Metamorphic Rocks

Metamorphic rocks form when heat and/or pressure are applied to earlier rocks. This happens most commonly deep inside the Earth along convergent plate boundaries, where some rocks are squeezed and pushed downward, resulting in metamorphic rocks that are often characterized by a non-random arrangment of mineral crystals, called foliation. Earlier rocks that are subjected only to high temperatures and not to high pressures (like those that are in contact with a body of hot magma or lava as it cools), may not develop foliation. Rocks that contain primarily a single mineral and/or contain few minerals that are "platy" in character, may not develop foliation even if they are subjected to high pressures - such rocks are nonfoliated. Metamorphic rocks may contain unusual minerals, like garnets, kyanite, or sillimanite - such minerals are more stable under the higher temperature/pressure conditions that exist deep inside the Earth where metamorphic rocks often form.

FOLIATED METAMORPHIC ROCKS
		GNEISS - a rock showing extreme foliation, as indicated by the presence of bands of different mineral composition. Likely to have formed within the "roots" of an old mountain range created along a convergent plate boundary. Might've formed from the intense deformation of a sandstone or intrusive igneous rock.
		SCHIST - a moderately foliated metamorphic rock characterized by the shiny surface resulting from parallel alignment of visible crystals of platy minerals (like micas). That alignment was likely created by the high pressures at a convergent plate boundary. Might've formed from the moderate deformation of a mudstone or extrusive igneous rock.
		SLATE - a weakly foliated metamorphic rock, with a strong tendency to fracture readily along certain directions as a result of the parallel alignment of tiny platy minerals (like micas). Might've formed as a result of weak deformation of a mudstone. Like mudstones, slates vary widely in color - pay attention to the slate stones that pave the walkways near the duck pond at Stanley Park the next time you visit!
NON-FOLIATED METAMORPHIC ROCKS
		MARBLE - a nonfoliated metamorphic rock produced by the deformation of limestone. Since marbles consist primarily of the mineral calcite (like limestones), they'll fizz with a drop of dilute hydrochloric acid. Marbles sometimes show color banding and are often used as decorative stones in buildings (check the foyer in Parenzo Hall).
		QUARTZITE - a nonfoliated metamorphic rock produced by the deformation of quartz sandstones. Consists primarily of quartz, and is therefore quite hard.

Igneous Rocks

Sedimentary Rocks

Common Rocks

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Please forward any suggestions or corrections to Mike Young at myoung@wsc.ma.edu