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Concept
Breccia
Summary
Breccia (ˈbrɛtʃiə,_ˈbrɛʃ-) is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". 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.
A megabreccia is a breccia composed of very large rock fragments, sometimes kilometers across, which can be formed by landslides, impact events, or caldera collapse.
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. Most of the rounding of rock fragments takes place within the first few kilometers of transport, though complete rounding of pebbles of very hard rock may take up to of river transport.
A megabreccia is a breccia containing very large rock fragments, from at least a meter in size to greater than 400 meters. In some cases, the clasts are so large that the brecciated nature of the rock is not obvious. Megabreccias can be formed by landslides, impact events, or caldera collapse.
Breccias are further classified by their mechanism of formation.
Sedimentary breccia is breccia formed by sedimentary processes. For example, scree deposited at the base of a cliff may become cemented to form a talus breccia without ever experiencing transport that might round the rock fragments.
Thick sequences of sedimentary (colluvial) breccia are generally formed next to fault scarps in grabens.
Sedimentary breccia may be formed by submarine debris flows. Turbidites occur as fine-grained peripheral deposits to sedimentary breccia flows.
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