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What causes foliation in metamorphic rocks?

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Quick Answer

Pressure that shapes rocks to align in particular directions causes foliation, according to the Mineralogical Society of America. When such differential pressure acts on the forming rocks, it produces layers of rocks that run in parallel to each other, creating the diagnostic banded appearance of foliated rocks.

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Full Answer

Different degrees of pressure and heat result in unique forms of foliated rock. When submitted to light pressure and low heat, metamorphic rock forms slate, which is a fragile rock that tends to break at the joints of layers. This produces thin, flat rocks.

Slightly higher pressures and temperatures produce phyllite, which is a harder rock that often has a shiny appearance. Phyllite often includes deposits of micas, which follow the direction of folliation, giving it its shiny appearance.

Still greater pressures result in the formation of schist. Schist has a larger grain than phyllite, but it shares phyllite's shiny luster from large mica deposits. These deposits are often larger in schist than in phyllite.

The greatest pressures and heats form gneiss. Gneiss has alternating bands of dark and light rock, which are formed when pressures form large mineral deposits into thinner bands. Gneiss has coarser bands than other foliated metamorphic rocks. Unlike phyllite and schist, gneiss contains very little mica. However, gneiss often contains quartz and feldspar.

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