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. 

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