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Metamorphism and Metamorphic Rocks
What are metamorphic rocks?
Increases in heat and pressure and changes in chemical environment
can alter the mineral composition and crystalline textures of
sedimentary and igneous rocks
Metamorphism is the solid-state (without melting) transformation
of pre-existing rock into texturally or mineralogically distinct new
rock as the result of high temperature, high pressure, or both.
Limestone
Sandstone
Marble
Quartzite (meta-quartzite)
Shale
Shale/sandstone
Shale
Shale/sandstone
Slate
Schist
Hornfels
Gneiss
Rock Cycle
Igneous
Erosion + Deposition
Sedimentary
Melting
Metamorphic
What are metamorphic rocks?
Causes of metamorphism
Heat & Pressure
Occurs at depths greater
than that of lithification
(Diagenesis ->
Sedimentary Process)
Surface
Sedimentary
15o C
5 km
Metamorphic
30 km
Igneous
200o C
600o C
The 3 “R”s of Metamorphism
Regrowth - crystals grow larger and more
uniform in shape.
Recrystallization - new minerals form from
existing minerals.
Reorientation - platy or elongated mineral
crystals become layered and aligned.
Factors Controlling
Metamorphism
Metamorphic changes bring a pre-existing rock into
equilibrium with new surroundings
Temperature
 Re-crystalisation – new mineral assemblages
 Segregation of new minerals – banding
Pressure
 Regrowth - Confining pressure – acts in all
directions – alters mineralogy by squeezing
atoms into a denser state
 Reorientation - Directed pressure – acting in a
particular direction – convergent plate
boundaries – causes preferred orientation foliation
Kinds of Metamorphism
Regional
Contact
Regional
Contact
Burial
Cataclastic
Hydrothermal
Kinds of Metamorphism
Regional Metamorphism
Occurs where both high temperature and high
pressure are imposed over large belts of the crust
Destroys all original sedimentary or igneous textures
through growth of new minerals
Occurs in deeper levels of the crust along convergent
plate boundaries particularly continental-continental
boundaries where active mountain building is taking
place.
Regional Metamorphism
Most metamorphic rocks
Occurs over a large
area as a result of
intense heat and
pressure
Most obvious at
convergent plate
boundaries where
rocks are intensely
deformed and
compacted during
subduction and
compression
Metamorphic grades seen through index minerals
Taconic Mountain Building Event
(450 million years ago)
Taconic Mountain Building Event
(450 million years ago)
Regional Metamorphism
Taconic Mountain Building Event
(450 million years ago)
Regional Metamorphism
Metamorphic Grade
Progression of metamorphism
Start with a shale and then hit
it with pressure and heat.
Slate
Phyllite
Schist
You end up with something that is really Gneiss!
Sedimentary Rock
3000 ATM
200° C
Low Grade Metamorphic Rock
High Grade Metamorphic Rock
600° C
Migmatite
Magma
Migmatite
• gneiss that begins to
partially melt into
granite.
Kinds of Metamorphism
Contact Metamorphism
Heat and pressure generated by igneous intrusions
Affects only a thin region of intruded rock.
At shallow depths the mineral transformations
generally associated with heat – pressure becomes
important for intrusion at great depth
A zone of alteration called an aureole forms in the
rock surrounding the magma
Types of Metamorphism
Contact Metamorphism:
The main metamorphic agent is heat.
Contact Metamorphism
Contact metamorphism
Kinds of Metamorphism
Dynamic / Cataclastic Metamorphism
Found along faults
Fault movement causes rocks on either side to
fragment
The product is a rock with a broken and pulverised
texture
Generally found in strongly deformed mountain belts
– often found with regionally metamorphosed rocks
Dynamic Metamorphism
Associated with Fault Zones
Low temp/high pressure
Mylonites – hard, dense, fine-grained rocks, with thin
laminations (less than 1 cm thick)
San Andreas Fault, CA and Northern Scotland
Granite away from active fault
Granite next to active fault
Kinds of Metamorphism
Hydrothermal Metamorphism
Often associated with mid-ocean ridges
Seawater heated by upwelling magma promotes
chemical reactions with basalt causing alteration
Alteration is also common in other igneous rocks
such as granite.
Alteration can result in the rock becoming weaker,
more compressible and more permeable – the end
product is very similar to that produced by chemical
weathering
Kinds of Metamorphism
Burial Metamorphism (precursor to Regional)
Diagenisis grades into burial metamorphism
Low grade metamorphisms resulting from heat and
pressure exerted by overlying sediments and
sedimentary rocks
Bedding and other sedimentary structures are
preserved
Burial metamorphism can grade into regional
metamorphism
Burial Metamorphism
Classification of Metamorphic
Rocks
Foliated Rocks
A set of flat or wavy parallel planes produced by the
preferred orientation of minerals, particularly platy
minerals like mica
Foliated Rocks
A foliation is any planar fabric in a
metamorphic rock. In this case, the
foliation is defined by aligned sheets
of muscovite sandwiched between
quartz grains.
This slide is indicative
of a phyllite.
The foliation in this rock is a
crenulation cleavage, and is
developed after the horizontal
foliation.
This slide is indicative
of a schist.
Classification of Metamorphic
Criteria
Rocks
Foliated Rocks
Nature of foliation
Grain size
Degree of banding
Metamorphic grade
Common metamorphic rocks
Foliated rocks
 Slate
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Very fine-grained
Excellent rock cleavage
Most often generated from low-grade
metamorphism of shale, mudstone, or
siltstone
Low Grade - Slate
Burial
Mountain Building
Common metamorphic rocks
Foliated rocks
 Phyllite
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Gradation in the degree of metamorphism between
slate and schist
Platy minerals not large enough to be identified with
the unaided eye
Glossy sheen and wavy surfaces
Exhibits rock cleavage
Composed mainly of fine crystals of muscovite
and/or chlorite
Phyllite (left) and Slate (right)
lack visible mineral grains
Intermediate Grade –
Phyllite
Common metamorphic rocks
Foliated rocks
 Schist
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Medium- to coarse-grained
Platy minerals predominate
Commonly include the micas
The term schist describes the texture
To indicate composition, mineral names are used
(such as mica schist)
A mica garnet schist
High Grade - Schist
Common metamorphic rocks
Foliated rocks
 Gneiss
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Medium- to coarse-grained
Banded appearance
High-grade metamorphism
Often composed of white or light-colored feldsparrich layers with bands of dark ferromagnesian
minerals
Gneiss typically displays
a banded appearance
High Grade - Gneiss
Classification of Metamorphic
Rocks
Foliated Rocks
Schist
Gneiss
Slate
Regional Revisited
Slate
Phyllite
Schist,
Gneiss
Metamorphic Grade
Non-foliated Metamorphic
Rocks
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granular texture
interlocking grains
composed primarily of one mineral
uniform grain size
Classification of Metamorphic
Rocks
Hornfels
Non-foliated Rocks
 Product of contact metamorphism
 Uniform grain size- little or no deformation – plate/elongate
crystals are randomly orientated
 Often harder and more brittle than parent rock
Marble
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Product of heat & pressure acting on limestone or dolomite
Often show irregular banding or mottling due to impurities
Granular texture
Coarse, crystalline
Parent rock was limestone or dolostone
Composed essentially of calcite or dolomite crystals
Used as a decorative and monument stone
Exhibits a variety of colors
Quartzite
 Derived from quartz-rich sandstones (contact or regional
metamorphism) often very hard and extremely strong
Marble – a nonfoliated
metamorphic rock
Nonfoliated - Marble
• metamorphosed limestone (CaCO3)
• bedding and fossils obliterated
Quartzite
Nonfoliated - Quartzite
• metamorphosed quartz-rich sandstone
• Pore spaces filled with SiO2
Metamorphic
Rocks
Metamorphic environments
associated with plate
tectonics
Now go to Metamorphic
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