METAMORPHISM: NEW
ROCKS FROM OLD
Objectives
• Explain how metamorphism differs from diagenesis
and lithification.
• Define the grade of metamorphism.
• Define foliation.
• Describe and identify the common metamorphic
products of shale and basalt.
• Identify several common metamorphic rocks.
• Describe contact, burial, and regional metamorphism.
What is Metamorphism?
• Metamorphism
– The mineralogical,
textural, chemical
and structural
changes that occur in
rocks
– A result of exposure
to elevated
temperatures and/or
pressure
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What is Metamorphism?
• Low-grade
– Rocks metamorphosed under temperature and pressure conditions up
to 400°C and 400 MPa
• High-grade
– Rocks metamorphosed under temperature and pressure conditions
higher than about 400°C and 400 MPa
What is
Metamorphism?
• Factors that influence
metamorphism
– Temperature
– Pressure
Metamorphism of shale
under increasingly
higher T and P: know the
sequence of rocks that is
formed.
What is Metamorphism?
• Factors that
influence
metamorphism
– Pore fluids
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• Factors that influence metamorphism
– Stress
• Foliation
– A planar arrangement of textural features in a metamorphic rock,
– Gives rock a layered or banded appearance
Granite:unfoliated
Gneiss: foliated
• Factors that influence
metamorphism
– Foliation
• Slaty cleavage
– Foliation in low-grade
metamorphic rocks that
causes such rock to break
into flat, plate like
fragments
• Schistosity
– Foliation in coarse grained
metamorphic rocks
What is Metamorphism?
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Metamorphism and Billiards
Metamorphic Rocks
• Rocks with foliation
– Slate
• A very fine grained metamorphic rock with slaty
cleavage; the metamorphic product of shale
– Phyllite
• A fine grained metamorphic rock with pronounced
foliation, produced by further metamorphism of slate
– Schist
• A coarse grained metamorphic rock with schistosity
– Gneiss
• A very coarse grained metamorphic rock with banding of
light and dark colored minerals.
Metamorphic Rocks
Grain size increases
from shale to gneiss
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Metamorphic Rocks
• Rocks without foliation
– Quartzite
• The product formed by re-crystallization of sandstone
– Marble
• The product formed by re-crystallization of limestone
Marble
Quartzite
Metamorphic Processes
• Processes that cause changes in texture and mineral
assemblages in metamorphic rock
– Mechanical deformation
• Grinding, crushing, bending and fracturing
Metamorphic Processes
• Processes that cause
changes in texture and
mineral assemblages in
metamorphic rock
– Chemical recrystallization
• Changes in mineral
composition, growth of new
minerals, recrystallization of
old minerals, and changes in
amount of pore fluid
• Typically caused by heating
and squeezing of rock
Contact metamorphism
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Metamorphic Processes
• Type of metamorphism
– Burial metamorphism
• Occurs after diagenesis, as
a result of the burial of
sediments in deep
sedimentary basins
– Contact metamorphism
• Rocks are heated and
chemically changed
adjacent to an intruded
body of hot magma
Metamorphic Processes
• Type of metamorphism
– Regional metamorphism
• Metamorphism of an extensive
area of the crust
• Associated with plate
convergence, collision, and
subduction
– Other types of metamorphism
occasionally occur in geologic
settings where high temperature
and/or elevated pressure or
stress exists
Metamorphic Facies
• The set of metamorphic
mineral assemblages
• Form in rocks of
different compositions
• Form under similar
temperature and stress
conditions
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Metamorphic Facies
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