Breccia ( or ) is a rock composed of broken fragments of minerals
or rock cemented
together by a fine-grained matrix
that can be similar to or different from the composition of the fragments.
The word has its origins in the Italian language, in which it means "rubble".
A breccia may have a variety of different origins, as indicated by the named types including sedimentary
breccia, and hydrothermal
A megabreccia is a breccia composed of very large rock fragments, sometimes kilometers across, which can be formed by landslides, impact events,
or caldera collapse.
Sedimentary breccia is a type of clastic sedimentary rock
which is made of angular to subangular, randomly oriented clasts
of other sedimentary rocks. A conglomerate
, by contrast, is a sedimentary rock composed of rounded fragments or clasts of pre-existing rocks. Both breccia and conglomerate are composed of fragments averaging greater than in size. The angular shape of the fragments indicates that the material has not been transported far from its source.
Collapse breccias also form by dissolution of underlying evaporite
breccia results 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 the introduction of mineral
matter in groundwater
Igneous clastic rocks can be divided into two classes:
# Broken, fragmental rocks associated with volcanic eruptions, both of the lava
# Broken, fragmental rocks produced by intrusive
processes, usually associated with plutons
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
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.
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 an ''alloclastic'' volcanic breccia.
are also commonly found in shallow subvolcanic intrusion
s such as porphyry stocks, granite
s 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
intrusions have been found 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 earthquake
* accumulation of xenolith
s within a feeder conduit or vent conduit.
Impact breccias are thought to be diagnostic of an impact event
such as an asteroid
striking the Earth and are normally found at impact crater
s. Impact breccia, a type of impactite
, forms during the process of impact crater
ing when large meteorite
s or comet
s impact with the Earth or other rocky planet
s or asteroid
s. 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 cone
s, impact glass, shocked minerals
, and chemical and isotopic
evidence of contamination with extraterrestrial material (e.g., iridium
anomalies). An example of an impact breccia is the Neugrund breccia
, which was formed in the Neugrund impact
Hydrothermal breccias usually form at shallow crustal
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 steam phase
as boiling continues, in particular carbon dioxide
. As a result, the chemistry of the fluid
s changes and ore
minerals rapidly precipitate
. Breccia-hosted ore
deposits are quite common.
The morphology of breccias associated with ore deposits varies from tabular sheeted veins and clastic dike
s associated with overpressured sedimentary strata, to large-scale intrusive diatreme
breccias (breccia pipe
s), or even some synsedimentary diatremes formed solely by the overpressure of pore fluid within sedimentary basin
s. 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 skarn
s 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 ''in situ'' breccia. Rounding of rock fragments is 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.
For thousands of years, the striking visual appearance of breccias has made them a popular sculptural
material. Breccia was used for column bases in the Minoan palace
on Crete in about 1800 BC
[C. Michael Hogan]
''Knossos fieldnotes'', Modern Antiquarian (2007)
/ref> 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.
Breccia 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 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).
| last = Sibson | first = R.H. |title = Fluid involvement in normal faulting
| journal = Journal of Geodynamics | volume = 29 | pages = 469–499 | year = 2000 | doi=10.1016/S0264-3707(99)00042-3
| issue = 3–5|bibcode = 2000JGeo...29..469S
it:Rocce sedimentarie clastiche#Brecce