image: basalt breccia with epidote groundmass, courtesy of Siim Sepp
Classification of breccias relates to their constituents, mode of occurrence, constituent fragment size, types of clasts, and source of clasts.
Many breccias comprise consolidated talus or scree material, and are made up of accumulations of rock fragments that have fallen down steep hill slopes or cliffs. Breccia are often found above uncomformities, and so are associated with conglomerate, arkose and sandstone. (Conglomerates have rounded fragments.) Other breccias are produced by the fragmentation of rocks during faulting (tectonic or monomictic breccia), or during volcanic eruptions (eruption breccia, vent breccia), or collapses such as in karst areas, or form upon meteorite impacts (often in breccia dikes). Monomictic breccias result from rock deformation by shearing and granulation (cataclasis) in the process of tectonism or dislocation metamorphism, while impact breccias have been called "monomictic movement breccias".
[images: hand-specimens: breccia, 2, 3, 4, chert breccia; breccia in copper, 2; impact breccia; thin-sections : volcanic breccia; fault breccia in the Antietam Formation; Breccia Pernice marble, Breccia Aurora marble; formations: cliffs of volcanic breccias formed by lahars; Wawa xenolithic breccia, Wawa Heterolithic Breccia; Ries impact structure, Iggenhausen quarry, monomictic movement breccia, close-up; Azuara impact structure close-up; grit brecciation and mortar texture, Rubielos de la Cérida impact basin, and heavily brecciated and polished scour surface; strongly brecciated chert nodule, Malmian limestone, Ries impact structure; breccia dikes in Liassic limestones south of Belchite (Deutsche)]