Physical Geology 101
Metamorphic Rocks


Meta- means:
Morph means:
Rocks and minerals change when they are exposed to conditions different from those when it formed.
Metamorphic rocks are other rocks which have changed form - Parent Rock.
Changes in temperature, pressure, chemistry.


Minerals form in certain stability fields.
– temperature range in which a mineral is stable.
An increase or decrease in temperature will remove a mineral from its stability field.
Mineral will be unstable and try to change.
Limits of metamorphism:
Lower limit of metamorphism - 200°C
Upper limit varies between 700-1200°C
Ultimate upper limit ?
Heat sources for metamorphism:


Pressure changes will change minerals and rock.
Confining Pressure:
Directed Pressure:

Chemical Changes

Water is often present - hydrothermal fluids.
- water, chemicals and gases.
These fluids can do one of two things:
1) remove chemicals from the rock, thus changing the overall chemistry of the rock.
2) add chemicals to a rock, thus changing the chemistry.

Classification of Metamorphic Rocks

Classification is based on the temperature and pressure of maximum metamorphism.

Metamorphic Facies
Amphibolite Facies
Blueshist Facies
Eclogite Facies
Granulite Facies
Greenschist Facies
Hornfels Facies
Zeolite Facies
Each facies has defined conditions of temperature and pressure
Facies can be determined by minerals present in rock and knowing temperature range for those minerals

Classification can also be done based on texture and composition
Two groups of Texture
Foliated and Non-foliated

Foliated Texture

Foliation is a distinct planar character which results from directed pressure.
- minerals show a preferred orientation or alignment
1) Slaty cleavage - very fine foliation
2) Phyllitic foliation - very fine foliation
3) Schistosity - medium scale foliation
4) Gneissic banding - alternating bands of light and dark colored minerals

Foliated Metamorphic Rock Types

Slate - Low grade (low T and P), slaty cleavage, minerals are too small to see, Parent Rock = shale, mudstone.

Phyllite - Low grade, slaty cleavage, slightly larger crystals than slate - still too small to see, appearance shows a slight sheen, Parent Rock = shale, mudstone.

Schist - Medium grade (moderate T and P), schistosity, platy minerals dominate (micas), Parent Rock = fine grained igneous rocks, slate, phyllite.
Schist Nomenclature
There are many different types of schist.
Mineral modifiers are used to distinguish schists.
A schist consisting dominantly of equal amounts of biotite and muscovite would be called a mica schist.
A schist which contains more biotite than muscovite would be called a muscovite-biotite schist.
Minerals are listed in reverse order of abundance.
garnet-biotite-muscovite schist
Two exceptions to this rule.
Quartz and feldspars (orthoclase, plagioclase) are not listed.
Hornblende schist is called amphibolite.

Gneiss - High grade (high T and P), gneissic banding, Parent Rock = schist, plutonic igneous rocks.

Non-Foliated Texture

Non-Foliated Texture - no preferred orientation of the minerals in the rock
Results from directed or confining pressure.

Non-Foliated Metamorphic Rock Types

Metaconglomerate - Parent Rock = conglomerate, pebbles have been flattened

Quartzite - fine to medium interlocking quartz grains, Parent Rock = sandstone.

Marble - fine to medium interlocking calcite crystals, Parent Rock = limestone.

Anthracite Coal - Low grade (if T is too high, the coal turns to graphite), Parent Rock = bituminous coal.


Larger mineral crystals set in matrix of much finer-grained minerals in metamorphic rock.

Types of Metamorphism

Environments of Metamorphism
Where does metamorphism occur?
What are the temperature and pressure conditions?

Contact Metamorphism

Thermal Metamorphism
Occurs along the margins of plutonic magma bodies
Increased temperature only
Localized in a halo around pluton
Generally less than 1 km from pluton
Presence of water (metasomatism) can extend this halo zone up to 10 km
Highest grade of metamorphism will be closest to the magma
Lower grade of metamorphism further from the magma
Non-foliated rocks are formed - Hornfels

Plate Tectonic Settings?

Regional Metamorphism

Associated with mountain-building
Collision of two plates results in:
Occurs on a large scale - 100's to 1000's of sq. kilometers
Foliated and non-foliated rocks are produced

Plate Tectonic Settings?

Cataclastic Metamorphism

Localized along fault zones - zones where the crust has fractured
Low to high pressure and temperature
Stress fractures the crust - breaks the rocks and grinds it up - Fault breccia
At depth, higher temperatures exist, the rock is deformed resulting in lineated and elongated mineral structures - Mylonites

Plate Tectonic Settings?

Burial Metamorphism

Accumulation of sediments results in increased pressure on the lowest buried sediments
Low pressure and low temperature
<200°C - diagenesis
Occurs at depths of 7-8 km below the surface typically

Plate Tectonic Settings?

Shock Metamorphism

Also called Impact Metamorphism
Occurs when high-speed planetesimals collide with a planetary surface.
Results in instantaneous, extremely high pressures and temperatures.
Produces pulverized, shattered and melted rock - impactiles, shocked quartz, tektites

Plate Tectonic Settings?


Hydrothermal Metamorphism
Chemical alteration of rock due to hot, ion-rich fluids (mostly water).
Increased temperature typically occurs along with chemical alteration

Plate Tectonic Settings?