Pyroclastic rock

Article By:

Tilling, Robert I. Branch of Igneous and Geothermal Processes, U.S. Geological Survey, Menlo Park, California.

Last reviewed:May 2022

DOI:https://doi.org/10.1036/1097-8542.559100

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• Pyroclastic rocks, published December 2019:Download PDF Get Adobe Acrobat Reader

Rock formed by the accumulation of fragmental volcanic materials (also called pyroclasts) ejected during explosive eruptions. Pyroclasts can vary widely in origin, fragment size and shape, composition, texture, and structure (such as their arrangement in the resulting deposit; Fig. 1). Despite such differences, all pyroclastic materials and the great variety of pyroclastic rocks they form share a common, unifying attribute—all are produced by explosive volcanic activity. Pyroclastic deposits are included under a broader category of volcaniclastic deposits, comprising all fragmented volcano-derived material. In some cases, the explosive fragmentation is caused by the interaction of water with hot volcanic ejecta, producing hydrovolcanic materials, which are commonly regarded as pyroclastic sensu lato and will be discussed as such. See also: Caldera; Pyroclast; Volcano

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