Ignimbrite
Encyclopedia
An ignimbrite is the deposit of a 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.
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. 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 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 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.
The en masse model was proposed by Steve 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.
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 phenocryst
s 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 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').
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 tuff
s and breccia
s. 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.
and bury watercourses and rivers. The water from such substrates will exist in the ignimbrite blanket in fumarole
s, geyser
s 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.
The factor that determines whether an ignimbrite has primary welding, secondary welding or no welding is debated:
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.
and the resulting explosive eruptions.
Ignimbrite occurs very commonly around the lower Hunter
region of the Australia
n 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 ignimbites mades up most of the post-erosional rocks in Gran Canaria
Island.
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.
Pyroclastic flow
A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...
, a hot suspension of particles and gases that flows rapidly from a volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
, driven by a greater density than the surrounding atmosphere.
Ignimbrites are made of a very poorly sorted mixture of volcanic ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
(or tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
when lithified) and pumice
Pumice
Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...
lapilli
Lapilli
Lapilli is a size classification term for tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. Lapilli means "little stones" in Latin. They are in some senses similar to ooids or pisoids in calcareous sediments.By definition lapilli range...
, 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 New Zealand geologist Patrick Marshall
Patrick Marshall
Dr. Patrick Marshall was a geologist who lived in New Zealand. For over forty years he was an outstanding figure among New Zealand scientists, and was well known to geologists in many lands as a very versatile and productive investigator. His research was also devoted to zoology...
derived the term 'ignimbrite' from ‘fiery rock dust cloud’ (from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
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 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
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
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 Steve Sparks
Steve Sparks (volcanologist)
Robert Stephen John Sparks, FRS, CBE , is Chaning Wills Professor of Geology in the Department of Earth Sciences at the University of Bristol. He is one of the world's leading volcanologists and has been widely recognised for his work in this field.-Career:Steve Sparks is a graduate of Imperial...
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 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 foliationFabric (geology)In geology, a rock's fabric describes the spatial and geometric configuration of all the elements that make it up.-Types of fabric:* Primary fabric — a fabric created during the original formation of the rock, e.g...
and a preferred stretching direction of pyroclastsPyroclastic rockPyroclastic rocks or pyroclastics are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic...
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, PantelleriaPantelleriaPantelleria , the ancient Cossyra, is an Italian island in the Strait of Sicily in the Mediterranean Sea, southwest of Sicily and just east of the Tunisian coast. Administratively Pantelleria is a comune belonging to the Sicilian province of Trapani...
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 depositVolcanic ashVolcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
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 fiammeFiammeFiamme are lens-shapes, usually mm to cm in size, seen on surfaces of some volcaniclastic rocks. They can occur in welded pyroclastic fall deposits and in ignimbrites, which are the deposits of pumiceous pyroclastic density currents. The name fiamme comes from the Italian word for flames,...
, 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
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....
) 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 phenocryst
Phenocryst
thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...
s 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 lense-shapes, known as fiamme
Fiamme
Fiamme are lens-shapes, usually mm to cm in size, seen on surfaces of some volcaniclastic rocks. They can occur in welded pyroclastic fall deposits and in ignimbrites, which are the deposits of pumiceous pyroclastic density currents. The name fiamme comes from the Italian word for flames,...
. 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
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, occasionally hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
, rarely pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
and in the case of phonolite
Phonolite
Phonolite is a rare igneous, volcanic rock of intermediate composition, with aphanitic to porphyritic texture....
tuffs, the feldspathoid
Feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks....
minerals such as nepheline
Nepheline
Nepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites...
and leucite
Leucite
Leucite is a rock-forming mineral composed of potassium and aluminium tectosilicate K[AlSi2O6]. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster in 1821, they are not optically isotropic, and are therefore pseudo-cubic. Goniometric measurements made by...
.
Commonly in most felsic ignimbrites the quartz polymorphs cristobalite
Cristobalite
The mineral cristobalite is a high-temperature polymorph of silica, meaning that it has the same chemical formula, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the quartz group which also include coesite, tridymite and stishovite...
and tridymite
Tridymite
Tridymite is a high-temperature polymorph of quartz and usually occurs as minute tabular white or colorless pseudo-hexagonal triclinic crystals, or scales, in cavities in acidic volcanic rocks. Its chemical formula is SiO2. Tridymite was first described in 1868 and the type location is in Hidalgo,...
are usually found within the welded tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
s and breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
s. In the majority of cases, it appears that these high-temperature polymorphs of quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties 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
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
, 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 soilSoil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
and bury watercourses and rivers. The water from such substrates will exist in the ignimbrite blanket in fumarole
Fumarole
A fumarole is an opening in a planet's crust, often in the neighborhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide. The steam is created when superheated water turns to steam as its pressure drops when it emerges from...
s, geyser
Geyser
A geyser is a spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapour phase . The word geyser comes from Geysir, the name of an erupting spring at Haukadalur, Iceland; that name, in turn, comes from the Icelandic verb geysa, "to gush", the verb...
s and the like, a process which may take several years, for example after the Novarupta
Novarupta
Novarupta, meaning "new eruption", is a volcano located on the Alaska Peninsula in Katmai National Park and Preserve, about southwest of Anchorage. Formed in 1912 during the largest volcanic eruption of the 20th century, Novarupta released 30 times the volume of magma as the 1980 eruption of...
tuff eruption. In the process of boiling off this water, the ignimbrite layer may become metasomatised
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
(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 viscosityViscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher 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
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
and welded ashflow.
Often, but not always, a caldera
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
will form as a result of a large ignimbrite eruption because the magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
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 magmaMagma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
and the resulting explosive eruptions.
Ignimbrite occurs very commonly around the lower Hunter
Hunter Valley
The Hunter Region, more commonly known as the Hunter Valley, is a region of New South Wales, Australia, extending from approximately to north of Sydney with an approximate population of 645,395 people. Most of the population of the Hunter Region lives within of the coast, with 55% of the entire...
region of the Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
n state of New South Wales
New South Wales
New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. 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
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
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
Coromandel, New Zealand
Coromandel is the name of a town and harbour on the western side of the Coromandel Peninsula, which is on the east coast of the North Island of New Zealand...
region of New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
, where the striking, orange-brown ignimbrite cliffs form a distinctive feature of the landscape. The nearby Taupo Volcanic Zone
Taupo Volcanic Zone
The Taupo Volcanic Zone is a highly active volcanic V shaped area in the North Island of New Zealand that is spreading east -west at the rate of about 8mm per year...
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
Sierra Madre Occidental
The Sierra Madre Occidental is a mountain range in western Mexico.-Setting:The range runs north to south, from just south of the Sonora–Arizona border southeast through eastern Sonora, western Chihuahua, Sinaloa, Durango, Zacatecas, Nayarit, Jalisco, Aguascalientes to Guanajuato, where it joins...
in western Mexico. In the western U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, massive ignimbrite deposits up to several hundred metres thick occur in the Basin and Range Province, largely in Nevada
Nevada
Nevada is a state in the western, mountain west, and southwestern regions of the United States. With an area of and a population of about 2.7 million, it is the 7th-largest and 35th-most populous state. Over two-thirds of Nevada's people live in the Las Vegas metropolitan area, which contains its...
, western Utah
Utah
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...
, southern Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
, and north-central and southern New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
, 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
Laramide orogeny
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 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...
, 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
Volcanic Explosivity Index
The Volcanic Explosivity Index was devised by Chris Newhall of the U.S. Geological Survey and Stephen Self at the University of Hawaii in 1982 to provide a relative measure of the explosiveness of volcanic eruptions....
of 8, comparable to Yellowstone Caldera
Yellowstone Caldera
The Yellowstone Caldera is the volcanic caldera located in Yellowstone National Park in the United States, sometimes referred to as the Yellowstone Supervolcano. The caldera is located in the northwest corner of Wyoming, in which the vast majority of the park is contained. The major features of...
and Lake Toba
Lake Toba
Lake Toba is a lake and supervolcano. The lake is 100 kilometres long and 30 kilometres wide, and 505 metres at its deepest point. Located in the middle of the northern part of the Indonesian island of Sumatra with a surface elevation of about , the lake stretches from to...
eruptions.
Successions of ignimbites mades up most of the post-erosional rocks in Gran Canaria
Gran Canaria
Gran Canaria is the second most populous island of the Canary Islands, with a population of 838,397 which constitutes approximately 40% of the population of the archipelago...
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.