English: Tectonic breccia from the Pyrenees Mountains of France. (cut surface; field of view 10.9 centimeters 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.

Tectonic breccias form by intense crushing of bedrock in or near fault zones. They may or may not have additional features, such as veining, serpentinization, chloritization, etc. These rocks consist of large angular pieces surrounded by a fine-grained matrix or finely-crystalline cement.

The tectonic breccia shown above is a decorative stone known as "French Grand Antique Marble" in the retail trade. This attractive rock is from France and consists of black, angular, micritic limestone clasts in a whitish carbonate cement. This material has been quarried for millennia - known Roman names for this rock include “Marmor Celticum” and “Marmor Aquitanicum”. The rock was exploited in Roman & Byzantine times, then abandoned and forgotten. Quarrying resumed after rediscovery in the 1700s, but the area is now exhausted - it’s an “extinct” quarry rock.

Locality: old quarry at Aubert, just southeast of Moulis, Lez River Valley, southwest of Saint-Girons, western Ariège Department, central Pyrenees Mountains, far-southern France.