PROPOSED CLASSIFICATION OF VOLCANICLASTIC SEDIMENTS AND ROCKS
Volcaniclastic sediments and rocks are divided here into autoclastic, pyroclastic, and epiclastic types with grain-size limits the same as non-volcanic epiclastic rocks. Autoclastic rocks contain fragments that are produced within (but not usually extruded from) volcanic vents, during movement of lava flows, or by gas explosions within flows that have ceased to flow. Pyroclastic rocks contain fragments produced by volcanic explosion and extruded as discrete particles from volcanic vents. Epiclastic volcanic rocks contain fragments produced by weathering and erosion of solidified or lithified volcanic rocks of any type. Volcaniclastic types may be mixed in all proportions with each other or with nonvolcanic fragments, although these mixtures are not designated within this classification. A non-genetic category, based only upon particle size and the presence of volcanic material, is included for rocks with clasts of unknown origin.