The word has its origins in the Italian language, in which it means "rubble".[1] A breccia may have a variety of different origins, as indicated by the named types includingsedimentary breccia,fault ortectonic breccia,igneous breccia,impact breccia, andhydrothermal breccia.
Amegabreccia is a breccia composed of very large rock fragments, sometimes kilometers across, which can be formed bylandslides,[2]impact events,[3] orcaldera collapse.[4]
Breccia is composed of coarse rock fragments held together by cement or a fine-grained matrix.[5] Likeconglomerate, breccia contains at least 30 percent ofgravel-sized particles (particles over 2mm in size), but it is distinguished fromconglomerate because the rock fragments have sharp edges that have not been worn down.[6] These indicate that the gravel was deposited very close to its source area, since otherwise the edges would have been rounded during transport.[1] Most of the rounding of rock fragments takes place within the first few kilometers of transport, though complete rounding of pebbles of very hard rock may take up to 300 kilometers (190 mi) of river transport.[7]
Amegabreccia is a breccia containing very large rock fragments, from at least a meter in size to greater than 400 meters. In some cases, the clasts are so large that the brecciated nature of the rock is not obvious.[8] Megabreccias can be formed bylandslides,[2]impact events,[3] orcaldera collapse.[4]
Breccias are further classified by their mechanism of formation.[5]
Sedimentary breccia is breccia formed by sedimentary processes. For example,scree deposited at the base of a cliff may become cemented to form a talus breccia without ever experiencing transport that might round the rock fragments.[9]Thick sequences of sedimentary (colluvial) breccia are generally formed next to fault scarps ingrabens.[10][11]
Sedimentary breccia may be formed by submarinedebris flows.Turbidites occur as fine-grained peripheral deposits to sedimentary breccia flows.[12]
In akarst terrain, a collapse breccia may form due to collapse of rock into asinkhole or incave development.[13][14] Collapse breccias also form by dissolution of underlyingevaporite beds.[15]
Fault or tectonic breccia results from the grinding action of two fault blocks as they slide past each other. Subsequentcementation of these broken fragments may occur by means of the introduction ofmineral matter ingroundwater.[16]
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 themagma conduit, or physically picked up by the ensuingpyroclastic surge.[17] Lavas, especiallyrhyolite anddacite flows, tend to form clastic volcanic rocks by a process known asautobrecciation. 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.[20]
Caldera collapse leads to the formation of megabreccias, which are sometimes mistaken for outcrops of the caldera floor.[8] These are instead blocks of precaldera rock, often coming from the unstable oversteepened rim of the caldera.[4] They are distinguished frommesobreccias whose clasts are less than a meter in size and which form layers in the caldera floor.[21] Some clasts of caldera megabreccias can be over a kilometer in length.[4]
Within the volcanic conduits of explosive volcanoes the volcanic breccia environment merges into the intrusive breccia environment. There the upwelling lava tends to solidify during quiescent intervals only to be shattered by ensuing eruptions. This produces analloclastic volcanic breccia.[22][23]
Clastic rocks are also commonly found in shallowsubvolcanicintrusions such as porphyry stocks,granites andkimberlite pipes, where they are transitional with volcanic breccias.[24] 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 laterapliteveins form a late-stagestockwork through earlier phases of the granite mass.[25][26] When particularly intense, the rock may appear as a chaotic breccia.[27]
Clastic rocks inmafic andultramafic intrusions have been found and form via several processes:
consumption and melt-mingling with wall rocks, where the wall rocks are softened and gradually invaded by the hotter ultramafic intrusion (producingtaxitic texture);[28]
accumulation of rocks which fall through the magma chamber from the roof, forming chaotic remnants;[29]
autobrecciation of partly consolidatedcumulate by fresh magma injections;[30]
accumulation ofxenoliths within a feeder conduit or vent conduit, forming adiatreme breccia pipe.[31]
Impact breccias are thought to be diagnostic of animpact event such as anasteroid orcomet striking the Earth and are normally found atimpact craters. Impact breccia, a type ofimpactite, forms during the process ofimpact cratering when largemeteorites orcomets impact with the Earth or other rockyplanets orasteroids. Breccia of this type may be present on or beneath the floor of the crater, in the rim, or in theejecta 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 asshatter cones, impact glass,shocked minerals, and chemical andisotopic evidence of contamination with extraterrestrial material (e.g.,iridium andosmium anomalies). An example of an impact breccia is theNeugrund breccia, which was formed in theNeugrund impact.
Hydrothermal breccia in the Cloghleagh Iron Mine, near Blessington in Ireland, composed mainly ofquartz andmanganese oxides, the result ofseismic activity about 12 million years ago
Hydrothermal breccias usually form at shallowcrustal levels (<1 km) between 150 and 350 °C, when seismic or volcanic activity 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. In addition, the sudden opening of a cavity causes rock at the sides of the fault to destabilise and implode inwards, and the broken rock gets caught up in a churning mixture of rock, steam and boiling water. Rock fragments collide with each other and the sides of the void, and the angular fragments become more rounded. Volatile gases are lost to the steamphase as boiling continues, in particularcarbon dioxide. As a result, the chemistry of thefluids changes andore minerals rapidlyprecipitate. Breccia-hostedore deposits are quite common.[32]
The morphology of breccias associated with ore deposits varies from tabular sheeted veins[33] andclastic dikes associated with overpressured sedimentary strata,[34] to large-scale intrusivediatreme breccias (breccia pipes),[35] or even some synsedimentary diatremes formed solely by the overpressure of pore fluid withinsedimentary basins.[36] Hydrothermal breccias are usually formed byhydrofracturing of rocks by highly pressuredhydrothermal fluids. They are typical of theepithermal ore environment and are intimately associated with intrusive-related ore deposits such asskarns,greisens andporphyry-related mineralisation. Epithermal deposits aremined for copper, silver and gold.[37]
In the mesothermal regime, at much greater depths, fluids underlithostatic pressure can be released during seismic activity associated with mountain building. The pressurised fluids ascend towards shallower crustal levels that are under lowerhydrostatic pressure. On their journey, high-pressure fluids crack rock byhydrofracturing, forming an angularin situ breccia. Rounding of rock fragments is less common in the mesothermal regime, as the formational event is brief. If boiling occurs,methane andhydrogen sulfide may be lost to the steam phase, and ore may precipitate. Mesothermal deposits are often mined for gold.[37]
For thousands of years, the striking visual appearance of breccias has made them a popularsculptural andarchitectural material. Breccia was used for column bases in theMinoanpalace ofKnossos on Crete in about 1800BC.[38] Breccia was used on a limited scale by theancient Egyptians; one of the best-known examples is the statue of the goddessTawaret in the British Museum.[39] Breccia was regarded by theRomans as an especiallyprecious stone and was often used in high-profile public buildings.[40] Many types ofmarble are brecciated, such as Breccia Oniciata.[41]
^Longhitano, S.G.; Sabato, L.; Tropeano, M.; Murru, M.; Carannante, G.; Simone, L.; Cilona, A.; Vigorito, M. (November 2015). "Outcrop reservoir analogous and porosity changes in continental deposits from an extensional basin: The case study of the upper Oligocene Sardinia Graben System, Italy".Marine and Petroleum Geology.67:439–459.Bibcode:2015MarPG..67..439L.doi:10.1016/j.marpetgeo.2015.05.022.hdl:11586/139746.
^Moore, P. R. (June 1989). "Kirks Breccia: a late cretaceous submarine channelised debris flow deposit, Raukumara Peninsula, New Zealand".Journal of the Royal Society of New Zealand.19 (2):195–203.Bibcode:1989JRSNZ..19..195M.doi:10.1080/03036758.1989.10426448.
^Lopes, Tuane V.; Rocha, Aline C.; Murad, Marcio A.; Garcia, Eduardo L. M.; Pereira, Patricia A.; Cazarin, Caroline L. (February 2020). "A new computational model for flow in karst-carbonates containing solution-collapse breccias".Computational Geosciences.24 (1):61–87.Bibcode:2020CmpGe..24...61L.doi:10.1007/s10596-019-09894-9.S2CID208144669.
^Blatt, Harvey; Middletone, Gerard; Murray, Raymond (1980).Origin of sedimentary rocks (2d ed.). Englewood Cliffs, N.J.: Prentice-Hall. pp. 546, 577.ISBN0136427103.
^Olianti, Camille A.E.; Harris, Chris (February 2018). "A low-δ18O intrusive breccia from Koegel Fontein, South Africa: Remobilisation of basement that was hydrothermally altered during global glaciation?".Lithos.300–301:33–50.Bibcode:2018Litho.300...33O.doi:10.1016/j.lithos.2017.12.006.
^Nurmi, P.A.; Haapala, I. (1986). "The Proterozoic granitoids of Finland: granite types, metallogeny and relation to crustal evolution".Bulletin of the Geological Society of Finland.58 (1):203–233.doi:10.17741/bgsf/58.1.014.
^Philpotts, Anthony R.; Ague, Jay J. (2009).Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. p. 80.ISBN9780521880060.
^Sherlock, Ross L.; Tosdal, Richard M.; Lehrman, Norman J.; Graney, Joseph R.; Losh, Steven; Jowett, E. Craig; Kesler, Stephen E. (1 December 1995). "Origin of the McLaughlin Mine sheeted vein complex; metal zoning, fluid inclusion, and isotopic evidence".Economic Geology.90 (8):2156–2181.Bibcode:1995EcGeo..90.2156S.doi:10.2113/gsecongeo.90.8.2156.
^Yahata, M.; Kurosawa, K.; Ohtsu, S.; Takahashi, T.; Tomagae, S.; Kawamori, H.; Mori, M. (1994). "Hydrothermal alteration and sedimentation at the formative period of a hot spring gold deposit".Shigen-Chishitsu.44.doi:10.11456/shigenchishitsu1992.44.1.
^Cartwright, Joe; Santamarina, Carlos (August 2015). "Seismic characteristics of fluid escape pipes in sedimentary basins: Implications for pipe genesis".Marine and Petroleum Geology.65:126–140.Bibcode:2015MarPG..65..126C.doi:10.1016/j.marpetgeo.2015.03.023.
^abJébrak, Michel (December 1997). "Hydrothermal breccias in vein-type ore deposits: A review of mechanisms, morphology and size distribution".Ore Geology Reviews.12 (3):111–134.Bibcode:1997OGRv...12..111J.doi:10.1016/S0169-1368(97)00009-7.
^Henderson, Julian; Morkot, Robert; Peltenberg, E.J.; Quirke, Stephen; Serpico, Margaret; Tait, John; White, Raymond (2000).Ancient Egyptian materials and technology. Cambridge: Cambridge University Press. p. 43.ISBN9780521452571. Retrieved2 April 2022.
^Lazzarini, Lorenzo (January 2010). "Six Coloured Types of Stone from Asia Minor Used by the Romans, and Their Specific Deterioration Problems".Studies in Conservation.55 (sup2):140–146.doi:10.1179/sic.2010.55.Supplement-2.140.S2CID194088642.
