Download Metamorphic Rocks

Lecture Outlines
Physical Geology, 15/e
Plummer, Carlson & Hammersley
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Metamorphism and
Metamorphic Rocks
Physical Geology 15/e, Chapter 7
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Metamorphism
Metamorphism refers to solid-state changes
to rocks in Earth’s interior
• produced by increased heat, pressure, or the
action of hot, reactive fluids
• old minerals, unstable under new conditions,
recrystallize into stable ones
Rocks produced from pre-existing or parent
rocks in this way are called metamorphic
rocks
Metamorphic rocks common in the old, stable
cores of continents, known as cratons
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Metamorphism
Texture and mineral content of metamorphic
rocks depend on:
• parent rock composition
• temperature and pressure during
metamorphism
• effects of tectonic forces
• effects of fluids, such as water
Parent rock composition
• usually no new material is added to rock
during metamorphism
• resulting metamorphic rock will have
similar composition to parent rock
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Factors Controlling the Characteristics of
Metamorphic Rocks
Temperature
• heat comes from Earth’s deep interior
• all minerals stable over finite
temperature range, if range exceeded,
new minerals result
• if temperature gets high enough, melting
will occur
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Factors controlling metamorphic rock
characteristics
Pressure
• confining pressure applied equally in all
directions
• pressure proportional to depth within the Earth
• high-pressure minerals more compact/dense
• Differential Stress – created by forces that
are not equal in all directions
• compressive stress causes flattening
perpendicular to stress
• shearing causes flattening by sliding
parallel to stress
• Foliation - planar rock texture of aligned
minerals produced by differential stress
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Factors controlling metamorphic rock
characteristics
Composition of the Parent Rock
•Composition may remain the same or change during metamorphism
depending upon the parent rock’s composition and whether new
chemicals are added during the process.
Fluids
•hot water (as vapor) is most important
•rising temperature causes water to be released from unstable minerals
•hot water very reactive; acts as rapid transport agent for mobile ions
Time
•metamorphism, particularly from high pressures, may take millions of
years
•longer times allow newly stable minerals to grow larger and increase
foliation
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Classification of Metamorphic Rocks
Nonfoliated Rocks
• non-foliated rocks are named based on
composition
• Marble – coarse grained rock
composed of interlocking calcite
crystals
• Quartzite – produced when grains of
quartz sandstone are welded together
• Hornfels – fine grained rock typically
composed of microscopically visible
micas formed from the clay particle in
shale.
Foliated Rocks
• foliated rocks named based on the type of
foliation (slaty, schistose, gneissic)
• Slate, Phyllite, Schist and Gneiss
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Types of Metamorphism
Contact Metamorphism
• high temperature is dominant factor
• produces non-foliated rocks
• occurs adjacent to magma bodies
intruding cooler country rock
• occurs in narrow zone (~1-100 m
wide) known as contact aureole
• rocks may be fine- (e.g., hornfels) or
coarse-grained (e.g., marble, quartzite)
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Types of Metamorphism
Hydrothermal Metamorphism–
rocks precipitated from or altered by hot
water
• common at divergent plate boundaries
Hydrothermal processes:
• metamorphism
• metasomatism
Hydrothermal Processes and Ore Deposits
• water passes through rocks and
precipitates new minerals on walls of
cracks and in pore spaces
• metallic ore deposits often form this
way (veins)
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Metallic Ore Vein deposit
Types of Metamorphism
Regional metamorphism
• high pressure is dominant factor
• results in rocks with foliated textures
• prevalent in intensely deformed
mountain ranges
• may occur over wide temperature
range
Shock metamorphism is produced
by rapid application of extreme pressure
•
•
meteor impacts produce this
shocked rocks are found around
and beneath impact craters
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Metamorphic Grade
Minerals present in a metamorphic rock
indicate its metamorphic grade.
Prograde Metamorphism – as a rock is buried to
greater depths it is subject to greater temperatures and
pressures causing recrystallization into higher grade
rocks.
Slate
Phyllite
Lower Grade
Schist
Gneiss
Higher Grade
Migmatites (partial melting) exhibit both intrusive
igneous and foliated metamorphic textures
Pressure and Temperature Paths in Time
Index minerals can be used to approximate temperature
and pressure conditions
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Migmatite
Plate Tectonics and Metamorphism
Foliation and Plate Tectonics
• Regional metamorphism associated
with convergent plate boundaries
• Differential stress
• Gravitational collapse and spreading
Pressure-Temperature Regimes
• Temperature varies laterally at convergent
boundaries
• isotherms bow down in sinking
oceanic plate and bow up where
magma rises
• wide variety of metamorphic
facies
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Plate Tectonics and Metamorphism
Hydrothermal Metamorphism and
Plate Tectonics
• Particularly important at mid-oceanic
ridges as seas water moves downward
into cracks in the sea floor
• Hydrothermal vents such as “black
smokers” occur as the water returns to
the ocean
• Dissolved metals and sulfur
precipitate to create mounds
around the vents
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End of Chapter 7
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