Chapter 8 – Metamorphic rocks
Metamorphic rocks are rocks that have changed from their original form (i.e. a
protolith rock) into a new form without completely melting or becoming a
sediment. These changes occur in a solid state. Remember, if a rock
completely melts and then recrystallizes, it is an igneous rock.
Four agents of metamorphism
(1) Heat
(2) hot groundwater
(3) pressure
(4) differential stress.
Metamorphic rocks possess:
(1) metamorphic mineral assemblage, or
(2) metamorphic foliation, or
(3) both.
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Making a cake from scratch is the analogy we’ll use. A cake batter is a mixture of
flour, sugar, eggs, butter, flavoring, and liquid that when heated, becomes a solid
(i.e.a cake).
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Similarly, a mixture of component minerals, when heated, becomes a metamorphic
rock. Heat causes the atoms making up the minerals to vibrate, breaking bonds and
allowing the atoms to migrate to new spaces and new crystals are formed
(recrystallization).
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This process can occur in less than one million years, a relatively short time frame
geologically speaking.
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Under conditions of very high temperatures and pressure, the migration of atoms
can cause compositional banding, when light and dark layers alternate in a rock.
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If the protolith rock is made up of many minerals, the metamorphic rocks produced
also contain many minerals.
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However, if a protolith is made up of basically a single mineral, such as a quartz
sandstone or a pure calcite limestone, it creates a metamorphic rock of the same kind
of mineral.
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For example: sandstone (made from quartz) forms the metamorphic rock called
quartzite and limestone (made from CaCO3) forms the metamorphic rock called
marble. Recrystallization does occur but the crystals have the same composition as
their protoliths and may originate from both the sand grains and the cementing
material in a piece of sandstone, for example.
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Metamorphism can change the size, texture, and arrangement of grains in a rock
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Metamorphism can replace the protolith’s original group of minerals with a new group
of minerals
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Metasomatism is when a rock’s chemical composition changes because of a reaction
with hot water. The dissolved ions may eventually precipitate elsewhere in the cracks
of the rock. Many veins in metamorphic rock consist of quartz because silica tends to
be left over after metamorphic reactions.
Red shale (clay flakes, quartz, hematite) was the protolith of this compositionally banded
gneiss. Notice light and dark banding and garnet inclusions.
An example of diffusion through a solid.
Under metamorphic processes of temperature and pressure, sandstone
becomes quartzite, a metamorphic rock.
Low pressure > open
lattices with widely
spaced atoms
Pressure squeezes atoms closer together
High pressure > atoms closer together
The stability field for 3 metamorphic minerals consisting of aluminum silicate (Al2SiO5).
At low pressures and temp’s, the mineral exists as andalusite, as pressure increases,
andalusite recrystallizes to form kyanite. A further increase in pressure and temp. causes
sillimanite to form. This group of minerals are called index minerals because they are
good indicators of metamorphic grade.
Differential stress changes the development of preferred mineral orientation. Normal
stress pushes (compression) or pulls (tension) perpendicular to a surface. Shear stress
moves one part of a material sideways relative to another.
Preferred orientation can develop in
four ways: (1) ductile (plastic)
deformation where compressed mineral
grains become flattened or elongate, (2)
pressure solution in wet rocks when
the compressed mineral grains dissolve
producing ions that precipitate
elsewhere on faces where compression
is less, (3) grain rotation where grains
rotate into parallelism as the rock
changes shape and, (4) inequant grains
may grow with a preferred orientation
due to differential stress.
Types of metamorphic rocks
Foliated metamorphic rocks: sometimes rocks look striped or streaked. Foliation is due
to: (1) a rock’s mineral crystals aligning parallel to one another (preferred mineral
orientation) or, (2) because the rock has compositional banding (alternating light and
dark stripes). Identifying characteristics of foliated rocks include composition, grain
size, and the nature of their foliation.
Compression of a bed can create slaty cleavage at an angle perpendicular to the
original bedding.
Pressure applied here
Metasandstone and metaconglomerate is caused by the flattening of quartz grains or
conglomerate clasts due to plastic deformation and pressure solution.
Phyllite: a fine-grained metamorphic rock with a foliation caused by the preferrred
orientation of mica (muscovite).
Schist is a medium to coarse-grained metamorphic rock that has a foliation called schistosity,
defined by the preferred orientation of large mica flakes. Schist forms at a higher temp than
phyllite and the mica grains are larger. The name given to a schist depends on the minerals
present.
Some metamorphic rocks
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Amphibolite is a mafic (high Fe content) metamorphic rock with a basalt protolith.
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Gneiss is a compositionally banded metamorphic rock.
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Migmatite is a metamorphic rock that has begun to partially melt and then re-form.
Migmatite is a mixture of igneous and metamorphic rock.
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Non-foliated metamorphic rocks include:
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hornfels, a rock that is created as a result of an igneous intrusion.
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quartzite is formed by the metamorphism of quartz sandstone.
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marble, which is a rock that results from the metamorphism of limestone.
The conditions of pressure and temperature for metamorphism.
Shale progressively
metamorphoses from low grade to
high grade.
Slate > phyllite > schist > gneiss.
Retrograde metamorphism only occurs if water is added back to the rock.
Retrograde metamorphism can occur below the ocean floor
Classic blueschists. The blue tinge results from the mineral glaucophane (an
amphibole), which here forms the rather stubby needles. This rock started life as a
volcanic rock of basic composition, part of the old ocean floor of Tethys. Blueschists
are comonly thought to be diagnostic of former subduction zones, because they imply
relatively high pressure conditions relative to the temperature (compared to normal
geothermal gradients).
Return to metamorphism page
Thermal or contact
metamorphism
Dynamic metamorphism
Point “A” undergoes dynamothermal metamorphism (both compression, shearing and
temp).
Exhumation is the process of returning buried rocks to the surface due to collision of 2
pieces of continental crust or, erosion.
Metamorphism due to rifting and transform faulting.
Metamorphism at mid-ocean ridges can cause retrograde metamorphism as happens
with ocean-floor basalt. Metamorphism in subduction zones can create blueschist, a rare
type of rock that occurs in accretionary prisms (wedges) where temperatures are
relatively low but pressures high.
You can find metamorphic rocks in mountain ranges due to uplift and exposure. They are also
visible where ancient mountain ranges once existed but have eroded down to the ground surface.
Older regions of continents have shields where large areas of Precambrian rock (> 545 my) make
up the surface because overlying younger rock has eroded away. Some places such as the Grand
Canyon or the Black Canyon of the Gunnison have rivers that eroded down through the
formations to exposure the basement rocks.
The Canadian shield
End of Chapter 8