page_title: Ignimbrite
section_title: Petrology
hierarchical_section_title: Ignimbrite / Petrology
caption_attribution_description: Microscope image of part of a thin section of an ignimbrite (welded tuff) showing shards (mostly brown glass) deformed during flow and compaction about crystal fragments (clear). Long dimension several mm. (New Mexico)
context_page_description: Ignimbrite is a variety of hardened tuff. 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.
  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 derived the term ignimbrite from "fiery rock dust cloud". 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 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 rock called lapilli-tuff.
context_section_description: Ignimbrite is primarily composed of a matrix of volcanic ash (tephra) which is composed of shards and fragments of volcanic glass, pumice fragments, and crystals. The crystal fragments are commonly blown apart by the explosive eruption. Most are phenocrysts that grew in the magma, but some may be exotic crystals such as xenocrysts, derived from other magmas, igneous rocks, or from country rock.
  The ash matrix typically contains varying amounts of pea- to cobble-sized rock fragments called lithic inclusions. They are mostly bits of older solidified volcanic debris entrained from conduit walls or from the land surface. More rarely, clasts are cognate material from the magma chamber.
  If sufficiently hot when deposited, the particles in an ignimbrite may weld together, and the deposit is transformed into a 'welded ignimbrite', made of eutaxitic lapilli-tuff.  When this happens, the pumice lapilli commonly flatten, and these appear on rock surfaces as dark lens shapes, known as fiamme.  Intensely welded ignimbrite may have glassy zones near the base and top, called lower and upper 'vitrophyres', but central parts are microcrystalline ('lithoidal').
caption: Light microscope image of a welded ignimbrite, composed of eutaxitic lapilli-tuff as seen in thin section (Long dimension is several mm). The glass shards (mostly brown) sometimes become welded together when the deposit is still hot, and can be deformed by flow and compaction about crystal fragments (clear).
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