Metamorphic rocks

Photograph of compositionally-layered gneiss outcrop southeast of San Gorgonio Mtn in the San Bernardino Mountains, southern California (Section 16 of T.1S., R.2E.); pencil is about 6 inches (15 cm) long. This outcrop shows metamorphic rocks that look considerably different today than when the geologic materials first were formed. The rocks began life either as sedimentary materials deposited in a marine environment, or as igneous materials of granitic composition.

Whatever their original origin, the parent rock (protolith) was subjected to high temperatures and strong directed forces (stress) that reconfigured the original mineral components into the conspicuous dark- and light-colored (mafic and felsic) layers that characterize the outcrop. This mineral segmentation into discrete layers is a type of foliation that geologists refer to as gneissose layering, and the rock generally is called "gneiss". Note the contortion of the mafic and felsic layers into small folds. Photo by J.C. Matti, USGS, June, 1978.

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It's not hard to see that this metamorphic rock, called gneiss, has been intensely folded! This rock had to have been under very high pressure and temperature to allow it to fold like this without breaking. Photo by Edward P. Klimasauskas, USGS.

Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot, mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates meet. In metamorphic rocks some or all of the minerals in the original rock are replaced, atom by atom, to form new minerals. Metamorphic rocks are often squished, smeared out, and folded.

Despite these uncomfortable conditions, metamorphic rocks do not get hot enough to melt, or they would become igneous rocks!