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.
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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