The Rock - BRECCIA

  • Sedimentary Rock Type: Clastic
  • Related to: Sandstone and conglomerate
  • Color: Variable
  • Texture: Angular pebble to cobble sized grains sometimes in a finer grained matrix
  • Origins: Debris flows, fault zones, cryptolithic explosion events and  impact site deposits
  • Common Minerals: Quartz, feldspars, micas, calcite and clays
  • Uses: Building material, decorative stones, tiles, tombstones, monuments, jewelry, aquifers, natural gas and petroleum reservoirs
 

Breccias are a relatively common clastic sedimentary rock. They form in many different violent situations where host rocks are broken and not transported far from their source. These situations include any scenario in which rocks can be broken and re-accumulate to form the angular sediment.  Landslides, fault zones, cryptolithic explosion events and impact craters can produce breccias. Landslides or debris flows can occur on continental shelves, on the sides of mountains or in karst environments such as sink holes or collapsed caves.  In fault zones, where rocks or even continents slide past each other, breccia zones can be created that can vary from inches across to tens of meters across.  Cryptolithic explosions are subterranean explosions that can send rocks flying into the air and the debris that falls back to Earth forms brecciated deposits.  Meteorite impact craters can form breccias as the meteor impacts the Earth and the debris is strewn across the country side or back into the crater.  However breccias are formed, it usually is an exciting event!

The sediment from which it forms is composed of angular pebble to cobble sized fragments often dispersed in a finer matrix. The only difference between breccias and conglomerates is the roundness of the grains. In conglomerates, the grains are rounded and usually indicate that they have been transported or worked more than the angular grains found in breccias. Distinguishing between breccias and conglomerates is usually very easy as the grains are mostly large enough to see with the unaided eye. If the rock has a smaller grain size (< 2.0mm) which is almost too small to see, then the rock is sandstone.

Like sandstone and conglomerates, breccias are cemented by various minerals. Normal cementing agents include calcite, quartz (silica), clays and gypsum. When the sediment is first deposited there are lots of open spaces or pores. Cement can affect the amount of pore space that is left in a rock as it solidifies. Breccias usually have significant pore space and they are generally a good rock to act as a reservoir for ground water, natural gas and petroleum.

Breccias have very unique angular textures and are prized as ornamental rocks for buildings, monuments, grave stones, tiles and many other ornamental uses.  They have been used by people for centuries for many ornamental uses and some breccias are even considered to be semi-precious and have found uses in jewelry.

Breccia is a term that has been applied to non-sedimentary rocks of igneous origin too.  At times there are situations in the formation of igneous rocks that produce angular fragments that solidify with a breccia like texture.  These rocks are sometimes referred to as breccia, but are not sedimentary and it is probably better to use the term as an adjective such as a brecciated gabbro for example instead of calling the rock a breccia. 

ROCKS
 IGNEOUS
  ANDESITE
  ANORTHOSITE
  BASALT
  CARBONATITE
  DACITE
  DIORITE
  DUNITE
  GABBRO
  GRANITE
  KIMBERLITE
  KOMATIITE
  LAMPROPHYRES
  MONZONITE
  OBSIDIAN
  PEGMATITE
  PERIDOTITE
  PUMICE
  PYROXENITE
  RHYOLITE
  SCORIA
  SYENITE
  
 METAMORPHIC
  GNEISS
  MARBLE
  QUARTZITE
  PHYLLITE
  SCHIST
  SERPENTINITE
  SLATE
  SOAPSTONE
 
 SEDIMENTARY
  ANHYDRITE
  BANDED IRON FORMATION
  BRECCIA
  CHALK
  CHERT
  COAL
  CONGLOMERATE
  COQUINA
  DOLOMITE
  GYPSUM
  HALITE
  LIMESTONE
  MUDSTONE
  PHOSPHORITE
  SANDSTONE
  SHALE
  SILTSTONE
  TILLITE
 
UNCONSOLIDATED SEDIMENTS
  ALLUVIAL DEPOSITS
  LAHARS
  MORAINES
  PEAT
  SANDS
  SOILS
  TEPHRA
  TILLS
  ORES
  MINING TALUS PILES
 

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