File:Migmatite (Precambrian; near Morrison, Colorado, USA) 1.jpg

English: 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 forms by very high-grade metamorphism of gneiss, accompanied by partial melting. 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 (= reddish speckled areas in the photo) formed by cooling of melt.

This specimen is out-of-place, but it may be from Colorado's Proterozoic-aged Idaho Springs Formation. Injection gneisses, which also occur in the Precambrian of Colorado, are similar in appearance to migmatites.

Locality: loose rock on the slopes of the Dakota Hogback, near Arthur Lakes' "Quarry 10", next to the town of Morrison, north-central Coloado, USA