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Earth Science 3.4
Metamorphic Rock
Metamorphism
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Recall that metamorphic rocks
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Metamorphism means
form when existing rocks are
changed by heat and pressure.
“ to change form”.

Rocks produced during
metamorphism often look much
different from the original parent
rocks.
Formation of Metamorphic Rocks
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
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Most metamorphic changes occur at
elevated temperatures and pressures.
These conditions are found a few
kilometers below Earth’s surface and
extend into the upper mantle.
Most metamorphism occurs in one of
two settings
- Contact Metamorphism
- Regional Metamorphism
Contact Metamorphism
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During contact metamorphism, hot
magma moves into rocks.
Contact metamorphism often produces
what is called low-grade
metamorphism, changes to rocks are
minor
Marble, like the marble used in
Michelangelo’s sculpture of David, is a
common contact metamorphic rock.
Marble often forms when magma
intrudes on limestone.
Regional Metamorphism

During mountain building, large areas of rocks are
subjected to extreme pressures and temperatures.
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The intense changes produced during these processes are described as
high-grade metamorphism.
Regional metamorphism
results in large-scale
deformation and high
grade metamorphism.
Agents of Metamorphism

The agents of
metamorphism are:
 Heat
 Pressure
 Hydrothermal
solutions

During metamorphism,
rocks are usually subjected to
all three at the same time.
Heat

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The most important agent is heat.
Heat provides the energy needed
to drive chemical reactions.
Some of these chemical reactions
cause minerals to recrystallize;
changing their structure.
Other reactions cause entirely new
minerals to form.
Heat

The heat for
metamorphism comes from
two sources mainly:
○ Magma
○ Change in
temperature with
depth
Heat



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Magma, molten rock deep within the
Earth, causes metamorphism to
happen when it comes in contact with
other cooler rock.
Magma basically “bakes” whatever
rocks it comes in contact with.
Heat also comes from the gradual
increase in temperature within the
crust.
In the upper crust, this increase
averages between 20 degrees
centigrade and 30 degrees
centigrade per kilometer of change.
Pressure or Stress

Pressure, like heat, also

Like the water pressure you
increases with depth.
feel underwater when you swim
in a pool, the pressure on rocks
in the Earth is applied in all
directions.
Pressure or Stress


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Pressure on rocks causes the
spaces between the mineral
grains to close.
The result is more compact
rock with greater density.
This pressure may also cause
minerals to recrystallize into
entirely new minerals.
Pressure or Stress

Increases in temperature and

Under these conditions, mineral
pressure cause rocks to flow
rather than fracture.
grains tend to flatten and
elongate.
Pressure or Stress

During mountain building,

This often produces intricate
horizontal forces caused by the
collision of plates metamorphose
large parts of the Earth’s crust.
deep folds in the rocks.
Reactions in Solutions

Water solutions containing other

Solutions that surround mineral grains
substances that readily change to
gases at the surface play an important
role in metamorphism.
aid in recrystallization by making it
easier for ions to move.
Reactions in Solutions

When these hot water-based solutions
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As a result of contact with hydrothermal
escape from a mass of magma, they are
called hydrothermal solutions.
solutions, a change in a rock’s overall
composition may occur.
Classification of Metamorphic Rock

Like igneous rocks,
metamorphic rocks can be
classified by texture and
composition into two
categories:
 Foliated
 Nonfoliated
Foliated
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When rocks undergo contact
metamorphism, they become
more compact and thus more
dense.
Under more extreme
conditions, certain minerals will
recrystallize.
Foliated
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Some minerals
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This alignment gives the
recrystallize with a
preferred orientation,
which is at right angles to
the direction of force.
rock a layered or striped
appearance which we call
a foliated appearance.
Nonfoliated Rocks
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
A
metamorphic rock that does not have a
banded texture is called a nonfoliated
metamorphic rock.
Most nonfoliated rocks contain only one
mineral.
Nonfoliated Rocks

Marble, for example, is a nonfoliated rock

When it’s parent rock, limestone, is
made of calcite.
metamorphosed, the calcite crystals combine
to form the larger interlocking crystals seen
in marble.