Volcanic breccias are grouped into three major categories based upon process of fragmentation: autoclastic, pyroclastic, and epiclastic. Autoclastic volcanic breccias result from internal processes acting during movement of semisolid or solid lava; they include flow breccia and intrusion breccia. Pyroclastic breccia is produced by volcanic explosion and includes vulcanian breccia, pyroclastic flow breccia, and hydrovolcanic breccia. Epiclastic volcanic breccias result from transportation of loose volcanic material by epigene geomorphic agents, or by gravity, and include laharic breccia, water-laid volcanic breccia, and volcanic talus breccia. Other volcanic breccia terms are discussed.
The rock types mentioned heretofore may also be designated by prefixing compositional terms as in basaltic block breccia and andesitic vulcanian breccia. The term volcanic breccia is used as a general term applying to all coarse-grained rocks composed of angular volcanic fragments.