Ignimbrite is a pyroclastic igneous rock that is an expansion of hardened tuff. It is made up by crystal and rock fragments in a glass-shard groundmass, althouugh the original texture of the groundmass is probably obliterated due to high degrees of welding. Forming of Ignimbrite is very hot ground-hugging cloud of volcanic ash, blocks, and gases known as pyroclastic flow or pyroclastic density current. Ignimbrite is synonymous with flood tuff, welded tuff, ash-flow tuff and pyroclastic flow deposit
Ignimbrites are consist of a mostly sorted aggregate of volcanic ash and and pumice lapilli, normally with scattered lithic fragments.The ash consists of glass shards and crystal fragments.The ash consists may be loose and unconsolidated or lithified rock known as lapilli-tuff.Near the volcanic source, ignimbrites normally incorporate thick accumulations of lithic blocks, and distally, many display meter-thick accumulations of rounded cobbles of pumice.
Name origin: The term “ignimbrite” (from the Latin igni- “fire” and imbri- “rain”) was coined by the New Zealand geologist Peter Marshall in 1935.
Colour: Typically light-coloured (e.g. pinkish-white, pale grey etc).
Texture:Aphanitic if not welded, eutaxitic if welded.
Mineral Content: Pumice clasts in a fine grained glassy matrix, may contain lithic clasts and / orphenocrysts of varying composition.
Silica (SiO 2) content – NA.
Alterations: Large hot ignimbrites can create some form of hydrothermal activity as they tend to blanket the wet soil and bury watercourses and rivers. The water from such substrates will exit the ignimbrite blanket in fumaroles, geysers and the like, a process which may take several years, for example after the Novarupta tuff eruption. In the process of boiling off this water, the ignimbrite layer may become metasomatised (altered). This tends to form chimneys and pockets of kaolin-altered rock.
Ignimbrite Classification and Petrology
Ignimbrite is main composed of a matrix of volcanic ash which is composed fragments of volcanic glass, pumice fragments, and crystals. The fragments are totally 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’).
An ignimbrite is a welded pyroclastic rock that contains abundant flattened juvenile clasts often originally pumice. The flattened clasts within ignimbrites are termed fiamme and range from lapilli-sized (>2 mm) to block-sized (>64 mm). The layered texture produced by fiamme is termed a eutaxitic texture. The groundmass of ignimbrites is usually dominated flattened vitric shards, but can contain lithic and crystal fragments. The fine-grained groundmass of many ignimbrites has a reddish colour due to high temperature oxidation of iron, in particular in the upper parts of a pyroclastic flow deposit. Less welded flows tend to be white or grey, whilst intensely welded flows are often dark grey to black. Recrystallisation and alteration of glass within ignimbrite is common, in particular in ancient examples
Chemical Composition of Ignimbrite
The mineralogy of an ignimbrite is controlled primarily by the chemistry of the source magma.
The typical range of phenocrysts in ignimbrites are biotite, quartz, sanidine or other alkali feldspar, occasionally hornblende, rarely pyroxene and in the case of phonolite tuffs, the feldspathoid minerals such as nepheline and leucite.
Commonly in most felsic ignimbrites the quartz polymorphs cristobalite and tridymite are usually found within the welded tuffs and breccias. In the majority of cases, it appears that these high-temperature polymorphs of quartz occurred post-eruption as part of an autogenic post-eruptive alteration in some metastable form. Thus although tridymite and cristobalite are common minerals in ignimbrites, they may not be primary magmatic minerals.
Ignimbrites form due to emplacement of high temperature pyroclastic flows that compact under their own weight. Exsolution of volatiles from pyroclasts after emplacement can cause alteration of the surrounding groundmass and generate vesicles. Rheomorphic flow of ignimbrites can occur after emplacement resulting in deformation of layering, clasts and vesicles. In thick ignimbrites columnar jointing may occur due to contraction during slow cooling.
Some ignimbrite deposits that are found worldwide are loose and unconsolidated rock formations. Others have three distinct layers. The top and bottom layers that were exposed to the ground and the air above the deposit cooled much faster and resemble sedimentary rock layers.
Where is it found?
Landscapes formed by erosion in hardned ignimbrite can be remarcably similar to those formed on granitic rocks.These landforms are inselbergs, flared slopes, domes, nubbins, tors, tafonis and gnammas.In addition, just like in granite landscapes landforms in ignimbrites may be influenced by joint systems. Ignimbrites occur worldwide associated with many volcanic provinces having high-silica content magma and the resulting explosive eruptions.
Ignimbrite Uses Area
- Yucca Mountain Repository, a U.S. Department of Energy terminal storage facility for spent nuclear reactor and other radioactive waste, is in a deposit of ignimbrite and tuff.
- The layering of ignimbrites is used when the stone is worked, as it sometimes splits into convenient slabs, useful for flagstones and in garden edge landscaping.
- In the Hunter region of New South Wales ignimbrite serves as an excellent aggregate or ‘blue metal’ for road surfacing and construction purposes.
Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
Wikipedia contributors. (2019, March 9). Ignimbrite. In Wikipedia, The Free Encyclopedia. Retrieved 14:57, April 11, 2019, from https://en.wikipedia.org/w/index.php?title=Ignimbrite&oldid=886940683