Pyroclastics were clastic rocks composed solely or primarily of volcanic materials.
In 2153, Skalaar told Jonathan Archer to locate a pyroclastic debris field on the Rykos shoreline and then to follow a lava tube to reach Ren-gham. (ENT: "Bounty")
In 2373, Enrique Muniz identified a rock Miles O'Brien showed him as a class five pyroclastic debris and ash, the same morphology as the rock formations they had encountered elsewhere on Torga IV. (DS9: "The Ship")
Also that year, The Doctor overenthusiastically reported that the cave walls of a Garan mining colony were comprised of granite with a mixture of pyroclastic infusions. (VOY: "Macrocosm")
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