Ignimbrite

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Rocks from the Bishop tuff from California, United States, uncompressed with pumice on left; compressed with fiamme on right BishopTuff.jpg
Rocks from the Bishop tuff from California, United States, uncompressed with pumice on left; compressed with fiamme on right
The caprock in this photo is the ignimbrite layer of the Rattlesnake Formation in Oregon. Rattlesnake Formation near Picture Gorge.jpg
The caprock in this photo is the ignimbrite layer of the Rattlesnake Formation in Oregon.

Ignimbrite is a variety of hardened tuff. [1] Ignimbrites are igneous rocks made up of crystal and rock fragments in a glass-shard groundmass, albeit the original texture of the groundmass might be obliterated due to high degrees of welding. The term ignimbrite is not recommended by the IUGS Subcommission on the Systematics of Igneous Rocks. [1]

Contents

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 and driven by being denser than the surrounding atmosphere. New Zealand geologist Patrick Marshall (1869-1950) derived the term ignimbrite from "fiery rock dust cloud" (from the Latin igni- (fire) and imbri- (rain)). Ignimbrites form 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. Near 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.

En masse model

The en masse model was proposed by volcanologist Stephen 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.

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 turbulent. 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. [2]

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. [2]

Rheomorphic flow model

Rheomorphic flow structures in a welded ignimbrite, Isle of Lipari, Italy Rheo flow.JPG
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 in order to say which process is the most common.

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 pyroclasts 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. [3] 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. [4]

Another model proposed is that the density current became stationary before the rheomorphic structures form. [5] 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 at that time the Green Tuff was believed to be a fall deposit which has no lateral transport. [6] 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. [7] Similar structures observed on Gran Canaria had been interpreted as syn-depositional flow. [6]

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. [8] It is 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. [9] Load compaction on an inclined slope is likely responsible for the majority of the rheomorphic structures.

Petrology

A block of ignimbrite Ignimbrite.jpg
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). Tuff welded.jpg
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

Rock sample of ignimbrite, collected at the foot of Mount Guna in Ethiopia Rock sample 8 Ignimbrite Guna Mika'el.jpg
Rock sample of ignimbrite, collected at the foot of Mount Guna in Ethiopia

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. [4]

The factor that determines whether an ignimbrite has primary welding, secondary welding or no welding is debated:

Morphology and occurrence

Landscapes formed by erosion in hardned ignimbrite can be remarkably similar to those formed on granitic rocks. In Sierra de Lihuel Calel, La Pampa Province, Argentina, various landforms typical of granites can be observed in ignimbrite. These landforms are inselbergs, flared slopes, domes, nubbins, tors, tafonis and gnammas. [12] In addition, just like in granite landscapes landforms in ignimbrites may be influenced by joint systems. [12]

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 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 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 before the last major Taupo 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.

Successions of ignimbrites make up a large part of post-erosional rocks in Tenerife and Gran Canaria islands.

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 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.

See also

Related Research Articles

Tuff Rock consolidated from volcanic ash

Tuff, also known as volcanic tuff, is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is compacted into a solid rock in a process called consolidation. Tuff is sometimes erroneously called "tufa", particularly when used as construction material, but geologically tufa is a limestone precipitated from groundwater. Rock that contains greater than 50% tuff is considered tuffaceous.

Volcanic cone Landform of ejecta from a volcanic vent piled up in a conical shape

Volcanic cones are among the simplest volcanic landforms. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature and size of the fragments ejected during the eruption. Types of volcanic cones include stratocones, spatter cones, tuff cones, and cinder cones.

Volcanic rock Volcanic rocks composing or associated with volcanoes, volcanic activity or volcanism

Volcanic rock is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "volcaniclastics," and these are technically sedimentary rocks.

Pyroclastic rock Clastic rocks composed solely or primarily of volcanic materials

Pyroclastic rocks or pyroclastics are sedimentary 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. Commonly associated with unsieved volcanic activity—such as Plinian or krakatoan eruption styles, or phreatomagmatic eruptions—pyroclastic deposits are commonly formed from airborne ash, lapilli and bombs or blocks ejected from the volcano itself, mixed in with shattered country rock. Tephra is any sized material formed by a volcanic eruption.

A pyroclastic surge, also referred as a dilute pyroclastic density current, is a flowing mixture of gas and rock fragments ejected during some volcanic eruptions. Pyroclastic surge refers to a specific type of pyroclastic current which moves on the ground as a turbulent flow with low particle concentration with support mainly from the gas phase. Pyroclastic surges are thus more mobile and less confined compared to dense pyroclastic flows, which allows them to override ridges and hills rather than always travel downhill.

Lapilli Small pyroclast debris thrown in the air by a volcanic eruption

Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. Lapilli is Latin for "little stones".

Agglomerate Coarse accumulation of large blocks of volcanic material that contains at least 75% bombs

Agglomerate is a coarse accumulation of large blocks of volcanic material that contains at least 75% bombs. Volcanic bombs differ from volcanic blocks in that their shape records fluidal surfaces: they may, for example, have ropy, cauliform, scoriaceous, or folded, chilled margins and spindle, spatter, ribbon, ragged, or amoeboid shapes. Globular masses of lava may have been shot from the crater at a time when partly molten lava was exposed, and was frequently shattered by sudden outbursts of steam. These bombs were viscous at the moment of ejection and by rotation in the air acquired their shape. They are commonly 1 to 2 feet in diameter, but specimens as large as 12 feet (3.7 m) have been observed. There is less variety in their composition at any one volcanic centre than in the case of the lithic blocks, and their composition indicates the type of magma being erupted.

La Garita Caldera supervolcanic caldera

La Garita Caldera is a large supervolcanic caldera in the San Juan volcanic field in the San Juan Mountains near the town of Creede in southwestern Colorado, United States. It is west of La Garita, Colorado. The eruption that created the La Garita Caldera is among the largest known volcanic eruptions in Earth's history, as well as being one of the most powerful known supervolcanic events.

Fiamme Small lens-shaped inclusions in volcaniclastic rocks

Fiamme are lens-shapes, usually millimetres to centimetres 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, describing their shape. The term is descriptive and non-genetic.

Lascar (volcano) volcano (stratovolcano)

Lascar is a stratovolcano within the Central Volcanic Zone of the Andes, a volcanic arc that spans the countries of Peru, Bolivia, Argentina and Chile. It is the most active volcano in the region, with records of eruptions going back to 1848. It is composed of two separate cones with several summit craters. The westernmost crater of the eastern cone is presently active. Volcanic activity is characterized by constant release of volcanic gas and occasional vulcanian eruptions.

Galán mountain in Argentina

Cerro Galán is a caldera in the Catamarca Province of Argentina. It is one of the largest exposed calderas in the world. It is part of the Central Volcanic Zone of the Andes, one out of several volcanic belts found in South America. It is one of several major caldera systems in the Central Volcanic Zone, some of which are grouped into the Altiplano–Puna volcanic complex.

Types of volcanic eruptions Basic mechanisms of eruption and variations

Several types of volcanic eruptions—during which lava, tephra, and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior has been observed. Some volcanoes may exhibit only one characteristic type of eruption during a period of activity, while others may display an entire sequence of types all in one eruptive series.

Mount Pleasant Caldera mountain in Canada

The Mount Pleasant Caldera is a large eroded Late Devonian volcanic caldera complex, located in the northern Appalachian Mountains of southwestern New Brunswick, Canada. It is one of few noticeable pre-Cenozoic calderas, and its formation is associated to a period of crustal thinning that followed the Acadian orogeny in the northern Appalachian Mountains. It sits relatively near to the coastline.

Phreatomagmatic eruption Volcanic eruption involving both steam and magma

Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions contain juvenile (magmatic) clasts. It is common for a large explosive eruption to have magmatic and phreatomagmatic components.

Taupo Volcano Caldera Volcano in New Zealand

Lake Taupo, in the centre of New Zealand’s North Island, is the caldera of a large rhyolitic supervolcano called the Taupo Volcano. This huge volcano has produced two of the world’s most violent eruptions in geologically recent times.

Volcanic ash volcanic material formed during explosive eruptions with the diameter of the grains less than 2 mm

Volcanic ash consists of fragments of rock, minerals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer to all explosive eruption products, including particles larger than 2 mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere. The force of the gasses shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash is also produced when magma comes into contact with water during phreatomagmatic eruptions, causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometres away.

El Toro volcanic field is part of the Central Volcanic Zone of the Andes in the northern Puna of Argentina. Three of the cones in the volcanic field are located southwest of the town of El Toro and the fourth is found north. Part of a field of monogenetic volcanoes associated with subduction of the Nazca plate beneath the South America plate, it is constructed from three main cones and an additional lava flow. The field formed between six and two million years ago.

Campanian Ignimbrite eruption

The Campanian Ignimbrite eruption was a major volcanic eruption in the Mediterranean during the late Quaternary, classified at 7 on the Volcanic Explosivity Index (VEI). The event has been attributed to the Archiflegreo volcano, the 13-kilometre-wide (8.1 mi) caldera of the Phlegraean Fields, located 20 km (12 mi) west of Mount Vesuvius under the western outskirts of the city of Naples and the Gulf of Pozzuoli, Italy. Estimates of the date, magnitude and the amount of ejected material have varied considerably during several centuries of investigation. This applies to most significant volcanic events that originated in the Campanian Plain, as it is one of the most complex volcanic structures in the world. However, continued research, advancing methods and accumulation of volcanological, geochronological, and geochemical data has amounted to ever more precise dating.

Kurile Lake lake in Russia

Kurile Lake is a caldera and crater lake in Kamchatka, Russia. It is also known as Kurilskoye Lake or Kuril Lake. It is part of the Eastern Volcanic Zone of Kamchatka which, together with the Sredinny Range, forms one of the volcanic belts of Kamchatka. These volcanoes form from the subduction of the Pacific Plate beneath the Okhotsk Plate and the Asian Plate.

Luingo is a caldera in the Andes of Argentina. It is located southeast of the Galan caldera. The caldera is not recognizable from satellite images and is associated with the Pucarilla-Cerro Tipillas volcanic complex.

References

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  11. Pérez, W.; Alvarado, G. E.; Gans, P. B. (2006). "The 322 ka Tiribí Tuff: stratigraphy, geochronology and mechanisms of deposition of the largest and most recent ignimbrite in the Valle Central, Costa Rica". Bulletin of Volcanology. 69 (1): 25–40. Bibcode:2006BVol...69...25P. doi:10.1007/s00445-006-0053-x.
  12. 1 2 Aguilera, Emilia Y.; Sato, Ana María; Llambías, Eduardo; Tickyj, Hugo (2014). "Erosion Surface and Granitic Morphology in the Sierra de Lihuel Calel, Province of La Pampa, Argentina". In Rabassa, Jorge; Ollier, Cliff (eds.). Gondwana Landscapes in southern South America. Springer. pp. 393–422.

Further reading