A general term of any fragment of lava ejected during an explosive volcanic eruption; by definition, pyroclastic deposits are composed of pyroclasts. pyroclastic Adjective of Greek derivation (from pyro, "fire"; klastos, "broken") relating to fragmental materials formed by the shredding of molten or semi-molten lava and (or) the shattering of pre-existing solid rock (volcanic and other) during explosive eruptions, and to the volcanic deposits they form