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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 to or different from the composition of 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.

Basalt breccia, green groundmass is composed of epidote.
Neugrundbreccia, composed mainly of gneiss and amphibolite, is a result of meteorite impact 540 million years ago.

Contents

Types

Sedimentary

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.

Sedimentary breccias can be described as rudaceous.

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 millimeters 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 because of gravity erosion. Talus slopes might become buried and the talus cemented in a similar manner.

Collapse

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

Tectonic breccias form where two tectonic plates create a crumbling of the interface, by their relative movements.

Fault

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

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

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.

Intrusive

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.[1]

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

Alamo bolide impact breccia (Late Devonian, Frasnian) near Hancock Summit, Pahranagat Range, Nevada.

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

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.

Breccia-hosted ore deposits are ubiquitous.[2]

Silicified and mineralized breccia. Light gray is mostly dolomite with a little translucent quartz. Dark gray is jasperoid and ore minerals. Veinlet along lower edge of specimen contains sphalerite in carbonates. Pend Oreille mine, Pend Oreille County, Washington.

The morphology of breccias associated with ore deposits varies from tabular sheeted veins and clastic dikes associated with overpressured sedimentary strata, to large-scale intrusive diatreme breccias, or even some synsedimentary diatremes formed solely by the overpressure of pore fluid within sedimentary basins. Hydrothermal breccias are usually formed by hydrofracturing of rocks by highly pressured hydrothermal fluids. They are typical of the epithermal ore environment and are intimately associated with intrusive-related ore deposits such as skarns, greisens and porphyry-related mineralisation. Epithermal deposits are mined for copper, silver and gold.

In the mesothermal regime, at much greater depths, fluids under lithostatic pressure can be released during seismic activity associated with mountain building. The pressurised fluids ascend towards shallower crustal levels that are under lower hydrostatic pressure. On their journey, high-pressure fluids crack rock by hydrofracturing, forming an angular jigsaw breccia. Rounding of rock fragments less common in the mesothermal regime, as the formational event is brief. If boiling occurs, methane and hydrogen sulfide may be lost to the steam phase and ore may precipitate. Mesothermal deposits are often mined for gold.

Ornamental uses

Natural patterns on the polished surface of selected pieces of breccia or "landscape marble" can resemble a city skyline or even trees, and were used as inlays for furniture etc, especially in Italy.
Breccia statue of the Ancient Egyptian goddess Tawaret

The striking visual appearance of breccias has for millennia made them a popular sculptural and architectural material. Breccia was employed for column bases in the Minoan palace of Knossos on Crete in about 1800 BC.[3] 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).

See also

Line note references

Bibliography

Sibson, R.H. (1987). "Earthquake rupturing as a mineralizing agent in hydrothermal systems". Geology 15: 701–704. doi:10.1130/0091-7613(1987)15<701:ERAAMA>2.0.CO;2. http://geology.geoscienceworld.org/cgi/content/abstract/15/8/701. Retrieved 2007. 

Sibson, R.H. (2000). "Fluid involvement in normal faulting". Journal of Geodynamics 29: 469–499. doi:10.1016/S0264-3707(99)00042-3. 


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

BRECCIA, in petrology, the name given to rocks consisting of angular fragments embedded in a matrix. They may be composed of volcanic rocks, limestones, siliceous charts, sandstones, in fact of any kind of material, and the matrix, which usually corresponds to some extent to the fragments it encloses, may be siliceous, calcareous, argillaceous, &c. The distinctive character of the group is the sharp-edged and unworn shapes of the fragments; in conglomerates the pebbles are rounded and water-worn, having been transported by waves and currents from some distance. There are many ways in which breccias may originate. Some are formed by ordinary processes of atmospheric erosion; frost, rain and gravity break up exposed surfaces of rock and detach pieces of all sizes; in this way screes are formed at the bases of cliffs, and barren mountain-tops are covered with broken debris. If such accumulations gather and are changed into hard rock by pressure and other indurating agencies they make typical breccias. Conglomerates often pass into rocks of this type, the difference being merely that the fragments are of purely local origin, and are unworn because they have not been transported. In caves breccias of limestone are produced by the collapse of part of the roof, covering the floor with broken masses. Coral reefs often contain extensive areas of limestone breccia, formed of detached pieces of rock which have been dislodged from the surface and have been carried down the steep external slopes of the reef. Volcanic breccias are very common near active or extinct craters, as sudden outbursts of steam bear fragments from the older rocks and scatter them over the ground.

Another group of breccias is due to crushing; these are produced in fissures, faults and veins, below the surface, and may be described as "crush-breccias" and "friction-breccias." Very important and well-known examples of this class occur as veinstones, which may be metalliferous or not. A fissure is formed, probably by slight crustal movements, and is subsequently filled with material deposited from solution (quartz, calcite, barytes, &c.). Very often displacement of the walls again takes place, and the infilling or "veinstone" is torn apart and brecciated. It may then be cemented together by a further introduction of mineral matter, which may be the same as that first deposited or quite different. In important veins this process is often repeated several times; detached pieces of the country rock are mingled with the shattered veinstone, and generally experience alteration by the percolating mineral solutions. Other crush-breccias occurring on a much larger scale are due to the folding of strata which have unequal plasticities. If, for example, shales and sandstones are bent into a series of arches, the sandstones being harder and more resistant will tend to crack, while the shales, which are soft and flow under great pressures, are injected into the crevices and separate the broken pieces from one another. Continued movement will give the brecciated fragments of sandstone a rounded form by rubbing them against one another, and, in this way, a crush-conglomerate is produced. Great masses of limestone in the Alps, Scottish Highlands, and all regions of intense folding are thus converted into breccias. Cherts frequently also show this structure; igneous rocks less commonly do so; but it is perhaps most common where there have been thin bedded alternations of rocks of different character, such as limestone and dolerite, limestone and quartzite, shale or phyllite and sandstone. Fault-breccias closely resemble vein-breccias, except that usually their fragments consist principally of the rocks which adjoin the fault and not of mineral deposits introduced in solution; but many veins occupy faults, and hence no hard and fast line can be drawn between these types of breccia.

A third group of breccias is due to movement in a partly consolidated igneous rock, and may be called "fluxion-breccias." Lava streams, especially when they consist of rhyolite, dacite and some kinds of andesite, may rapidly solidify, and then become exceedingly brittle. If any part of the mass is still liquid, it may break up the solid crust by pressure from within and the angular fragments are enveloped by the fluid lava. When the whole comes to rest and cools, it forms a typical "volcanic-fluxion-breccia." The same phenomena are sometimes exemplified in intrusive sills and sheets. The fissures which are occupied by igneous dikes may be the seat of repeated injections following one another at longer or shorter intervals; and the latter may shatter the earlier dike rocks, catching up the fragments. Among the older formations, especially when decomposition has gone on extensively, these fluxion and injection-breccias are often very hard to distinguish from the commoner volcanic-breccias and ash-beds, which have been produced by weathering, or by the explosive power of superheated steam. (J. S. F.)


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Simple English

File:Basalt
Basalt breccia (section)
File:Breccia
Breccia. Notice the angular nature of the large clasts

[[File:|thumb|210px|Tertiary breccia at Resting Springs Pass, Mojave Desert, California.]]

File:Umberto tomb
Tomb of Umberto I in the Pantheon.

[[File:|right|thumb|100px|Breccia statue of the Ancient Egyptian goddess Tawaret.]]

Breccia [1] is a type of rock formed of broken fragments of minerals or rock cemented together by a fine-grained matrix.[2] The matrix may be similar to or different from the composition of the fragments.

Rocks formed by sticking together pieces of previous rocks (clasts) are called clastic rocks. There are two types of clastic rock: conglomerates and breccias. What divides these two categories is the amount of rounding. The particles that make up conglomerates are well rounded, whereas in breccias they are angular.

Origins

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.

The basic sequence is as follows. Sediment is laid down in the sea, and is gradually transformed into sedimentary rock. After a long period of time something violent happens to the rock, breaking it up into many jagged pieces. Later the pieces are cemented together by another deposition, forming a new sedimentary rock. This is a breccia. On examination, the rock is seen to include fragments of the original rock.

Ornament

Brecciated marble can have a marvellous appearance. The Pantheon in Rome is full of the most wonderful marble, many of which are breccias. It includes 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).

References

  1. Italian: meaning 'breach' pronounced 'brechia'
  2. Meteorlab [1]







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