Ignimbrite is a volcanic pyroclastic rock, often of dacitic or rhyolitic composition. Plate tectonics and hotspots Divergent plate boundaries At the Pyroclastic rocks or pyroclastics (derived from the Greek πῦρ, meaning fire and κλαστός, meaning broken are Clastic rocks Dacite ( deɪsaɪt) is an igneous, volcanic rock with a high iron content This page is about a volcanic rock For the ghost town see Rhyolite Nevada, and for the satellite system see Rhyolite/Aquacade.
"Ignimbrite" is the deposit of a pumice rich pyroclastic density current, or 'pyroclastic flow', a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere. Pumice is a textural term for a volcanic rock that is a solidified frothy lava composed of highly microvesicular glass Pyroclastic with very thin translucent A pyroclastic flow (also known as a pyroclastic density current) is a common and devastating result of some volcanic eruptions. 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. Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome.
Ignimbrites may be white, grey, pink, beige, brown or black depending on their composition and density. Many pale ignimbrites are dacitic or rhyolitic. Dacite ( deɪsaɪt) is an igneous, volcanic rock with a high iron content This page is about a volcanic rock For the ghost town see Rhyolite Nevada, and for the satellite system see Rhyolite/Aquacade. Darker coloured ones may be densely welded vitrophyre (i. e. glassy) or, less commonly, mafic in composition. Mafic is an adjective describing a Silicate mineral or rock that is rich in magnesium and iron the term was derived by contracting "magnesium" and "ferric"
Contents |
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 Sparks 1976. Sparks attributed the poor sorting in ignimbrites to laminar flows of very high particle concentration. Laminar flow, sometimes known as streamline flow occurs when a fluid flows in parallel layers with no disruption between the layers 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 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. Geology the term fabric describes the spatial and geometric configuration of all the elements that make up a particular rock. Pyroclastic rocks or pyroclastics (derived from the Greek πῦρ, meaning fire and κλαστός, meaning broken are Clastic rocks 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, Pantalleria 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. Volcanic ash consists of small Tephra, which are bits of pulverized rock and glass created by volcanic eruptions less than in diameter 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. Fiamme are lens-shapes usually mm to cm in size seen on surfaces of some Volcaniclastic rocks 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
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. Tephra is air-fall material produced by a volcanic eruption regardless of composition or fragment size 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. A phenocryst is a relatively large and usually conspicuous Crystal distinctly larger than the grains of the rock groundmass of a Porphyritic Igneous
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 lense-shapes, known as fiamme. Fiamme are lens-shapes usually mm to cm in size seen on surfaces of some Volcaniclastic rocks 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. Feldspar is the name of a group of rock-forming Minerals which make up as much as 60% of the Earth 's crust. Hornblende is a complex inosilicate series of Minerals Hornblende is not a recognized mineral in its own right but the name is used as a general or field term to The pyroxenes are a group of important rock-forming Silicate minerals found in many Igneous and metamorphic rocks. Phonolite is an evolved lava which is considered as forming in shallow Magma chambers Phonolite is an igneous, volcanic ( Extrusive) rock The feldspathoids are a group of tectosilicate Minerals which resemble Feldspars but have a different structure and much lower Silica content Nepheline, also called nephelite (from Greek: nephos, "cloud" is a Feldspathoid: a silica-undersaturated aluminosilicate Na Leucite is a rock -forming Mineral composed of Potassium and Aluminium tectosilicate K
Commonly in most felsic ignimbrites the quartz polymorphs cristobalite and tridymite are usually found within the welded tuffs and breccias. The mineral cristobalite is a high-temperature polymorph of Quartz and Tridymite. Tridymite is a high-temperature polymorph of Quartz and usually occurs as minute tabular white or colorless pseudo-hexagonal Triclinic crystals or scales Tuff (from the Italian "tufo" is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption Breccia (ˈbrɛtʃiə ˈbrɛʃiə breach is a rock composed of angular fragments of several Minerals or rocks in a matrix, that is a cementing material 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. Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in 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. Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock.
Alteration
Large hot ignimbrites can create some form of hydrothermal activity as they tend to blanket the wet soil and bury watercourses and rivers. Hydrothermal circulation in its most general sense is the circulation of hot water 'hydros' in the Greek meaning water and 'thermos' meaning heat Soil, often typeset as SOiL, is a four piece rock band from Chicago Illinois United States founded by Shaun Glass Tom Schofield Tim King and Adam Zadel The water from such substrates will exist in the ignimbrite blanket in fumaroles, geysers and the like, a process which may take several years, for example after the Novarupta tuff eruption. A geyser is a Hot spring characterized by intermittent discharge of water ejected turbulently and accomplished by a vapour phase Novarupta, meaning "new eruption" is a Volcano located on the Alaska Peninsula in the Katmai, about southwest of Anchorage. In the process of boiling off this water, the ignimbrite layer may become metasomatised (altered). Metasomatism is the chemical alteration of a rock by Hydrothermal and other fluids This tends to form chimneys and pockets of kaolin-altered rock. Kaolinite is a Clay mineral with the chemical composition Al 2 Si 2 O 5( OH)4
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. Regolith ( Greek: "blanket rock" is a layer of loose Heterogeneous material covering solid rock. 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. Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress.
- 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'. A pyroclastic flow (also known as a pyroclastic density current) is a common and devastating result of some volcanic eruptions.
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. Tuff (from the Italian "tufo" is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption
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. A caldera is a cauldron-like volcanic feature formed by the collapse of land following a volcanic eruption Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet
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. Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet
Ignimbrite occurs very commonly around the lower Hunter region of the Australian state of New South Wales. The Hunter Region, more commonly known as the Hunter Valley, is a region of New South Wales, extending from approximately to north of Sydney, Australia For a topic outline on this subject see List of basic Australia topics. 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). The Carboniferous is a geologic period and system that extends from the end of the Devonian period about 359 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. Education Coromandel Area School is a state composite (years 1-13 school with a decile rating of 3 and a roll of 304 New Zealand is an Island country in the south-western Pacific Ocean comprising two main landmasses (the North Island and the South Island The nearby Taupo Volcanic Zone is covered in extensive, flat sheets of ignimbrite erupted from caldera volcanoes during the Pleistocene and Holocene. The Taupo Volcanic Zone is a highly active volcanic area in the North Island of New Zealand.
Huge deposits of ignimbrite and form large parts of the Sierra Madre Occidental in western Mexico. The Sierra Madre Occidental is a mountain range in western Mexico and the extreme southwest of the United States, extending 1500 km from southeast Arizona 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 United States of America —commonly referred to as the The Basin and Range Province is a large geologic province which includes parts of the southwestern United States and northwestern Mexico, typified by Basin and Nevada ( is a state located in the western region of the United States of America. The State of Utah (ˈjuːtɔː or) is a western state of the United States. The State of Arizona ( is a state located in the southwestern region of the United States. New Mexico ( is a state located in the southwestern region of the United States of America. 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. The Laramide orogeny was a period of Mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago and ended 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
References
- The Mid-Tertiary Ignimbrite Flare-Up (Western US)
- Branney M. Bluestone is the name given to several stones (1 a feldspathic sandstone in the U A pyroclastic flow (also known as a pyroclastic density current) is a common and devastating result of some volcanic eruptions. Lava is molten rock expelled by a Volcano during an eruption When first expelled from a volcanic vent it is a Liquid at Temperatures Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet J. , Kokelaar P. (1992) A reappraisal of ignimbrite emplacement: progressive aggradation and changes from particulate to non-particulate flow during emplacement of high-grade ignimbrites. Bulletin of Volcanology 54, 504–520
- Branney M. J. , Kokelaar, B. P. (2002) Pyroclastic Density Currents and the Sedimentation of Ignimbrites. Geological Society London Memoir 27, 143 pp.
- Branney M. J. , Barry T. L. , Godchaux M. (2004) Sheathfolds in rheomorphic ignimbrites. Bulletin of Volcanology 66, 485-491
- Chapin C. E. , Lowell G. R. (1979) Primary and secondary flow structures in ash-flow tuffs of the Gribbles Run paleovalley, central Colorado. Geology Society of America Special Paper 180, 137–154
- Freundt A. (1998) Formation of high grade ignimbrites. Part II A pyroclastic suspension current model with implications for low grade ignimbrites. Bulletin of Volcanology 60, 545-567
- Kobberger G. , Schmincke H. U. (1999) Deposition of rheomorphic ignimbrite D (Morgan Formation), Gran Canaria, Canary Islands, Spain. Bulletin of Volcanology 60, 465-485
- Schmincke, H. -U, Swanson D. A. (1967) Laminar viscous flowage structures in ash-flow tuffs from Gran Canaria, Canary Islands. Geology 75, 641–664
- Perez W. , Alvarado G. E. , Gans P. B (2006) The 322 ka Tiribi Tuff: stratigraphy, geochronology and mechanisms of deposition of the largest and most recent ignimbrite in Valle Central, Costa Rica. Bulletin of Volcanology 69, 25-40
- Ragan D. M. , Sheridan M. F. (1972) Compaction of the Bishop Tuff. Geological Society of America Bulletin 83, 95-106
- Sparks, R. S. J. (1976) Grain size variations in ignimbrites and implications for the transport of pyroclastic flows. Sedimentology 23, 147-188.
- Wolff J. A. , Wright J. V. (1981) Rheomorphism of welded tuffs. Journal of Volcanology and Geothermal Research 10, 13–34
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic