Breccia is a rock made of smaller rocks, like conglomerate. But breccia contains sharp, broken clasts while conglomerate has smooth, round clasts. Breccia (pronounced /ˈbrɛtʃiə, ˈbrɛʃiə/, Italian: breach) is a rock composed of angular fragments of minerals or rocks in a matrix (cementing material), that may be similar or different in composition to the fragments. The word is a loan from Italian, and in that language indicates both loose gravel and stone made by cemented gravel. A breccia may have a variety of different origins, as indicated by the named types including sedimentary breccia, tectonic breccia, igneous breccia, impact breccia and hydrothermal breccia.
Breccia is usually listed under sedimentary rocks, but igneous and metamorphic rocks may become shattered, too. It is safest to think of brecciation as a process rather than breccia as a rock type. As a sedimentary rock, breccia is a variety of conglomerate.
There are many different ways to make breccia, and usually geologists add a word to signify the kind of breccia they’re talking about. A sedimentary breccia arises from things like talus or landslide debris. A volcanic or igneous breccia forms during eruptive activities. A collapse breccia forms when rocks are partly dissolved, such as limestone or marble. One created by tectonic activity is a fault breccia. And a new member of the family, first described from the Moon, is impact breccia.
Sedimentary breccias are a type of clastic sedimentary rock which are composed of angular to subangular, randomly oriented clasts of other sedimentary rocks. They are formed by either submarine debris flows, avalanches, mud flow or mass flow in an aqueous medium. Technically, turbidites are a form of debris flow deposit and are a fine-grained peripheral deposit to a sedimentary breccia flow.
The other derivation of sedimentary breccia is as angular, poorly sorted, immature fragments of rocks in a finer grained groundmass which are produced by mass wasting. These are, in essence, lithified colluvium. Thick sequences of sedimentary (colluvial) breccias are generally formed next to fault scarps in grabens.
In the field, it may at times be difficult to distinguish between a debris flow sedimentary breccia and a colluvial breccia, especially if one is working entirely from drilling information. Sedimentary breccias are an integral host rock for many SEDEX ore deposits.
Collapse breccias form where there has been a collapse of rock, typically in a karst landscape. Collapse breccias form blankets in highly weathered regolith due to the removal of rock components by dissolution.
Tectonic breccias form where two tectonic plates create a crumbling of the interface, by their relative movements.
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.
Igneous clastic rocks can be divided into two classes.
Broken, fragmental rocks associated with volcanic eruptions, both of lava and pyroclastic type
Broken, fragmental rocks produced by intrusive processes, usually associated with plutons or porphyry stocks
Volcanic pyroclastic rocks are formed by explosive eruption of lava and any rocks which are entrained within the eruptive column. This may include rocks plucked off the wall of the magma conduit, or physically picked up by the ensuing pyroclastic surge. Lavas, especially rhyolite and dacite flows, tend to form clastic volcanic rocks by a process known as autobrecciation. This occurs when the thick, nearly solid lava breaks up into blocks and these blocks are then reincorporated into the lava flow again and mixed in with the remaining liquid magma. The resulting breccia is uniform in rock type and chemical composition.
Lavas may also pick up rock fragments, especially if flowing over unconsolidated rubble on the flanks of a volcano, and these form volcanic breccias, also called pillow breccias.
The volcanic breccia environment is transitional into the plutonic breccia environment in the volcanic conduits of explosive volcanoes, where lava tends to solidify and may be repeatedly shattered by ensuing eruptions. This is typical of volcanic caldera settings.
Clastic rocks are also commonly found in shallow subvolcanic intrusions such as porphyry stocks, granites and kimberlite pipes, where they are transitional with volcanic breccias.
Intrusive rocks can become brecciated in appearance by multiple stages of intrusion, especially if fresh magma is intruded into partly consolidated or solidified magma. This may be seen in many granite intrusions where later aplite veins form a late-stage stockwork through earlier phases of the granite mass. When particularly intense, the rock may appear as a chaotic breccia.
Clastic rocks in mafic and ultramafic intrusions are known and form via several processes;consumption and melt-mingling with wall rocks, where the felsic wall rocks are softened and gradually invaded by the hotter ultramafic intrusion (termed taxitic texture by Russian geologists)
Accumulation of rocks which fall through the magma chamber from the roof, forming chaotic remnants
Autobrecciation of partly consolidated cumulate by fresh magma injections or by violent disturbances within the magma chamber (e.g. postulated earthquakes)
Accumulation of xenoliths within a feeder conduit or vent conduit
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. Impact breccia, a type of impactite, forms during the process of impact cratering when large meteorites or comets impact with the Earth or other rocky planets or asteroids. Breccia of this type may be present on or beneath the floor of the crater, in the rim, or in the ejecta expelled beyond the crater. Impact breccia may be identified by its occurrence in or around a known impact crater, and/or an association with other products of impact cratering such as shatter cones, impact glass, shocked minerals, and chemical and isotopic evidence of contamination with extraterrestrial material (e.g. iridium and osmium anomalies).
Hydrothermal breccia, Cloghleagh Iron Mine, near Blessington in Ireland, composed mainly of quartz and manganese oxides, the result of seismic activity about 12 million years ago.
Hydrothermal breccias usually form at shallow crustal levels (<1 km) between 150 to 350oC, when seismic activity (an earthquake) causes a void to open along a fault deep underground. The void draws in hot water and as pressure in the cavity drops, the water violently boils – akin to an underground geyser. In addition, the sudden opening of a cavity causes rock at sides of the fault to destabilise and implode inwards, the broken rock gets caught up in a churning mixture of rock, steam and boiling water. Rock fragments hit each other and sides of the fault, and attrition quickly rounds angular breccia fragments. Volatile gases are lost to the steam phase as boiling continues, in particular CO2. As a result, the chemistry of the fluids change and ore minerals rapidly precipitate.