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Geology of the Pacific Northwest

Metamorphic Rock Classification Table
Foliated Metamorphic Rocks
Crystal Size Mineralogy Parent Rock Metamorphism Rock Name
very fine clay, quartz, muscovite, chlorite shale low grade regional slate
fine muscovite, quartz, chlorite shale low grade regional phyllite
medium to coarse serpentine, magnetite, talc, chlorite peridotite, dunite low grade regional serpentinite
fine to medium blue amphibole, green mica basalt low grade regional, high pressure blueschist
fine to medium green amphibole, green mica, plagioclase basalt medium grade regional greenschist
medium to coarse biotite, muscovite, quartz, garnet, plagioclase shale, basalt medium grade regional schist
medium to coarse amphibole, plagioclase, biotite, quartz basalt medium grade regional amphibolite
medium to coarse plagioclase, orthoclase, quartz, biotite, amphibole, pyroxene basalt, granite, shale high grade regional gneiss
Nonfoliated Metamorphic Rocks
Crystal Size Mineralogy Parent Rock Metamorphism Rock Name
fine to coarse quartz sandstone regional or contact quartzite
fine to coarse calcite limestone regional or contact marble
fine pyroxene, amphibole, plagioclase shale contact hornfels

Metamorphic rocks form from pre-existing rocks ("parent rocks") due to changes in either temperature, pressure, or volatiles within the earth, often by a combination of all three. Volatiles are those chemical substances, including water and carbon dioxide, that easily turn into gas or fluid and are mobile enough to move in and out of solid rock inside the earth.

Except for certain types of gneiss and migmatite, there is no melting involved in the formation of metamorphic rocks. Metamorphism involves chemical reactions of minerals and fluids without any melting of the rock.

Though pressure and volatiles are important factors in determining the type of metamorphism that occurs, in simple terms it is temperature that determines the metamorphic grade. The higher the temperature of metamorphism, the higher the metamorphic grade.

Most metamorphic rocks form during regional metamorphism, where whole sectors of the crust are pressurized and heated, forcing large volumes of rock to recrystallize into new minerals that are stable under the new conditions. As a result, rocks are changed into metamorphic rock across a region that may be many miles wide.

Regional metamorphic rocks recrystallize under tectonic pressure, directed pressure - the rocks are squeezed more in some directions than others. Directed pressure during metamorphism causes flat or elongate minerals to grow at right angles to the direction of maximum compression. This results in the metamorphic rock being foliated, or layered, due to the way the flat or elongate minerals grew during metamorphism. (However, if no flat or elongate minerals grow, the rock will not be foliated, even if forms under directed pressure during regional metamorphism.)

Some metamorphic rocks form due to contact metamorphism, as a result of heat from a nearby intrusion of magma. Contact metamorphic rocks are found in contact with igneous intrusions (or near such a contact), hence the name.

Because no significant amount of directed pressure is added to a rock's environment during contact metamorphism - just heat and possibly volatiles - contact metamorphic rocks do not develop foliation (do not develop layering) during metamorphism.


Geology of the Pacific Northwest
Metamorphic Rock Classification Table
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 5/31/13