Metamorphic Rocks


Metamorphic rocks have undergone changes in mineralogy, texture and/or chemical composition as a result of changes in temperature and pressure



Metamorphic Reactions

я      Many reactions occur in the presence of fluids

я      Original rock may have been:

ё   Igneous

ё   Sedimentary

ё   Another metamorphic rock



Driving Forces of Metamorphism

я      The pressure and heat that drive metamorphism are a consequence of three forces:

ё   Internal heat of the Earth

ё   Weight of the overlying rock

ё   Horizontal or tectonic forces that cause the rock to deform



Contact Metamorphism

я      Occurs at high temperatures

я      Restricted to a small area around the margins of a magmatic intrusion

я      Geothermal gradients are high



Hydrothermal Metamorphism

я      Occurs along mid-ocean ridges where heated seawater percolates through hot, fractured basalt

я      Chemical reactions between heated seawater and basalt

я      Crustal rocks on continents can react with invading, hot fluids associated with igneous intrusions



Burial Metamorphism

я      Sedimentary rocks that had undergone diagenesis are buried even deeper

я      A relatively mild type of metamorphism

я      Diagenesis grades into burial metamorphism

я      Results in partial alteration of mineralogy and texture

я      Original bedding and sedimentary structures usually preserved



Regional Metamorphism

я      When temperatures & pressures increase beyond range of burial metamorphism

я      Takes place under high temperatures & pressures over large areas

я      Intense alteration of rock mineralogy and texture

я      Original sedimentary structures destroyed

я      Caused by major tectonic forces



Cataclastic Metamorphism

я      Results from the crushing and shearing of rock during tectonic movement

я      Generally localized along fault planes

я      Produces sheared, highly deformed rocks called mylonites



Metamorphic Facies

я      Describes the grouping of rocks formed under different temperatures and pressures

я      Encompass different regions of P-T space


Facies are named after certain characteristic minerals usually formed through the metamorphism of basalt

Zeolite Facies

Greenschist Facies

Amphibolite Facies

Granulite Facies

Blueschist Facies

Eclogite Facies

Hornfel Facies




Retrograde Metamorphism

Serpentine Minerals Products of Retrograde Metamorphism of Ultramafic Rocks


Common Minerals in Metamorphic Rocks

я      Quartz

я      Feldspar

я      Chlorite

я      Mica

я      Staurolite

я      Garnet

я      Pyroxene

я      Kyanite

я      Andalusite

я      Sillimanite




я      Metamorphism of shale and clay-rich sandstone (greywacke) produces flat or wavy parallel planes called foliation

я      Produced in rocks containing platy minerals:

ё   Micas

ё   Chlorite



Foliation Can Cut at an Angle to the Original Bedding or Be Coincident With Bedding


Foliated Rocks Like Slate Split Along Foliation Planes In What Is Known As Slaty Cleavage



Metamorphism of Shale & Greywacke

я      Slate

ё   Low grade metamorphism

ё   Platy minerals too small to be seen

ё   Exhibits slaty cleavage

я      Phyllite

ё   Upper low-grade metamorphism

ё   Flakes have grown larger

ё   Increase in luster

я      Schist

ё   Upper low-grade to medium grade metamorphism

ё   Increased size of platy minerals

ё   Platy minerals clearly visible to naked eye

я      Gneiss

ё   High grade metamorphism

ё   Light and dark minerals segregated into bands, lenses and streaks

ё   Mafic minerals (biotite & amphibole) concentrated in the dark bands

ё   Felsic minerals (quartz & feldspar) concentrated in light bands




я      Medium-grade metamorphism of mafic igneous rocks

я      Long, thin crystals of amphiboles are aligned in a common direction (lineation)

я      Sometimes separate light bands of feldspar are evident




я      Metamorphism reaches a temperature (>700o C) where rock begins to melts

я      Felsic minerals preferentially melt

я      Mafic minerals remain solid

я      Resulting silica-rich liquid invades partially melted rock as veins and stringers




я      High- to very-high grade metamorphism of shale, greywacke and igneous rocks

я      Display granular texture consisting of

ё   Quartz

ё   Plagioclase

ё   Pyroxene

ё   Garnet

ё   Al-silicates (sillimanite)




The metamorphism of shale forms different minerals (index minerals) at different metamorphic grades


Index Minerals Indicate the Degree of Metamorphism Experienced by theн Rocks That Contain Them


нIsograds Represent the First Appearance of Different Index Minerals inн the Field



Non-Foliated Metamorphic Rocks

я      Contact Metamorphism

я      Hydrothermal Metamorphism

я      Metamorphism of rocks without platy minerals


Quartzite Forms From the Metamorphism of Quartz Sandstone


Marble Forms Through Metamorphism of Limestone


Greenstones Form Through Low-grade Metamorphism of Basalt



Sources of Metamorphic Rocks


Parent Rockнннннннннннннннннннннннннннннннннннннн Metamorphic Rock

Limestoneнннннннннннннннннннннннннннннннннннннннннн Marble

Shaleнннннннннннннннннннннннннннннннннннннннннннннннннн Slate/Phyllite/Schist

Sandstoneнннннннннннннннннннннннннннннннннннннннннн Quartzite

Basaltнннннннннннннннннннннннннннннннннннннннннннннннн Schist or Greenstone

Mixed Sedimentaryнннннннннннннннннннннннннн Gneiss

Graniteнннннннннннннннннннннннннннннннннннннннннннннн Gneiss