Pyroclast

Article By:

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

Last reviewed:December 2019

DOI:https://doi.org/10.1036/1097-8542.995100

The basic building block of volcanic deposits formed by explosive eruptions. A pyroclast (or pyroclastic fragment) is any rock or mineral fragment—regardless of its size, shape, composition, origin, or mode of deposition—that is ejected during explosive volcanic activity (see illustration). The term pyroclast itself is of Greek origin, meaning "fire-broken" (pyro = fire, klastos = broken). The term pyroclast is more inclusive than the term tephra, which applies only to airborne pyroclastic materials (such as volcanic bombs or blocks, scoria, cinders and volcanic ash) that travel through the air and fall to form deposits. In contrast, pyroclasts include both tephra and ground-hugging fragments blasted from the volcanic vent that do not necessarily rise high into the air. Volcanologists recognize many varieties of pyroclasts based on their origin, size and shape, chemical and mineralogic composition, texture and other characteristics. See also: Pyroclastic rocks; Tuff; Volcano; Volcanology

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