11/7/2012 1 METAMORPHIC ROCKS Processes, Conditions and Rocks Sedimentary rocks may be buried deep enough in the Earth that they are transformed by high temperatures and pressures into metamorphic rocks. The process to form metamorphic rocks is known as metamorphism. Metamorphism may be taken to the point where the rock melts, forming magma, and the whole process starts over.
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METAMORPHIC ROCKS · preexisting igneous, sedimentary, or even other metamorphic rocks by heat and/or pressure. Types of Metamorphic Rocks 2 geologic settings for metamorphism: 1.
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11/7/2012
1
METAMORPHIC ROCKS
Processes, Conditions and
Rocks
Sedimentary rocks may
be buried deep enough
in the Earth that they are
transformed by high
temperatures and
pressures into
metamorphic rocks.
The process to form
metamorphic rocks is
known as
metamorphism.
Metamorphism may be
taken to the point where
the rock melts, forming
magma, and the whole
process starts over.
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Metamorphism and Metamorphic
Rocks
Origins of Metamorphic Rocks • What is Metamorphism?
• Types of Metamorphism
• What Drives Metamorphism?
Classification of Metamorphic Rocks • Texture
• Composition
Interpretation of Metamorphic Rocks • Temperature and Pressure
• Relationship to Plate Tectonics
What is Metamorphism?
“Change of form”
Change from one rock (parent) to another • Change of mineral composition (not overall chemistry
except when fluids added)
• Change of rock texture (generally more compact)
• Without melting (solid state of re-bonding)
Change to a more stable chemical and physical condition for circumstances
Change occurs incrementally, from slight change (low-grade) to dramatic change (high-grade) from the parent rock.
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What is a Metamorphic Rock?
Rock formed by the transformation of
preexisting igneous, sedimentary, or
even other metamorphic rocks by heat
and/or pressure.
Types of Metamorphic Rocks
2 geologic settings for metamorphism:
1. Contact metamorphism - rocks intruded
by hot magma that "bakes" the
surrounding rock.
2. Regional metamorphism –
(dynamothermal metamorphism) large-
scale deformation in rocks that are
subjected to higher pressure and
temperatures from burial and/or during
"mountain building" processes .
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Two “Roof Pendants” – Contact
Metamorphism
Country rock intruded by granite, Sierra Nevada, California
Regional Metamorphism
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What Drives Metamorphism?
Agents of Metamorphism
HEAT
PRESSURE (stress)
CHEMICALLY ACTIVE FLUIDS (H20 and
CO2 )
During metamorphism, rocks are typically treated to all 3
agents at once.
Heat as an Agent of Metamorphism Heat is the most important agent since it provides the energy to drive
the chemical changes that result in recrystallization of minerals in
the rock.
Two primary ways that heat is an metamorphic agent:
1. During contact metamorphism, the intense heat of an intruded
magma may "bake" the adjacent rock.
2. During regional metamorphism, rocks near the surface of the Earth
may be thrust downward and buried where they are subjected to
increased temperatures and stresses.
Geothermal gradient in crust = ~20° - 30°C/km.
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Two “Roof Pendants” – Contact
Metamorphism
Country rock intruded by granite, Sierra Nevada, California
Pressure and Stress as Metamorphic Agents
Pressure increases with depth
~280 bar/km
Two types of pressure/stress:
1. Lithostatic (confining)
pressure - same in all
directions (burial).
2. Directed pressure
(differential stress) -tectonic
forces during mountain
building processes result in
increase pressure in one
direction relative to the
others (collision of two
continents or at a subduction
zone).
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Chemically Active Fluids as a Metamorphic Agent
Circulating Fluids enhance metamorphic
processes - act as a catalyst by aiding in the
migration of ions.
Water (with dissolved ions) is plentiful
because it is contained in the pore spaces of
virtually every rock.
During deep burial, the water is squeezed out
and becomes available for chemical reactions.
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Metamorphic setting
P T Chemically active fluids
Metamorphic setting
P T Chemically active fluids
Contact metamorphism (C)
Low High: increasing toward magma
High – from magma and from heated surface waters
Metamorphic setting
P T Chemically active fluids
Contact metamorphism (C)
Low High: increasing toward magma
High – from magma and from heated surface waters
Regional metamorphism: Deep burial (B)
High: steadily increasing with depth
High: steadily increasing with depth
Low – liberated from hydrous minerals and small amounts in cracks/pores
Metamorphic setting
P T Chemically active fluids
Contact metamorphism (C)
Low High: increasing toward magma
High – from magma and from heated surface waters
Regional metamorphism: Deep burial (B)
High: steadily increasing with depth
High: steadily increasing with depth
Low – liberated from hydrous minerals and small amounts in cracks/pores
Regional metamorphism: Converging continents (R)
High: increasing with depth
Low to medium: increasing with depth
Low – liberated from hydrous minerals and small amounts in cracks/pores
Metamorphic setting
P T Chemically active fluids
Contact metamorphism (C)
Low High: increasing toward magma
High – from magma and from heated surface waters
Regional metamorphism: Deep burial (B)
High: steadily increasing with depth
High: steadily increasing with depth
Low – liberated from hydrous minerals and small amounts in cracks/pores
Regional metamorphism: Converging continents (R)
High: increasing with depth
Low to medium: increasing with depth
Low – liberated from hydrous minerals and small amounts in cracks/pores
Subduction zone metamorphism (S)
High Low: slowly increasing with depth
High – from hydrous minerals in hydrothermally altered ocean crust and water trapped in pores/cracks