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Breccia () is a rock composed of large angular broken fragments of minerals or
rock Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wales ...
s cemented together by a fine-grained
matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...
. 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, tectonic breccia, igneous breccia,
impact Impact may refer to: * Impact (mechanics), a high force or shock (mechanics) over a short time period * Impact, Texas, a town in Taylor County, Texas, US Science and technology * Impact crater, a meteor crater caused by an impact event * Impact ...
breccia, and hydrothermal breccia. A megabreccia is a breccia composed of very large rock fragments, sometimes kilometers across, which can be formed by
landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated grade (slope), slope failures, mudflows, and debris flows. Landslides occur in a variety of ...
s,
impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...
s, or
caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
collapse.


Types

Breccia is composed of coarse rock fragments held together by cement or a fine-grained matrix. Like
conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** Co ...
, breccia contains at least 30 percent of
gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...
-sized particles (particles over 2mm in size), but it is distinguished from
conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** Co ...
because the rock fragments have sharp edges that have not been worn down. These indicate that the gravel was deposited very close to its source area, since otherwise the edges would have been rounded during transport. 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 of river transport. A ''megabreccia'' 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. Megabreccias can be formed by
landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated grade (slope), slope failures, mudflows, and debris flows. Landslides occur in a variety of ...
s,
impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...
s, or
caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
collapse. Breccias are further classified by their mechanism of formation.


Sedimentary

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. Thick sequences of sedimentary (colluvial) breccia are generally formed next to fault scarps in
graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...
s. Sedimentary breccia may be formed by submarine debris flows. Turbidites occur as fine-grained peripheral deposits to sedimentary breccia flows. In a karst terrain, a collapse breccia may form due to collapse of rock into a sinkhole or in cave development. Collapse breccias also form by dissolution of underlying evaporite beds.


Fault

Fault 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

Igneous clastic rocks can be divided into two classes: # Broken, fragmental rocks associated with volcanic eruptions, both of the lava and
pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...
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 Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
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.
Caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
collapse leads to the formation of megabreccias, which are sometimes mistaken for outcrops of the caldera floor. These are instead blocks of precaldera rock, often coming from the unstable oversteepened rim of the caldera. They are distinguished from ''mesobreccias'' whose clasts are less than a meter in size and which form layers in the caldera floor. Some clasts of caldera megabreccias can be over a kilometer in length. 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.


Intrusive

Clastic rocks Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...
are also commonly found in shallow
subvolcanic A subvolcanic rock, also known as a hypabyssal rock, is an intrusive igneous rock that is emplaced at depths less than within the crust, and has intermediate grain size and often porphyritic texture between that of volcanic rocks and plutonic r ...
intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
s 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 Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated b ...
form a late-stage
stockwork In geology, a stockwork is a complex system of structurally controlled or randomly oriented veins. Stockworks are common in many ore deposit types and in greisen Greisen is a highly altered granitic rock or pegmatite, usually composed predom ...
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 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 (producing ''taxitic texture''); * accumulation of rocks which fall through the magma chamber from the roof, forming chaotic remnants; * autobrecciation of partly consolidated
cumulate Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group o ...
by fresh magma injections; * accumulation of xenoliths within a feeder conduit or vent conduit, forming a diatreme breccia pipe.


Impact

Impact breccias are thought to be diagnostic of an
impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...
such as an
asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...
or comet striking the Earth and are normally found at impact craters. Impact breccia, a type of impactite, forms during the process of impact cratering when large
meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...
s or comets impact with the Earth or other rocky planets or
asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...
s. Breccia of this type may be present on or beneath the floor of the crater, in the rim, or in the
ejecta Ejecta (from the Latin: "things thrown out", singular ejectum) are particles ejected from an area. In volcanology, in particular, the term refers to particles including pyroclastic materials (tephra) that came out of a volcanic explosion and magma ...
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 Osmium (from Greek grc, ὀσμή, osme, smell, label=none) is a chemical element with the symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mos ...
anomalies). An example of an impact breccia is the
Neugrund breccia Neugrund breccia is a type of rock consisting of gneissic breccia and amphibolite originating from the Neugrund crater. Neugrund breccia is different from Ordovician breccia, which is found in a similar region but was formed millions of years lat ...
, which was formed in the Neugrund impact.


Hydrothermal

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 Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...
as boiling continues, in particular carbon dioxide. As a result, the chemistry of the
fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...
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 dikes associated with overpressured sedimentary strata, to large-scale intrusive diatreme breccias (
breccia pipe A breccia pipe, also referred to as a chimney, is a mass of breccia (rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix), often in an irregular and cylindrical shape. Characteristics When exposed a ...
s), or even some synsedimentary diatremes formed solely by the overpressure of pore fluid within sedimentary basins. Hydrothermal breccias are usually formed by
hydrofracturing Frost weathering is a collective term for several mechanical weathering processes induced by stresses created by the freezing of water into ice. The term serves as an umbrella term for a variety of processes such as frost shattering, frost wedgi ...
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 Pressure is force magnitude applied over an area. Overburden pressure is a geology term that denotes the pressure caused by the weight of the overlying layers of material at a specific depth under the earth's surface. Overburden pressure is also ca ...
can be released during seismic activity associated with mountain building. The pressurised fluids ascend towards shallower crustal levels that are under lower
hydrostatic Fluid statics or hydrostatics is the branch of fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body "fluids at hydrostatic equilibrium and the pressure in a fluid, or exerted by a fluid, on an imme ...
pressure. On their journey, high-pressure fluids crack rock by
hydrofracturing Frost weathering is a collective term for several mechanical weathering processes induced by stresses created by the freezing of water into ice. The term serves as an umbrella term for a variety of processes such as frost shattering, frost wedgi ...
, 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 Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
may be lost to the steam phase, and ore may precipitate. Mesothermal deposits are often mined for gold.


Ornamental uses

For thousands of years, the striking visual appearance of breccias has made them a popular sculptural and
architectural Architecture is the art and technique of designing and building, as distinguished from the skills associated with construction. It is both the process and the product of sketching, conceiving, planning, designing, and constructing buildings o ...
material. Breccia was used for column bases in the
Minoan The Minoan civilization was a Bronze Age Aegean civilization on the island of Crete and other Aegean Islands, whose earliest beginnings were from 3500BC, with the complex urban civilization beginning around 2000BC, and then declining from 1450B ...
palace A palace is a grand residence, especially a royal residence, or the home of a head of state or some other high-ranking dignitary, such as a bishop or archbishop. The word is derived from the Latin name palātium, for Palatine Hill in Rome which ...
of Knossos on Crete in about 1800 BC. 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. Breccia 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.


See also

* * * * * * *


References


Further reading

* * {{Authority control it:Rocce sedimentarie clastiche#Brecce ja:礫岩#角礫岩