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Geology 110


Metamorphic Rocks



 

Metamorphic Rock can be defined as rocks that have undergone the process of metamorphism.

Metamorphism: the process of changing a rock into a new and different rock with the addition of heat and/or pressure.  Metamorphism occurs WITHOUT melting much like clay will change into pottery when fired in a kiln.

If the original rock is composed of primarily one mineral then the metamorphic equivalent will have the same composition but the crystals are usually larger and interlocking.  EXAMPLE: a sandstone composed of quartz grains (silica) held together by a silica cement.....after metamorphism the rock will be quartzite which consists of large interlocking silica crystals. Also, Limestone, which is composed of calcium carbonate will metamorphose into marble.

If the original rock is composed of more than one mineral then the individual mineral grains within the original rock can coalesce to form larger crystals and align themselves into bands; this can give the rock a "striped" appearance.  The minerals within the rock can also recombine to produce entirely new minerals in the metamorphic equivalent.  Garnet is an example of a mineral produced through the process of metamorphism.  

USGS site on metamorphism 


Causes of Metamorphism:
 

Temperature: either the geothermal gradient or from the presence of a magma.  The original rock DOES NOT melt but is restructured in the solid state.

Pressure: tectonic pressures in the crust puts extreme pressure on rock material causing atoms within the mineral to reconfigure; deformation or folding can occur.

 

Migrating Fluids (Metasomatism): heated fluids can cause metamorphism through chemical alteration of a rock.  During this process minerals are often leached out of a rock and deposited into veins elsewhere.  This is the process that deposits gold, silver and other economically important elements into concentrated veins that are mined.
 

Metamorphism usually occurs along plate tectonic boundaries or near magma bodies.


 


Different forms of metamorphism:
 

CONTACT METAMORPHISM: caused primarily from the heat of a nearby magma body, little or no deformation is usually associated with contact metamorphism, affects a localized area adjacent to the magma.

REGIONAL METAMORPHISM: caused by large tectonic movements and associated with folding, faulting and extreme deformation.  Usually affects a large regional area.

HYDROTHERMAL ALTERATION (METASOMATISM): caused by the movement of chemically active, heated fluids through rock units.  Often accompanied by leaching, transport and replacement of minerals into veins.


Metamorphic rocks are classified by texture.

FOLIATED: the rock has a banded appearance.  This can be very obvious as "stripes", or more subtle producing a "sheen" or "flaky" appearance. Foliation can also be expressed as rock cleavage as in a slate.

Gneiss: visible banding
Schist: flaky appearance
Slate: no visible banding but rock cleavage

NON-FOLIATED (MASSIVE): no banding evident.

Marble: composed of calcium carbonate
Quartzite: composed of silica
 



 
 LINK TO ROCK CHART FOR LAB