Ignimbrite

Rocks from the Bishop tuff from California, United States, uncompressed with pumice on left; compressed with fiamme on right.

An 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 driven by having a greater density than the surrounding atmosphere. New Zealand geologist Patrick Marshall derived the term 'ignimbrite' from ‘fiery rock dust cloud’ (from the Latin igni- (fire) and imbri- (rain)), formed 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. Proximal to the volcanic source, ignimbrites commonly 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.

Deposition

There are two main models that have been proposed to explain the deposition of ignimbrites from a pyroclastic density current, the en masse deposition and the progressive aggradation models.

The en masse model was proposed by volcanologist Robert Stephen John Sparks in 1976. Sparks attributed the poor sorting in ignimbrites to laminar flows of very high particle concentration. Pyroclastic flows were envisioned as being similar to debris flows, with a body undergoing laminar flow and then stopping en masse. The flow would travel as a plug flow, with an essentially non-deforming mass travelling on a thin shear zone and the en masse freezing occurs when the driving stress falls below a certain level. This would produce a massive unit with an inversely graded base.

Branney et al. 2002 suggest that as an ignimbrite is a deposit, its characteristics cannot completely represent the flow. They suggest that the deposit only records the depositional process. They highlight a number of problems with en masse deposition. Vertical chemical zonation in ignimbrites is interpreted as recording incremental changes in the deposition and the zonation rarely correlate with flow unit boundaries and may occur within flow units. Branney et al. suggest 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 the case the base of the flow cannot be turbulent. They also suggest that 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. For a flow to stop simultaneously across its entire length would cause local compression and extension, there would be evidence of this recorded, in the form of tension cracks and small scale thrusting, and it is not seen in most ignimbrites. In response they suggest 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.

Rheomorphic flow

Rheomorphic flow structures in a welded ignimbrite, Isle of Lipari, Italy

Rheomorphic structures are only observed in high grade ignimbrites. There are two types of rheomorphic flow; post depositional re-mobilization and late stage viscous flow. While there is currently debate in the field of the relative importance of either mechanism, there is agreement that both mechanisms have an effect. A vertical variation in orientation of the structures is compelling evidence against post depositional re-mobilization being responsible for the majority of the structures but more work needs to be carried out to discover if the majority of ignimbrites have these vertical variations or not in order to say which process is the most common.

A model proposed by Schmincke et al. (1967) and later supported by Chapin et al. (1979), based on observations on the Wall Mountain Tuff, suggests that the rheomorphic structures such as a pervasive foliation and a preferred stretching direction of pyroclasts were formed during laminar viscous flow as the density current comes to a halt. Schmincke et al. (1967) suggested that there was a change from particulate flow to a viscous fluid involving the entire cooling unit in the last few metres en masse. This disagrees with Chapin et al. (1979) who suggest transformation at a boundary layer at the base of the flow and that all the materials pass through this layer during deposition.
Another model proposed is that the density current became stationary prior to the formation of the rheomorphic structures (Ragan et al. 1972). They suggest that structures such as pervasive foliation are a result of load compaction. The other structures are the result of remobilization by load and deposition on inclined topography. Ragan et al. (1972) argue that a number of the structures cited by Schmincke et al. (1967) as evidence for late stage primary viscous flow are compatible with compaction structures. Ragan et al. (1972) also suggest that any viscous flow of the deposit must occur post compaction as the Wagontire Mountain tuff shows evidence of late stage viscous flow but has a foliation almost identical to the Bishops Tuff. These tuffs have a similar chemistry and so must have undergone the same compaction process to have the same foliation.
The Green Tuff, Pantelleria contains rheomorphic structures and held to be as a result of post depositional re-mobilization because at that time the Green Tuff was believed to be a fall deposit which has no lateral transport (Wolff et al. 1981). Similarities between the structures in the Green Tuff and ignimbrites on Gran Canaria led Wolff et al. (1981) to interpret these as post depositional re-mobilization.
This interpretation of the deposition of the Green Tuff was disputed by Branney et al. (1992) who proposed that the Green Tuff was an ignimbrite. They also held that 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 by Wolff et al. (1981).
Branney et al. (2004) interpreted sheathfolds and other rheomorphic structures to be the result of a single stage of shear. They suggest that the shear 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.
Kobberger et al. (1999) dispute 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 they concede it could be responsible for some of the structures such as imbricate fiamme. They agree with Chapin et al. (1976) that load compaction on an inclined slope is responsible for the majority of the rheomorphic structures.

Petrology

A block of Ignimbrite
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).

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').

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 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.

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 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.

Welding

Welding is a common form of ignimbrite alteration. There are two types of welding, primary and secondary. If the density current is sufficiently hot the particles will agglutinate and weld at the surface of sedimentation to form a viscous fluid, this is primary welding. If during transport and deposition the temperature is low, then the particles will not agglutinate and weld, although welding may occur later if compaction or other factors reduce the minimum welding temperature to below the temperature of the glassy particles, this is secondary welding. This secondary welding is most common and suggests that the temperature of most pyroclastic density currents is below the softening point of the particles (Chapin et al. 1979). The factor that determines whether an ignimbrite has primary welding, secondary welding or no welding is debated:

  • Schmincke et al. (1967) suggest that different chemical compositions will lower the viscosity and enable primary welding.
  • Chapin et al. (1979) do not believe that there is enough variation in the composition of primary and secondary welded ignimbrites for this to be a major factor.
  • Freundt (1999) suggests that cooling during transport is negligible so if the eruption temperature is high enough then primary welding will occur. Lateral variations in degree of welding are not a result of cooling during transport.
  • Perez et al. (2006) suggest that lithostatic load is responsible for the intensity of welding because the Tiribi ignimbrite is most densely welded where the thickness is greatest. They noticed that the correlation was not perfect and concede that other factors may have an influence.
  • Branney et al. (2002) outline two lines of evidence for the relative unimportance of lithostatic load in determining the intensity of welding; lateral changes in the degree of welding irrespective of thickness and cases where the degree of welding correlates with the chemical zoning. Welding is determined by a combination of factors including compositional changes, volatile content, temperature, grain size population and lithic content.

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 density 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. The exposed ignimbrite cliffs at Hinuera (Waikato) mark the edges of the ancient Waikato River course which flowed through the valley prior to the last major Taupo eruption 1800 years ago. The west cliffs are quarried to obtain blocks of Hinuera Stone, the name given to welded Ignimbite used for building cladding. The stone is light grey with traces of green and is slightly porous.

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, giving them a Volcanic Explosivity Index of 8, comparable to Yellowstone Caldera and Lake Toba eruptions.

Successions of ignimbrites make up most of the post-erosional rocks in Gran Canaria Island.

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

References

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