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.
If the original rock is composed of more than one mineral then reactions and recombinations can occur that will produce entirely new minerals in the metamorphic equivalent. Garnet is a mineral produced through metamorphism. Individual mineral grains within the original rock can also coalesce to form larger crystals and align themselves into bands giving the rock a "striped" appearance.
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 and deformation or folding can occur.
Migrating Fluids (Metasomatism): heated fluids can cause metamorphism often accompanied by mineral replacement.
Metamorphism usually occurs along plate tectonic boundaries or near magma bodies.
Link to the complete rock cycle.
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
