Tuff is a type ofrock made ofvolcanic ash ejected from avent during avolcanic eruption. Following ejection and deposition, the ashlithifies into solid rock.[1][2] Rock that contains greater than 75% ash is considered tuff, while rock containing 25% to 75% ash is described astuffaceous (for example,tuffaceous sandstone).[3] Apyroclastic rock containing 25–75%volcanic bombs orvolcanic blocks is calledtuff breccia.[4] Tuff composed of sandy volcanic material can be referred to asvolcanic sandstone.[5]
Tuff is a relatively soft rock, so it has been used for construction since ancient times.[6] Because it is common in Italy, the Romans used it often for construction.[7] TheRapa Nui people used it to make most of themoai statues onEaster Island.[8]
Lava, the name ofmagma when it emerges and flows over the surface
Tephra, particles of solid material of all shapes and sizes ejected and thrown through the air
Light-microscope image of tuff as seen in thin section (long dimension is several mm): The curved shapes of altered glass shards (ash fragments) are well preserved, although the glass is partly altered. The shapes were formed about bubbles of expanding, water-rich gas.
Tephra is made when magma inside the volcano is blown apart by the rapid expansion of hot volcanic gases. Magma commonly explodes as the gas dissolved in it comes out of solution as the pressure decreaseswhen it flows to the surface. These violent explosions produce particles of material that can then fly from the volcano. Solid particles smaller than 2 mm in diameter (sand-sized or smaller) are called volcanic ash.[9][3]
Volcanic ash is further divided into fine ash, with particle sizes smaller than 0.0625 mm in diameter, and coarse ash, with particle sizes between 0.0625 mm and 2 mm in diameter. Tuff is correspondingly divided into coarse tuff (coarse ash tuff) and fine tuff (fine ash tuff or dust tuff). Consolidated tephra composed mostly of coarser particles is called lapillistone (particles 2 mm to 64 mm in diameter) or agglomerate or pyroclasticbreccia (particles over 64 mm in diameter) rather than tuff.[3]
Volcanic ash can vary greatly in composition, and so tuffs are further classified by the composition of the ash from which they formed. Ash from high-silica volcanism, particularly in ash flows, consists mainly of shards ofvolcanic glass,[10][11] and tuff formed predominantly from glass shards is described as vitric tuff.[12] The glass shards are typically either irregular in shape or are roughly triangular with convex sides. They are the shattered walls of countless small bubbles that formed in the magma as dissolved gases rapidly came out of solution.[13]
Tuffs formed from ash consisting predominantly of individual crystals are described as crystal tuffs, while those formed from ash consisting predominantly of pulverized rock fragments are described as lithic tuffs.[12]
The chemical composition of volcanic ash reflects the entire range of volcanic rock chemistry, from high-silicarhyolitic ash to low-silicabasaltic ash, and tuffs are likewise described as rhyolitic, andesitic, basaltic, and so on.[14]
Transport and lithification
The most straightforward way for volcanic ash to move away from the vent is as ash clouds that are part of aneruption column. These fall to the surface asfallout deposits that are characteristicallywell-sorted and tend to form a blanket of uniform thickness across terrain.Column collapse results in a more spectacular and destructive form of transport, which takes the form ofpyroclastic flows andsurges that characteristically are poorly sorted and pool in low terrain. Surge deposits sometimes showsedimentary structures typical of high-velocity flow, such asdunes andantidunes.[15] Volcanic ash already deposited on the surface can be transported as mud flows (lahars) when mingled with water from rainfall or through eruption into a body of water or ice.[16]
Particles of volcanic ash that are sufficiently hot will weld together after settling to the surface, producing awelded tuff. Welding requires temperatures in excess of 600 °C (1,100 °F). If the rock contains scattered, pea-sized fragments orfiamme in it, it is called a weldedlapilli tuff. Welded tuffs (and welded lapilli tuffs) can be of fallout origin, or deposited from ash flows, as in the case ofignimbrites.[17] During welding, the glass shards and pumice fragments adhere together (necking at point contacts), deform, and compact together, resulting in aeutaxitic fabric.[18] Welded tuff is commonly rhyolitic in composition, but examples of all compositions are known.[19][20]
A sequence of ash flows may consist of multiplecooling units. These can be distinguished by the degree of welding. The base of a cooling unit is typically unwelded due to chilling from the underlying cold surface, and the degree of welding and of secondary reactions from fluids in the flow increases upwards towards the center of the flow. Welding decreases towards the top of the cooling unit, where the unit cools more rapidly. The intensity of welding may also decrease towards areas in which the deposit is thinner, and with distance from source.[21]
Cooler pyroclastic flows are unwelded and the ash sheets deposited by them are relatively unconsolidated.[18] However, cooled volcanic ash can quickly become lithified because it usually has a high content of volcanic glass. This is a thermodynamically unstable material that reacts rapidly with groundwater or seawater, which leachesalkali metals andcalcium from the glass. New minerals, such aszeolites,clays, andcalcite, crystallize from the dissolved substances and cement the tuff.[22]
Tuffs are further classified by their depositional environment, such as lacustrine tuff, subaerial tuff, or submarine tuff, or by the mechanism by which the ash was transported, such as fallout tuff or ash flow tuff. Reworked tuffs, formed by erosion and redeposition of ash deposits, are usually described by the transport agent, such as aeolian tuff or fluvial tuff.[23]
Bandelier Tuff at San Diego Canyon, New Mexico, US. The lower Otowi Member is a single massive cooling unit, while the upper Tshirege Member is composed of multiple cooling units.
Occurrences
Tuffs have the potential to be deposited wherever explosive volcanism takes place, and so have a wide distribution in location and age.[24]
High-silica volcanism
Rhyolite tuffs contain pumiceous, glassy fragments and small scoriae withquartz,alkalifeldspar,biotite, etc. Iceland,[25] Lipari,[26] Hungary,[27] theBasin and Range of the American southwest, andNew Zealand[28] are among the areas where such tuffs are prominent. In the ancient rocks ofWales,[29]Charnwood,[30] etc., similar tuffs are known, but in all cases, they are greatly changed by silicification (which has filled them withopal,chalcedony, and quartz) and by devitrification.[31] The frequent presence of rounded corroded quartz crystals, such as occur in rhyolitic lavas, helps to demonstrate their real nature.[9]
Welded ignimbrites can be highly voluminous, such as theLava Creek Tuff erupted fromYellowstone Caldera inWyoming 631,000 years ago. This tuff had an original volume of at least 1,000 cubic kilometers (240 cu mi).[32] Lava Creek tuff is known to be at least 1,000 times as large as the deposits of the1980 eruption of Mount St. Helens, and it had aVolcanic Explosivity Index (VEI) of 8, greater than any eruption known in the last 10,000 years.[33] Ash flow tuffs cover 7,000 square kilometers (2,700 sq mi) of theNorth Island ofNew Zealand and about 100,000 square kilometers (39,000 sq mi) ofNevada. Ash flow tuffs are the only volcanic product with volumes rivaling those offlood basalts.[28]
The Tioga Bentonite of the northeastern United States varies in composition from crystal tuff to tuffaceous shale. It was deposited as ash carried by wind that fell out over the sea and settled to the bottom. It isDevonian in age and likely came from a vent in centralVirginia, where the tuff reaches its maximum thickness of about 40 meters (130 ft).[34]
Alkaline volcanism
Trachyte tuffs contain little or no quartz, but muchsanidine oranorthoclase and sometimes oligoclase feldspar, with occasional biotite, augite, and hornblende. In weathering, they often change to soft red or yellowclaystones, rich inkaolin with secondary quartz.[9] Recent trachyte tuffs are found on theRhine (atSiebengebirge),[35] inIschia[36] and nearNaples.[37] Trachyte-carbonatite tuffs have been identified in theEast African Rift.[38] Alkaline crystal tuffs have been reported fromRio de Janeiro.[39]
Intermediate volcanism
Andesitic tuffs are exceedingly common. They occur along the whole chain of theCordilleras[40][41] andAndes,[42] in theWest Indies, New Zealand,[43] Japan,[44] etc. In theLake District,[45] North Wales,Lorne, thePentland Hills, theCheviots, and many other districts ofGreat Britain, ancient rocks of exactly similar nature are abundant. In color, they are red or brown; their scoriae fragments are of all sizes from huge blocks down to minute granular dust. The cavities are filled with many secondary minerals, such ascalcite,chlorite, quartz,epidote, or chalcedony; in microscopic sections, though, the nature of the original lava can nearly always be made out from the shapes and properties of the little crystals which occur in the decomposed glassy base. Even in the smallest details, these ancient tuffs have a complete resemblance to the modern ash beds ofCotopaxi,Krakatoa, and Mont Pelé.[9]
Mafic volcanism typically takes the form ofHawaiian eruptions that are nonexplosive and produce little ash.[46] However, interaction between basaltic magma and groundwater or seawater results in hydromagmatic explosions that produce abundant ash. These deposit ash cones that subsequently can become cemented into tuff cones.Diamond Head, Hawaii, is an example of a tuff cone, as is the island ofKa'ula. The glassy basaltic ash produced in such eruptions rapidly alters topalagonite as part of the process of lithification.[47]
Although conventional mafic volcanism produce little ash, such ash as is formed may accumulate locally as significant deposits. An example is the Pahala ash of Hawaii island, which locally is as thick as 15 meters (49 ft). These deposits also rapidly alter to palagonite, and eventually weather tolaterite.[48]
Basaltic tuffs are also found inCounty Antrim,Skye,Mull, and other places, wherePaleogene volcanic rocks are found; in Scotland,Derbyshire, and Ireland among theCarboniferous strata, and among the still older rocks of the Lake District, the southern uplands of Scotland, and Wales. They are black, dark green, or red in colour; vary greatly in coarseness, some being full of round spongy bombs a foot or more in diameter; and being often submarine, may contain shale, sandstone, grit, and other sedimentary material, and are occasionally fossiliferous. Recent basaltic tuffs are found inIceland, theFaroe Islands,Jan Mayen, Sicily, theHawaiian Islands,Samoa, etc. When weathered, they are filled with calcite, chlorite,serpentine, and especially where the lavas containnepheline orleucite, are often rich inzeolites, such asanalcite,prehnite,natrolite,scolecite,chabazite,heulandite, etc.[9]
Ultramafic volcanism
Ultramafic tuffs are extremely rare; their characteristic is the abundance ofolivine or serpentine and the scarcity or absence offeldspar andquartz.[49]
Kimberlites
Occurrences of ultramafic tuff include surface deposits ofkimberlite atmaars in thediamond-fields of southern Africa and other regions. The principal variety of kimberlite is a dark bluish-green, serpentine-rich breccia (blue-ground) which, when thoroughly oxidized and weathered, becomes a friable brown or yellow mass (the "yellow-ground").[9] These breccias were emplaced as gas–solid mixtures and are typically preserved and mined indiatremes that form intrusive pipe-like structures. At depth, some kimberlite breccias grade into root zones of dikes made of unfragmented rock. At the surface, ultramafic tuffs may occur in maar deposits. Because kimberlites are the most common igneous source of diamonds, the transitions from maar to diatreme to root-zone dikes have been studied in detail. Diatreme-facies kimberlite is more properly called an ultramafic breccia rather than a tuff.
In course of time, changes other than weathering may overtake tuff deposits. Sometimes, they are involved in folding and becomesheared andcleaved. Many of the greenslates of the EnglishLake District are finely cleaved ashes. InCharnwood Forest also, the tuffs are slaty and cleaved. The green color is due to the large development of chlorite. Among the crystallineschists of many regions, green beds or green schists occur, which consist of quartz, hornblende, chlorite or biotite,iron oxides, feldspar, etc., and are probably recrystallized ormetamorphosed tuffs. They often accompany masses of epidiorite and hornblende – schists which are the corresponding lavas andsills. Some chlorite-schists also are probably altered beds of volcanic tuff. The "Schalsteins" ofDevon and Germany include many cleaved and partly recrystallized ash-beds, some of which still retain their fragmental structure, though their lapilli are flattened and drawn out. Their steam cavities are usually filled with calcite, but sometimes with quartz. The more completely altered forms of these rocks are platy, green chloritic schists; in these, however, structures indicating their original volcanic nature only sparingly occur. These are intermediate stages between cleaved tuffs and crystalline schists.[9]
Importance
The primary economic value of tuff is as a building material. In the ancient world, tuff's relative softness meant that it was commonly used for construction where it was available.[6]
Italy
Tuff is common in Italy, and theRomans used it for many buildings and bridges.[7] For example, the whole port of the island ofVentotene (still in use), was carved from tuff. TheServian Wall, built to defend the city ofRome in the fourth century BC, is also built almost entirely from tuff.[52] The Romans also cut tuff into small, rectangular stones that they used to create walls in a pattern known asopus reticulatum.[53]
Peperino has been used in Rome and Naples as a building stone, is atrachyte tuff.Pozzolana also is a decomposed tuff, but of basic character, originally obtained nearNaples and used as acement, but this name is now applied to a number of substances not always of identical character. In the historical architecture of Naples, Neapolitan yellow tuff is the most used building material.[54]Pipernoignimbrite tuff was also used widely in Naples and Campania.
Germany
In theEifel region of Germany, a trachytic, pumiceous tuff calledtrass has been extensively worked as a hydraulicmortar.[9] Tuff of the Eifel region ofGermany has been widely used for construction of railroad stations and other buildings in Frankfurt, Hamburg, and other large cities.[55] Construction using theRochlitz Porphyr, can be seen in theMannerist-style sculpted portal outside the chapel entrance inColditz Castle.[56] The trade nameRochlitz Porphyr is the traditional designation for adimension stone ofSaxony with an architectural history over 1,000 years in Germany. The quarries are located near Rochlitz.[57]
Tuff from Rano Raraku was used by the Rapa Nui people of Easter Island to make the vast majority of their famousmoai statues.[8]
Ahu Tongariki on Easter Island, with 15moai made of tuff fromRano Raraku crater: The second moai from the right has aPukao ("topknot") which is made of redscoria.
Tuff is used extensively inArmenia andArmenian architecture.[60] It is the dominant type of stone used in construction in Armenia's capitalYerevan,[61][62]Gyumri, Armenia's second largest city, andAni, the country's medieval capital, now in Turkey.[63] A small village in Armenia was renamedTufashen (literally "built of tuff") in 1946.[64]
Pilar Formation outcrop showing metatuff beds used for radiometric dating
Tuffs are deposited geologically instantaneously and often over a large region. This makes them highly useful as time-stratigraphic markers. The use of tuffs and other tephra deposits in this manner is known as tephrochronology and is particularly useful forQuaternary chronostratigraphy. Individual tuff beds can be "fingerprinted" by their chemical composition and phenocryst assemblages.[66] Absolute ages for tuff beds can be determined byK-Ar,Ar-Ar, orcarbon-14 dating.[67]Zircon grains found in many tuffs are highly durable and can survive even metamorphism of the host tuff to schist, allowing absolute ages to be assigned to ancient metamorphic rocks. For example, dating of zircons in a metamorphosed tuff bed in thePilar Formation provided some of the first evidence for thePicuris orogeny.[68]
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![Cathedral of Ani, early 11th century, in the medieval Armenian capital of Ani (modern-day Turkey) was built in tuff.[65]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAni-Cathedral%2c_Ruine.jpeg)
