Weathered amphibolite from the Precambrian of Montana, USA.

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.

Amphibolite is a dark-colored, amphibole-dominated metamorphic rock. It is usually weakly foliated but can be crystalline-textured. Amphibolites are principally composed of hornblende amphibole and plagioclase feldspar. Many amphibolites form by metamorphism of basalt or gabbro (= mafic igneous rocks).

Age: Archean, metamorphic date probably 3.1-3.2 Ga (= age of peak metamorphism during intrusion of Spanish Peaks area trondhjemites and tonalites)