Ignimbrite

Ignimbrite is a volcanic pyroclastic rock, often of dacitic or rhyolitic composition.

"Ignimbrite" is the deposit of a pumice-rich pyroclastic density current, or 'pyroclastic flow' that flows rapidly from a volcano as a hot suspension of particles and gases, driven by a greater density than the surrounding atmosphere. An ignimbrite is 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 source, ignimbrites commonly contain thick accumulations of lithic blocks, and distally, many show m-thick accumulations of rounded blocks (or cobbles) of pumice. The term 'ignimbrite' derives from ‘fiery rock dust cloud’ (from the Latin igni- (fire) and imbri- (rain)), and forms as the result of immense explosions of pyroclastic ash, lapilli and blocks flowing down the sides of volcanoes.

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 ones may be densely welded vitrophyre (i.e. glassy) or, less commonly, mafic in composition.

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Petrology

    Some ignimbrites are welded. 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 .

Within the ash matrix, often varying amounts of pea to cobble sized rock fragments, mostly previously cooled debris entrained from conduit walls or the land surface), but possibly cognate material (from the magma chamber).

The ash may also contain crystal fragments 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.

If sufficiently hot when deposited, the particles in an ignimbrite may weld together, and the deposit is transformed into a solid 'welded ignimbrite', made of eutaxitic lapilli-tuff. When this happens, the pumice lapilli common flatten, and these appear on rock surfaces as dark lense-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').

Mineralogy

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, albite or other feldspar usually of orthoclase composition, 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 coesite and tridymite are usually found within the welded tuffs and brecciass. 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 coesite are common minerals in ignimbrites, they may not be primary magmatic minerals.

Geochemistry

Most ignimbrites are silicic, with generally over 65% SiO₂. The chemistry of the ignimbrites, like all felsic rocks, and the resultant mineralogy of phenocryst populations within them, is related mostly to the varying contents of sodium, potassium, calcium, the lesser amounts of iron and magnesium.

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.

Alteration

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 exist 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.

Morphology and occurrence

Ignimbrite originates from explosive eruptions caused by vigorous exsolution of magmatic gases. The escaping gas accelerates the magma up the conduit, resulting in fragmentation to produce pumice and ash, which dispersed in gas will flow downslope or spread where the dispersal is denser than the atmosphere, as pyroclastic desnity current, sometimes known as a pyroclastic flow'.

Ignimbrites form sheets that can cover as much as thousands of square kilometers. Some examples create thick, valley-filling deposits, while others form a landscape-mantling veneer that locally thickens in valleys and other palaeotopographic depressions.

Many igimbrites are loose, unconsolidated deposits, but some exhibit welding, giving the ignimbrite the texture of a solid rock mass, hence the terms commonly used to describe these examples: welded tuff and welded ashflow.

Often, but not always, a caldera will form as a result of a large ignimbrite eruption because the magma chamber underneath will drain and thus can no longer support the weight of the rock above.

Ignimbrite deposits can be voluminous - examples with up to hundreds or even thousands of cubic kilometers are known from individual eruptions in the geological past.

Distribution

Ignimbrites occur worldwide associated with many volcanic provinces having high-silica content magma and the resulting explosive eruptions.

Ignimbrite occurs very commonly around the lower Hunter region of the Australian state of New South Wales. The ignimbrite quarried in the Hunter region at locations such as Martins Creek, Brandy Hill, Seaham (Boral) and at the now disused 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 Taupo Volcanic Zone is covered in extensive, flat sheets of ignimbrite erupted from caldera volcanoes during the Pleistocene and Holocene.

Huge deposits of ignimbrite and form large parts of the Sierra Madre Occidental in western Mexico. In the western U.S., 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 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.

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.

The layering of ignimbrites is utilized 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.

See also

• Bluestone
• Pyroclastic flow
• Lava
• Magma

References

• The Mid-Tertiary Ignimbrite Flare-Up (Western US)