Breccia

Categories: Sedimentary rocks | Rocks | Petrology

Breccia, derived from the Latin word for "broken," is typically a sedimentary rock composed of angular fragments in a matrix that may be of a similar or a different material. A conglomerate by contrast is a sedimentary rock composed of rounded fragments or clasts of pre-existing rocks. Both breccias and conglomerates are composed of fragments averaging greater than 2 mm in size. The angular shape of the fragments indicate that the material has not been transported far from its source. Breccias indicate accumulation in a juvenile stream channel or accumulations due to gravity erosion. Talus slopes may become buried and the talus cemented in a similar manner.

Fault breccias result from the grinding action of two fault blocks as they slide past each other. Subsequent cementation of these broken fragments may occur by means of mineral matter introduced by groundwater.

Volcanic breccias result from the cementation or welding (when the material is glass) of fragments that have been broken by volcanic action. Sometimes the surface of a lava flow will harden while the interior will remain liquid; the fracturing of the surface layer and its subsequent cementation by uncooled lava produces flow breccia.

The intrusion of plutonic rocks will often shatter the invaded country rock, producing a shatter breccia. In the case of plutonic rocks partly cooled and subsequently broken by further invasions of magma, intrusive breccias are formed. Explosive release of volatiles can form masses or pipe like breccias above an intrusion . These shatter breccias or breccia pipes may be mineralized by later hydrothermal fluids forming metal rich sulfide ores.

Impact breccias are thought to be diagnostic of an impact event such as an asteroid or comet striking the earth, and are usually found at impact craters.

Ornamental uses of breccia

The striking visual appearance of breccias has for millennia made them a popular sculptural and architectural material. Breccia was used on a limited scale by the ancient Egyptians - one of the best-known examples is the statue of the goddess Tawaret in the British Museum). It was regarded by the Romans as an especially precious stone and was often used in high-profile public buildings. Many types of marble are brecciated, such as Breccia Oniciata or Breche Nouvelle.

It is most often used as an ornamental or facing material in walls and columns. A particularly striking example can be seen in the Pantheon in Rome, which features two gigantic columns of pavonazzetto, a breccia coming from Phrygia (in modern Turkey). Pavonazzetto obtains its name from its extremely colourful appearance, which is reminiscent of a peacock's feathers (pavone is "peacock" in Italian).de:Brekzie et:Bretša pl:Brekcja ru:Брекчия