Pyroclastic materials are individual fragments of magma and rock that are created by explosions during volcanic eruptions. When unconsolidated, these fragments are called tephra. When they group together and form consolidated rocks, they are called pyroclastic rocks.
Tephra fall into three classes, based on the grain of the pyroclastic fragments. The smallest fragments of 2 mm or less are considered ash; those between 2 mm and 64 mm are lapilli; and those greater than 64 mm are called blocks or bombs. Finer distinctions between tephra types are made based on the physical attributes of the fragments.
Pyroclastic rocks are classified based on their general composition. Ash tuff and lapilli tuff consist primarily of ash and lapilli, respectively. Tuff and pyroclastic breccia include blocks and bombs, and are differentiated by the proportion of these compared to the amount of ash and lapilli. Finally, agglomerate and agglutinate have more consistent chunks of rock, with agglutinate being primarily single large chunks of tephra.