ignimbrite

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ignimbrite A poorly sorted, pyroclastic rock body formed by deposition from a pumiceous pyroclastic flow. The passage of one pyroclastic flow deposits one ignimbrite flow unit with a number of layers directly related to the components of the flow. Ignimbrite flow units deposited by the passage of successive flows can accumulate into a loosely consolidated, composite sheet. Where the rock-body temperature and the accumulating overburden load are both sufficiently high, sintering and flattening of groundmass shards and pumice clasts can occur in the lower part of the body, producing a welded zone within the ignimbrite body which is then characterized by the development of a eutaxitic texture. Hot gases escaping through the ignimbrite deposit can precipitate minerals between the loosely packed shards and pumice in the upper parts of the flow, creating a lithified sillar horizon in which the glass of the pumice and shards is devitrified. Ignimbrites are found on all scales, from a few hundred metres to more than 100 km long, and from one metre to tens of metres thick. The geometry of the ignimbrite sheet is descried using the aspect ratio. See HIGH-ASPECT-RATIO IGNIMBRITE; and LOW-ASPECT-RATIO IGNIMBRITE.