Monday, 16 March 2015

Breccia

Breccia

Breccia is a clastic sedimentary rock which consists of large angular fragments. The angular fragments when accumulated, pore spaces are also formed which are then filled by matrix usually of fine grained silt and clay material. This matrix is the binding material holding the angular fragments together.

Breccia is formed by the accumulation of broken rock which are angular fragments. The possible locations for accumulation of breccia are:


  • At the base of an outcrop where rock breaks by the mechanical weathering and accumulates at a single place.
  • In stream deposits near the outcrop where it will accumulate as alluvial fan deposits.
  • It can also be formed as debris flow.
The angular fragments in breccia are the indicators that they are not subjected to transportation or else it would be round rather than angular. When these angular fragments accumulate at an area then the pore spaces are filled by small fine grained particles or cementing material. These bind them together.

Difference between breccia and conglomerate

Both the breccia and conglomerate are clastic sedimentary rock which have fragments over 2 millimetre size. The difference between them lies in the shape of the fragments. The particles of breccia would be angular and those of the conglomerate will be round. If any of these rock is however far from the source rock it can always be differentiated by the particle shape.

Breccia composition

The breccia is the accumulation of rock fragments, so therefore the lithic fragments will describe the type of breccia. As the composition of breccia is of different types this influence on type of rock fragments such as, sandstone breccia, limestone breccia, granite breccia etc. Other breccia which contains different  rock fragments are called polymictic breccia.

Colour of breccia

Breccia can be of different colour depending on the type of angular fragments colour. The colour of the matrix and rock fragments determine the colour of the breccia.

Different type of breccia

Breccia can be of different types by the origin
  • Collapse breccia derived from the cavern or magma chamber collapse.
  • Fault breccia is derived from the fault drag which breaks fragments from the fault block.
  • Flow breccia is lava texture when flow is broken.
  • Igneous breccia is from the igneous rock derived angular fragments.
  • Pyroclastic breccia derived from the igneous rock debris that was ejected from volcanic blast or pyroclastic flow.

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