A clastic sedimentary rock that is composed of large (over two millimeter diameter) angular fragments. The spaces between the large fragments can be filled with a matrix of smaller particles or a mineral cement which binds the rock together. Breccia is a rock consisting of angular fragments of any kind, united by a matrix. The shape of the components indicates that they have been produced by fracture, and have not been subjected to rounding by attrition. Fault breccia is often found between the two walls of a geological fault, and is due to the breaking down of the rocky walls when grinding on one another. Mineral veins are often formed in fissures, and are brecciated later by movement of the walls. Another kind of breccia is produced when hot molten lava enters a lake or a stream; it is suddenly cooled and solidified, being at the same time shattered by the clouds of steam that are formed. Braccias differ from conglomerates in the angular nature of their fragments, and in the method of their origin.
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