Metamorphism
Temperature, fluids, pressure,
deviatoric stress
Metamorphic grade
Metamorphic facies
Contact metamorphism
Regional metamorphism
The Rock Cycle
Rocks are classified by their mode of formation. There
are three major rock forming processes on Earth,
producing three kinds of rocks.
Igneous Rocks
Formed when magma
(molten rocks) solidifies
Environment: Hot
enough to melt rock,
pressure varies
Sedimentary Rocks
All non-igneous rocks
formed by processes
acting on the surface of
the Earth
Environment: normal for
Earth’s surface
Metamorphic Rocks
Formed by chemically
and physically altering
rocks under heat and
pressure deep within the
Earth’s crust.
Environment: High
pressure, not hot enough
to melt rock
Metamorphic Rocks
Metamorphic rocks form deep within
Earth’s crust under high pressure and
temperature. The buried rock (protolith)
undergoes solid state transformation,
altering the mineral composition and
texture of the rock. The rock can not
melt, or it would become an igneous
rock after cooling.
Metamorphism that occurs around
igneous intrusions is called contact
metmorphism.
Metamorphism occurring due to stresses caused by mountain building is called
regional metamorphism. These complex metamorphic environments may include
zones of contact metamorphism if igneous intrusions occur.
The type of metamorphic rock produced is controlled by the composition of the
protolith and the temperature, pressure and duration of the metamorphism.
Metamorphic rocks come in two flavors – foliated and non-foliated.
Factors in Metamorphism
Metamorphism is effected by four main factors:
Fluids
Temperature
Pressure
Deviatoric stress
Factors in Metamorphism
Temperature
Controlled by:
Geothermal gradient (increasing temperature with
greater depth in Earth)
Tectonic setting (higher in areas of igneous activity)
Most rocks melt at less than 2000oC, the upper limit of
metamorphism (rocks formed from melt are igneous!)
Factors in Metamorphism
Fluids
Moving fluids within the rock body can effect:
Pressure (increases with temperature)
Heat (moves it around)
Dissolved ions and gases (moves them around)
Factors in Metamorphism
Pressure
A uniform compressive stress created by
Load (weight of overlying rocks)
Fluid pressure
Increases greatly deeper in the crust.
With temperature, controls what minerals are stable.
Factors in Metamorphism
Deviatoric Stress
Stress imbalance causes compression in some
directions and expansion in others. Controls:
Foliation (orientation of minerals into layers)
Lineation (linear orientation of minerals)
Hydrostatic Pressure vs Deviatoric Stress
Hydrostatic Pressure
Deviatoric Stress
Foliated Metamorphic Rocks Metamorphic rock with aligned flat minerals
Foliation occurs when a protolith
with flat minerals like mica is
subjected to deviatoric stress. The
flat minerals align perpendicular to
the direction of maximum stress.
Increasing metamorphism causes
more severe reordering and
alteration of the rock’s
metamorphic texture.
Non- Foliated Metamorphic Rocks
Metamorphic rock without minerals that can
be foliated
Marble
Quartzite
Anthracite
Amphibolite
Protolith
limestone
quartz
sandstone
coal
mafic rocks
Mineralogy
carbonates
quartz
none
amphiboles
http://www.mii.org/
http://www.mii.org/
http://www.mii.org/
Marble
Quartzite
Anthracite
http://www.smccd.edu/
Amphibolite
Deviatoric Stress
Maximum crystal growth occurs
perpendicular to direction of
maximum stress
http://www.tulane.edu/~sanelson/geol212/contactmeta.htm
Deviatoric Stress
Deviatoric stress is the primary control on foliation and lineation
Foliation - alignment of flat minerals
Slaty Cleavage – Alignment of small mica flakes
Schistosity – Alignment of large mica flakes
Gneissic Banding – segregation of different
minerals into light and dark bands
Lineation - alignment of pointy minerals
Deviatoric Stress
Foliation and
lineation
http://www.aqd.nps.gov/grd/usgsnps/deform/gfoliation.html
Foliation
Foliation - Slaty Cleavage
Orientation of existing
minerals
http://www.tulane.edu/~sanelson/geol212/contactmeta.htm
Foliation - Slaty Cleavage
Slate
slaty cleavage.
Foliation - Slaty Cleavage
Slate
slaty cleavage.
Protolith– fine grained
rock like shale, mudstone,
or siltstone
FoliationSchist
- Schistosity
Orientation of existing minerals
and crystallization of new
minerals
http://www.tulane.edu/~sanelson/geol212/contactmeta.htm
Foliation - Schistosity
Schist
exhibits schistosity
Protolith – fine grained rock like
shale, mudstone, or siltstone
Foliation - Schistosity
Schist
New minerals
crystallize - in this
case garnets
Foliation - Gneissic Banding
Orientation and crystallization
continue, minerals become
segregated into bands
http://www.tulane.edu/~sanelson/geol212/contactmeta.htm
Foliation - Gneissic Banding
Gneiss
shows gneissic banding
Arizona State University
http://www.slu.edu
Foliated Metamorphic Rocks Metamorphic rock with aligned flat minerals
http://hyperphysics.phy-astr.gsu.edu/hbase/geophys
Slate
Schist
Gneiss
Low Grade
Foliation perpendicular to
direction of maximum
differential stress
High Grade
Foliation and crystallization
perpendicular to direction of
maximum differential stress
Foliation, crystallization and
differentiation perpendicular to
direction of maximum differential
stress
http://www.tulane.edu/~sanelson/geol212/contactmeta.htm
Metamorphic Grade
Metamorphic grade increases with both increasing temperature
and pressure, both of which increase with depth in the Earth
http://www.tulane.edu/~sanelson/geol212/contactmeta.htm
Metamorphic Grade
Materials with the same chemical formula can have different
crystalline structures, and thus different physical properties,
depending on the conditions of formation
Pressure
Diamond
Graphite
1000
Temperature (K)
3000
Rocks containing diamonds formed under different
P/T conditions than rocks containing graphite
Metamorphic Grade
Polymorphs - minerals with the same chemical formula but different crystallize
structures (=different chemical and physical properties)
Finding one polymorph in a
rock narrows down the P/T
conditions at metamorphism
Metamorphic Grade
kyanite
sillimanite
andalusite
http://www.tulane.edu/~sanelson/geol212/contactmeta.htm
Metamorphic Grade
Metamorphic Grade
Index Mineral – a mineral in a metamorphic rock which indicates a certain
metamorphic grade
Isograd – line marking the first appearance of an index mineral on a map or
cross-section
Contact Metamorphism
Contact Metamorphism
Metamorphism adjacent to
an igneous intrusion
Shale Rock
Igneous Intrusion
Contact Metamorphism
low grade
high grade
igneous
Finds first specimens of
“yellow” metamorphic
index mineral
high grade
Map
low grade
Finds first specimens of
“green” metamorphic
index mineral
Finds igneous
intrusion
Metamorphic Grade
Index Mineral – a mineral in a metamorphic terrain which
indicates a certain metamorphic grade
Isograd – line marking the first appearance of an index
mineral on a map.
A
Map
B C
C
B
A
Contact Metamorphism
Metamorphic Grade
igneous
high grade
unmetamorphosed
low grade
Metamorphic Grade
300oC
700oC
http://www.sun.ac.za/geology/METCOURSE/2nd%20year/metamorphic%20zones.htm
Contact Metamorphism
Isograds of index minerals
Regional Metamorphism
Large-scale metamorphism is usually related to tectonic events (rifting,
mountain building, etc.)
A single regional event can produce many different kinds of
metamorphism, including smaller zones of contact metamorphism.
Metamorphic Facies
Metamorphic Facies – Rocks that have been
exposed to the same degree of metamorphism (P/T
regime), and thus are the same metamorphic grade
http://www.tulane.edu/~sanelson/geol212/contactmeta.htm
Metamorphic Facies
All rocks that have experienced
this much metamorphism belong
to Greenschist facies.
http://www.dc.peachnet.edu/~janderso/images/metafaci.jpg
Metamorphic Facies
Minerals in Metamorphic Rock
Protoliths
Mudstone
Limestone
Granite
Gabbro
Schist
Marble
Schist
Schist
Quartz
K-feldspar
Plagioclase
Muscovite
Biotite
Chlorite
Epidote
Calcite
Magnetite
Actinolite
Metamorphic Rocks
However, all the rocks have undergone the same amount of
metamorphism, and all belong to the Greenschist
facies.
Metamorphic Facies
Carolina Slate Belt
Greenschist facies
http://www.tulane.edu/~sanelson/geol212/regionalmetamorph.htm
The Rock Cycle
The Rock
Cycle
Igneous Process
making igneous rocks
1. Melt (form magma)
2. Cool (crystallize)
geothermal gradient, Bowen’s Reaction Series, felsic, intermediate, mafic, magma,
lava, texture, basalt, gabbro, andesite, diorite, rhyolite, granite
Sedimentary Processes
making sedimentary rocks
1. Chemical and physical
weathering (breaks down rock)
2. Transport clasts and/or ions
(ice)
(air/water)
Sedimentary Processes
making sedimentary rocks
3. Deposition/
precipitation in basin
4. Lithification
weathering, transportation, deposition, lithification, clasts, clastic rocks, chemical
rocks, depositional environment
Metamorphic Processes
making metamorphic rocks
Add heat and pressure
to pre-existing rock
(protolith)
metamorphism, temperature, fluids, pressure, deviatoric stress, metamorphic
grade, metamorphic facies, index minerals, isograds, contact metamorphism,
regional metamorphism
Igneous Process
igneous rock
Metamorphic Process
1. Heat (melt)
2. Cool (crystallize)
original rock mass
The Rock Cycle
Add heat and pressure
metamorphic rock
sedimentary rock
Sedimentary Process
1. Breakdown (weather)
2. Transport
3. Deposit/precipitate
4. Lithify