Migmatite (8.1 centimeters across at its widest)
Metamorphic rocks result from intense alteration of any previously existing rocks by heat and/or pressure and/or chemical change. This can happen as a result of regional metamorphism (large-scale tectonic events, such as continental collision or subduction), burial metamorphism (super-deep burial), contact metamorphism (by the heat & chemicals from nearby magma or lava), hydrothermal metamorphism (by superheated groundwater), shear metamorphism (in or near a fault zone), or shock metamorphism (by an impact event). Other categories include thermal metamorphism, kinetic metamorphism, and nuclear metamorphism. Many metamorphic rocks have a foliated texture, but some are crystalline or glassy.
Migmatite is the result of very high-grade metamorphism and the partial melting of gneiss. Migmatite is a half-metamorphic, half-igneous rock with a half-gneiss, half-granite appearance. It has both foliated and crystalline textures. The granite portions of migmatite (see the pinkish/orangish/salmon-colored bands in the above specimen) formed from cooling of partially melted gneiss.
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