritical fluid is a hydrothermal fluid that is at great depth so it has the
physical characteristics of both a liquid and a gas.
This process by which a metamorphic rock’s chemical composition changes due
to the interaction with hydrothermal fluids is called metasomatism.
Foliated metamorphic rocks are classified based on the intensity of the
metamorphism. The hotter the metamorphic reaction, the more well developed
the foliation will be in a metamorphic rock.
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Slate: Fine-grained, low-grade metamorphic rock, formed by the metamorphism of
shale.
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slaty cleavage : The foliation typical of slate, and reflective of the preferred
orientation of slate’s clay minerals, that allows slate to be split into thin sheets.
Forms perpendicular to differential stress applied to the rock.
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Phyllite: A fine-grained metamorphic rock with a foliation caused by the preferred
orientation of very fine-grained mica. Forms at higher temperatures and
pressures than slate.
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Schists: A medium-to-coarse-grained metamorphic rock that possesses
schistosity. At higher temperatures and pressures than slate and phyllite.
Schistosity: Foliation caused by the preferred orientation of large mica flakes.
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Gneiss: A compositionally banded metamorphic rock typically composed of
alternating dark- and light-colored layers. Formed at higher temperatures and
pressures than slate, phyllite, and schists. Usually formed from the melting of a
protolith.
Compositional layering: A type of metamorphic foliation, found in gneiss, defined
by alternating bands of light and dark minerals.
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This diagram shows how compositional layering can form in a gneiss through
progressive deformation and recrystallization under high pressures and
temperatures..
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Migmatite: A rock formed when gneiss is heated high enough so that it begins to
partially melt, creating layers, or lenses, of new igneous rock that mix with layers
of the relict gneiss. Formed at the highest temperatures and pressures of any
foliated metamorphic rock.
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As you increase temperature and pressure, you increase metamorphic grade.
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Metamorphic grade: the amount or intensity of change in a protolith as the result
of temperature and pressure increase.
Metamorphic mineral assemblage: A group of minerals that form in a rock as a
result of metamorphism.
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Metamorphic grade can range from low grade to high grade .
Nonfoliated metamorphic rocks include:
Amphibolites: metamorphic mafic rocks that consists of the minerals hornblende
and plagioclase.
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Quartzite: A metamorphic rock composed of quartz and transformed from a
protolith of quartz sandstone.
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Hornfels: Rock that undergoes metamorphism simply because of a change in
temperature, without being subjected to differential stress.
This is due to metamorphic aureole: The region around a pluton, stretching tens
to hundreds of meters out, in which heat transferred into the country rock and
metamorphosed the country rock.
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An example of hornfels produced from the contact with a pluton is the Onawa
pluton in Maine. Far way from the pluton as slates that were deformed before the
pluton was emplaced.
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Closer to the pluton, low-grade hornfels minerals start to form. And just adjacent
to the pluton occurs the highest-grade hornfels mineral assemblages.
Marble: A nonfoliated metamorphic rock composed of calcite and transformed
from a protolith of limestone.
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Metamorphic grade and metamorphic mineral assemblages leads to the
classification of different metamorphic temperatures and pressures into
metamorphic facies: A set of metamorphic mineral assemblages indicative of
metamorphism under a specific range of pressures and temperatures.
At a convergent plate boundary, multiple metamorphic facies occur as a result of
the differing temperatures and pressures created at these zones.
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Prograde metamorphism: metamorphism that occurs with increasing temperature
and pressure.
Retrograde metamorphism : metamorphism that occurs with decreasing
temperature and pressure.
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Metamorphic sequences are the result of index metamorphic minerals that reflect
different metamorphic temperatures and pressures.
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As more rocks form on top of each other, rocks become buried deeper into the
earth. Due to the geothermal gradient (~24 °C/km) rocks will undergo burial
metamorphism as a result of this process..
Metamorphism that
occurs along faults
results from shearing
forces applied to the
rock. This results in
dynamic metamorphism:
Metamorphism that
occurs as a
consequence of
shearing alone, with no
change in temperature
or pressure.
A metamorphic rock
type that is produced as
a result of dynamic
metamorphism are
mylonites (Fine-grained
rock formed by the
nonbrittle subdivision of
larger grains).
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Dynamothermal metamorphism: Metamorphism that involves heat, pressure, and
shearing. This type of metamorphism is the result of collisions of one tectonic
plate with another tectonic plate.
This type of metamorphism effects large areas so it is also called regional
metamorphism.
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When cold sea water interacts with the hot rising magma at mid-ocean ridges, the
sea water heats up and produces a low-grade hydrothermal metamorphism.
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exhumation:
exhumation The process
(involving uplift and erosion)
that returns deeply buried
rocks to the surface.
This is how metamorphic
rocks become exposed at the
surface of the earth.
This process occurs over a
long period of time.
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Shield : An older, interior region of a continent. This is typically where
metamorphic rocks are exposed.
Now that we have learned about igneous, sedimentary and metamorphic rocks, it
is important to know that they are related together in what is known as the
rock cycle: The succession of events that results in the transformation of Earth
materials from one rock type to another, then another, and so on.
•Sedimentary rocks can consists of fragments derived from older metamorphic,
igneous and sedimentary rocks.
•Metamorphic rocks can form through the deformation of older sedimentary,
igneous and metamorphic rocks.
•Igneous rocks can form from the partial melting of older sedimentary,
metamorphic and igneous rocks.
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