Ignimbrite

Ignimbrite is a type of volcanic rock. More specifically, ignimbrite is a variety of hardened tuff.[1] Ignimbrites are igneous rocks made up of crystal and rock fragments in a glass-shard groundmass, albeit the original texture of the groundmass might be obliterated due to high degrees of welding. The term ignimbrite is not recommended by the IUGS Subcommission on the Systematics of Igneous Rocks.[1]

Rocks from the Bishop tuff from California, United States, uncompressed with pumice on left; compressed with fiamme on right
The caprock in this photo is the ignimbrite layer of the Rattlesnake Formation in Oregon.

Ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, which is a hot suspension of particles and gases flowing rapidly from a volcano and driven by being denser than the surrounding atmosphere. New Zealand geologist Patrick Marshall (1869-1950) derived the term ignimbrite from "fiery rock dust cloud" (from the Latin igni- [fire] and imbri- [rain]). Ignimbrites form as the result of immense explosions of pyroclastic ash, lapilli, and blocks flowing down the sides of volcanoes.

Ignimbrites are made of a very poorly sorted mixture of volcanic ash (or tuff when lithified) and pumice lapilli, commonly with scattered lithic fragments. The ash is composed of glass shards and crystal fragments. Ignimbrites may be loose and unconsolidated, or lithified (solidified) rock called lapilli-tuff. Near the volcanic source, ignimbrites often contain thick accumulations of lithic blocks, and distally, many show meter-thick accumulations of rounded cobbles of pumice.

Ignimbrites may be white, grey, pink, beige, brown, or black depending on their composition and density. Many pale ignimbrites are dacitic or rhyolitic. Darker-coloured ignimbrites may be densely welded volcanic glass or, less commonly, mafic in composition.


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