Gneiss from the Precambrian of India. (field of view ~7.35 cm across)

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.

Gneiss is a high-grade, foliated metamorphic rock. It differs from lower grade foliated metamorphics, such as slate, phyllite, and schist, by having alternating bands of light and dark minerals. Metamorphism has been so intense that minerals have segregated into discrete zones. Most gneisses form by metamorphism of schists or granites.

The gneiss shown above is from the Himalayan Mountains of India and has well-defined foliation with crenulations. The dark bands are rich in biotite mica. The light-colored bands are dominated by quartz and feldspar. The small red crystals are garnet.

Stratigraphy: Joshimath Formation, lower Vaikrita Group, Higher Himalayan Crystalline Belt, Proterozoic

Locality: outcrop in the town of Joshimath, Uttarakhand State, Himalayan Mountains, northern India

Some info. from: