Saturday, May 22, 2010

Breccia is a sedimentary rock which consists of broken fragments of rock cemented together by a fine-grained matrix, that may be similar to or different from the composition of the fragments. It is safest to think of brecciation as a process rather than breccia as a rock type. As a sedimentary rock, breccia is a variety of conglomerate. Breccia is composed of angular rock fragments (2 mm to many meters in diameter) set in a fine- to medium grained matrix. In some breccias the fragments can be seen to match along their opposed sides, indicating only modest disturbance.

The word is a loan from Italian, and in that language indicates both loose gravel and stone made by cemented gravel. A breccia may have a variety of different origins, as indicated by the named types including sedimentary breccia, tectonic breccia, igneous breccia, impact breccia and hydrothermal breccia.

Types of Breccias

Sedimentary breccias are a type of clastic sedimentary rock which are composed of angular to subangular, randomly oriented clasts of other sedimentary rocks. Collapse breccias form where there has been a collapse of rock, typically in a karst landscape; collapse breccias form blankets in highly weathered regolith due to the removal of rock components by dissolution. Tectonic breccias form where two tectonic plates create a crumbling of the interface, by their relative movements. Fault breccias result 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 mineral matter introduced by groundwater. Intrusive rocks can become brecciated in appearance by multiple stages of intrusion, especially if fresh magma is intruded into partly consolidated or solidified magma.