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SECTION 14-4
NOTES
Origins of Metamorphic Rock
■ Metamorphic rocks are rocks in which the structure, texture, or
composition of the rock have changed.
■ Rocks undergo metamorphism when the temperature or pressure in
the new environment are different from the original environment of
the rock.
■ The temperature at which most metamorphism occurs ranges from
50oC to 1,000oC.
■ Most metamorphic rock changes due to pressure take place at depths
greater than 2 km.
■ Mineral grains align in parallel bands due to added pressure on a rock
during metamorphism.
Contact Metamorphism
■ Contact metamorphism occurs when a rock undergoes
metamorphism by being heated by nearby magma.
■ The effect of heat on rock decreases as the rock’s distance
from the magma increases and its temperature decreases.
Regional Metamorphism
■ Regional metamorphism occurs when pressure builds up in
rock that is buried deep below other rock formations or
when large pieces of the Earth’s crust collide.
■ The increased pressure can cause thousands of square
miles of rock to become deformed and chemically changed.
Composition of Metamorphic Rock
■ As rocks undergo metamorphism, the original minerals in a
rock change into new minerals that are more stable in new
pressure and temperature conditions.
■ An index mineral is a metamorphic mineral that forms only
at certain temperatures and pressures and therefore can be
used by scientists to estimate the temperature, pressure,
and depth at which a rock undergoes metamorphosis.
■ Index minerals include biotite mica, chlorite, garnet, kyanite,
muscovite mica, sillimanite, and staurolite.
Foliated Metamorphic Rock
■ The texture of metamorphic rock in which the mineral grains
are arranged in planes or bands is called foliated.
■ Foliated metamorphic rock usually contains aligned grains
of flat minerals, such as biotite mica or chlorite.
■ Examples of Foliated Metamorphic Rock: slate, phyllite,
schist, gneiss
Nonfoliated Metamorphic Rock
■ The texture of metamorphic rock in which the mineral grains
are not arranged in planes or bands is called nonfoliated.
■ Nonfoliated rocks are commonly made of one or only a few
minerals.
■ During metamorphism, the crystals of these minerals may
change in size or the mineral may change in composition in
a process called recrystallization.
Metamorphic Rock Structures
■ Deformation is a change in the shapes of a rock caused by a
force placed on it.
■ These forces may cause a rock to be squeezed or stretched.
■ Folds, or bends, in metamorphic rock are structures that
indicated that a rock has been deformed.