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Ignimbrite is a type of volcanic rock, consisting of a typically welded tuff. Ignimbrites form from the deposits of , which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrounding atmosphere. New Zealand geologist Patrick Marshall (1869–1950) coined the term ignimbrite from the Latin igni- fire and imbri- rain.
Ignimbrites are made of a very poorly sorted mixture of volcanic ash (or tuff when Lithification) and pumice and/or scoria lapilli, commonly with scattered lithic fragments. The ash is composed of glass shards and crystal fragments. Ignimbrites may be fairly loose and unconsolidated, or a poor to strongly 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 dacite or rhyolite. Darker-coloured ignimbrites may be densely welded volcanic glass or, less commonly, mafic in composition.
There are several problems with the en masse model. Since ignimbrite is a deposit, its characteristics cannot completely represent the flow, and the deposit may only record the depositional process. Vertical chemical zonation in ignimbrites is interpreted as recording incremental changes in the deposition, and the zonation rarely correlates with flow unit boundaries and may occur within flow units. It has been posited that the chemical changes are recording progressive aggradation at the base of the flow from an eruption whose composition changes with time. For this to be so, the base of the flow cannot be Turbulence. The instantaneous deposition of an entire body of material is not possible because displacement of the fluid is not possible instantaneously. Any displacement of the fluid would mobilize the upper part of the flow, and en masse deposition would not occur. Instantaneously cessation of the flow would cause local compression and extension, which would be evident in the form of tension cracks and small scale thrusting, which is not seen in most ignimbrites. (2026). 9781862390973, The Geological Society. ISBN 9781862390973
An adaptation of the en masse theory suggests that the ignimbrite records progressive aggradation from a sustained current and that the differences observed between ignimbrites and within an ignimbrite are the result of temporal changes to the nature of the flow that deposited it.
A model based on observations at the Wall Mountain Tuff at Florissant Fossil Beds National Monument in Colorado suggests that the rheomorphic structures such as foliation and Pyroclastic rock were formed during laminar viscous flow as the density current comes to a halt. A change from particulate flow to a viscous fluid could cause the rapid en masse cooling in the last few meters. It is also theorized that transformation occurs at a boundary layer at the base of the flow and that all the materials pass through this layer during deposition. (1979). 9780813721804 ISBN 9780813721804
Another model proposed is that the density current became stationary before the rheomorphic structures form. Structures such as pervasive foliation are a result of load compaction, and other structures are the result of remobilization by load and deposition on inclined topography. The tuff deposited at Wagontire Mountain in Oregon and Bishop Tuff in California show evidence of late stage viscous flow. These tuffs have a similar chemistry and so must have undergone the same compaction process to have the same foliation.
The Green Tuff in Pantelleria contains rheomorphic structures which are held to be a result of post-depositional re-mobilization because it is believed the Green Tuff is a Volcanic ash that has no lateral transport. Similarities between the structures in the Green Tuff and ignimbrites on Gran Canaria suggest post-depositional re-mobilization. This interpretation of the deposition of the Green Tuff has been disputed, suggesting that it is an ignimbrite, and structures such as imbricate fiamme, observed in the Green Tuff, were the result of late stage primary viscous flow. Similar structures observed on Gran Canaria had been interpreted as syn-depositional flow.
Sheathfolds and other rheomorphic structures may be the result of a single stage of shear. Shear possibly occurred as the density current passed over the forming deposit. Vertical variations in the orientations of sheathfolds are evidence that rheomorphism and welding can occur syn-depositionally. It has been disputed that the shear between the density current and the forming deposit is significant enough to cause all of the rheomorphic structures observed in ignimbrites, although the shear could be responsible for some of the structures such as imbricate fiamme.
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').
The typical range of phenocrysts in ignimbrites are biotite, quartz, sanidine or other 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 and . 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.
Some rare ignimbrites are andesitic, and may even be formed from volatile saturated basalt, where the ignimbrite would have the geochemistry of a normal basalt.
An example of an ignimbrite with a varying composition is the Pleistocene aged Diliman Tuff. It is a basaltic trachyandesitic to trachydacite with a composition of 54 to 65 wt.% SiO2 in a single fragment of pumice from the tuff.Arpa, M. C. B.; et al. (2008). “The basaltic to trachydacitic upper Diliman Tuff in Manila”. *Journal of Asian Earth Sciences*. doi:10.1016/j.jseaes.2008.10.010
The factor that determines whether an ignimbrite has primary welding, secondary welding or no welding is debated:
Ignimbrite occurs very commonly around the lower Hunter Region of the state of New South Wales. The ignimbrite quarried in the Hunter region at locations such as Martins Creek, Brandy Hill, Seaham (Boral) and at abandoned quarry at Raymond Terrace is a volcanic sedimentation rock of Carboniferous age (280–345 million years). It had an extremely violent origin. This material built up to considerable depth and must have taken years to cool down completely. In the process the materials that made up this mixture fused together into a very tough rock of medium density.
Ignimbrite also occurs in the Coromandel region of New Zealand, where the striking orange-brown ignimbrite cliffs form a distinctive feature of the landscape. The nearby Taupō Volcanic Zone is covered in extensive flat sheets of ignimbrite erupted from caldera volcanoes during the Pleistocene and Holocene. The exposed ignimbrite cliffs at Hinuera (Waikato) mark the edges of the ancient Waikato River course which flowed through the valley before the last major Taupō eruption 1,800 years ago (the Hatepe eruption). The west cliffs are quarried to get blocks of Hinuera Stone, the name given to welded ignimbrite used for building cladding. The stone is light grey with traces of green and is slightly porous.
Huge deposits of ignimbrite form large parts of the Sierra Madre Occidental in western Mexico. In the western United States, massive ignimbrite deposits up to several hundred metres thick occur in the Basin and Range Province, largely in Nevada, western Utah, southern Arizona, and north-central and southern New Mexico, and the Snake River Plain. The magmatism in the Basin and Range Province included a massive flare-up of ignimbrite which began about 40 million years ago and largely ended 25 million years ago: the magmatism followed the end of the Laramide orogeny, when deformation and magmatism occurred far east of the plate boundary. Additional eruptions of ignimbrite continued in Nevada until roughly 14 million years ago. Individual eruptions were often enormous, sometimes up to thousands of cubic kilometres in volume, giving them a Volcanic Explosivity Index of 8, comparable to Yellowstone Caldera and Lake Toba eruptions.
The Diliman Tuff that underlies manila is a basaltic trachyandesitic to trachydacite that underlies Metro Manila It consists of an ignimbrite along with reworked and airfall pumiceous tuff units. Magma mixing and mingling is shown by banded textures in some of its pumice fragments with a groundmass composition of 54 to 65 wt.% SiO2 in a single pumice fragment. Despite the proximity to nearby volcanoes, the source volcano of this deposit has not yet been discovered. It is chemically distinct from adjacent Taal Volcano to the south and the Laguna Caldera to the southeast.Arpa, M. C. B.; et al. (2008). “The basaltic to trachydacitic upper Diliman Tuff in Manila”. *Journal of Asian Earth Sciences*. doi:10.1016/j.jseaes.2008.10.010 The two main types are the mixed scoria and pumice pyroclastic flow and the predominantly fine grained and more pumice rich ignimbrite. There are not just two but several more units of ignimbrites that underlie Manila.
Successions of ignimbrites make up a large part of post-erosional rocks in Tenerife and Gran Canaria islands.
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.
/ref> There are two additional types of tuff found in Quezon City.
/ref> The lower layer of this formation is primarily composed of pumiceous welded tuff with scoria. It is primarily found in Diliman in Quezon City in which this is likely the source name of the tuff. It is the lower part of Diliman tuff. It is more coarse in the lower part and outcrops of this formation can be found in the north and the east of Diliman containing a lot of large scoria. In the upper part of the lower layer of the Diliman tuff, the scoriacous breccia decreases and pumice breccia increases. The upper layer of the formation In this rock formation, like the lower pyroclastic rock bed, the scoria decreases and the pumice breccia becomes dominant. The average diameter of the pumice clasts becomes less than 1 cm.
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Use
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.
See also
Further reading
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