Breccia is composed of coarse rock fragments held together by cement or a fine-grained matrix. Like
conglomerate, breccia contains at least 30 percent of
gravel-sized particles (particles over 2mm in size), but it is distinguished from
conglomerate 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. 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 or tectonic 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 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. 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 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 by fresh magma injections; • accumulation of
xenoliths within a feeder conduit or vent conduit, forming a
diatreme breccia pipe.
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 normally 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). An example of an impact breccia is the
Neugrund breccia, which was formed in the
Neugrund impact.
Hydrothermal 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 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
fluids changes and
ore minerals rapidly
precipitate. Breccia-hosted
ore deposits are quite common. 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 (
breccia pipes), 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
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. ==Ornamental uses==