Breccia

Breccia (pronounced BRETCH-ee-ə or BRESH-, from the Italian word for "rubble") is a type of clastic rock made up of angular, coarse fragments that are held together by a finer-grained matrix or cement. Unlike conglomerate, which has rounded pebbles, breccia's sharp-edged pieces show that the material was deposited very close to where it came from—if it had traveled far, the edges would have been worn smooth. A megabreccia is a version of this rock containing huge fragments, sometimes over a kilometer across, which can result from landslides, asteroid or comet impacts, or the collapse of a volcanic caldera.

Breccias are classified by how they form. Sedimentary breccia, such as talus breccia, forms when scree at the base of a cliff becomes cemented without being transported. Thick sequences of this type often develop next to fault scarps in grabens, and they can also come from submarine debris flows, with turbidites forming fine-grained deposits around them. In karst terrain, collapse breccia appears when rock falls into sinkholes or caves, or when underlying evaporite beds dissolve.

Fault or tectonic breccia is created by the grinding of two fault blocks sliding past each other; the broken fragments are later cemented by minerals from groundwater.

Igneous breccias fall into two groups: those from volcanic eruptions (lava and pyroclastic types) and those from intrusive processes, often linked to plutons or porphyry stocks. Volcanic pyroclastic breccias form during explosive eruptions, incorporating rocks from the magma conduit or picked up by pyroclastic surges. Lava flows, especially rhyolite and dacite, can undergo autobrecciation, where thick, nearly solid lava breaks into blocks that mix back into the still-liquid magma, creating a breccia uniform in composition. Caldera collapse produces megabreccias—blocks of precaldera rock, sometimes over a kilometer long, from the unstable rim—while smaller mesobreccias (clasts under a meter) form layers in the caldera floor. In volcanic conduits, the environment merges with intrusive breccias as upwelling lava solidifies and is shattered by later eruptions, forming alloclastic volcanic breccia.

Intrusive breccias are common in shallow subvolcanic intrusions like porphyry stocks, granites, and kimberlite pipes. They can result from multiple stages of intrusion, such as later aplite veins cutting through earlier granite, creating a chaotic appearance. In mafic and ultramafic intrusions, breccias form through processes like consumption and melt-mingling with wall rocks (producing taxitic texture), accumulation of roof rock falling through the magma chamber, autobrecciation of partly consolidated cumulate by fresh magma, or accumulation of xenoliths in a feeder conduit to form a diatreme breccia pipe.

Impact breccias are diagnostic of asteroid or comet impacts and are found at impact craters. This type of impactite forms during cratering, appearing on or beneath the crater floor, in the rim, or in the ejected material. It can be identified by its location at a known impact crater and by associated features such as shatter cones, impact glass, and shocked minerals.