Tuff
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{{Unreferenced|date=August 2007}}
{{distinguish2|[[Travertine|calcareous tuff]]}}
[[Image:Welded tuff at Golden Gate in Yellowstone-750px.JPG|thumb|right|250px|Welded tuff at Golden Gate in [[Yellowstone National Park]].]]
[[image:Servian Wall-Termini Station.jpg|thumb|250px|Remains of the ancient [[Servian Walls]] in Rome, made of tuff bricks.]]
'''Tuff''' (from the [[Italian language|Italian]] "tufo") is a type of [[Rock (geology)|rock]] consisting of consolidated volcanic [[ash (volcanic)|ash]] ejected from vents during a volcanic eruption. ''Tuff'' should not be confused with ''[[tufa]]'', another type of rock. <!-- follow that link to see what tufa is; recommend we don't confuse the matter here with too much non-tuff detail. -->
==Volcanic ash==
The products of a [[volcano|volcanic eruption]] are volcanic gases, [[lava]], [[steam]], and [[tephra]]. <!-- magma is the input, lava is the product, no? --> Magma is blown apart when it interacts violently with volcanic gases and steam. Solid material produced and thrown into the air by such volcanic eruptions is called [[tephra]], regardless of composition or fragment size. If the resulting pieces of ejecta are small enough, the material is called volcanic ash, defined as such particles less than 2 mm in diameter, [[sand|sand-sized]] or smaller. These particles are small, [[slag]]gy pieces of [[magma]] and rock that have been tossed into the air by outbursts of steam and other gases; magma may have been torn apart as it became [[Vesicular texture|vesicular]] by the expansion of the gases within it.
==Breccias==
Among the loose beds of ash that cover the slopes of many [[volcano]]es, three classes of materials are represented. In addition to true ashes of the kind described above, there are lumps of the old lavas and tuffs forming the walls of the crater, etc., which have been torn away by the violent outbursts of steam, and pieces of [[sedimentary rock]]s from the deeper parts of the volcano that were dislodged by the rising lava and are often intensely baked and [[Recrystallization|recrystallized]] by the heat to which they have been subjected.
In some great volcanic explosions nothing but materials of the second kind were emitted, as at [[Mount Bandaisan]] in [[Japan]] in [[1888]]. There have been many eruptions also in which the quantity of broken sedimentary rocks that mingled with the ash is very great; as instances we may cite the volcanoes of the [[Eifel]] and the [[Devonian period|Devonian]] tuffs, known as "Schalsteins," in [[Germany]]. In the [[Scotland|Scottish]] coalfields some old volcanoes are plugged with masses consisting entirely of sedimentary debris: in such a case we must suppose that no lava was ejected, but the cause of the eruption was the sudden liberation and expansion of a large quantity of steam. These accessory or adventitious materials, however, as distinguished from the true ashes, tend to occur in angular fragments; and when they form a large part of the mass the rock is more properly a "volcanic [[breccia]]" than a tuff. The ashes vary in size from large blocks twenty feet or more in diameter to the minutest impalpable dust. The large masses are called "[[volcanic bomb]]s"; they have mostly a rounded, elliptical or pear-shaped form owing to rotation in the air while they were still viscous. Many of them have ribbed or nodular surfaces, and sometimes they have a crust intersected by many cracks like the surface of a loaf of bread. Any ash in which they are very abundant is called an [[agglomerate]].
In those layers and beds of tuff that have been spread out over considerable tracts of country and which are most frequently encountered among the sedimentary rocks, smaller fragments preponderate greatly and bombs more than a few inches in diameter may be absent altogether. A tuff of recent origin is generally loose and incoherent, but the older tuffs have been, in most cases, cemented together by pressure and the action of infiltrating water, making rocks which, while not very hard, are strong enough to be extensively used for building purposes (e.g. in the neighborhood of [[Rome]]). If they have accumulated [[subaerial]]ly, like the ash beds found on [[Mt. Etna]] or [[Vesuvius]] at the present day, tuffs consist almost wholly of volcanic materials of different degrees of fineness with pieces of wood and vegetable matter, land shells, etc. But many volcanoes stand near the sea, and the ashes cast out by them are mingled with the sediments that are gathering at the bottom of the waters. In this way ashy [[mud]]s or [[sand]]s or even in some cases ashy [[limestone]]s are being formed. As a matter of fact most of the tuffs found in the older formations contain admixtures of [[clay]], sand, and sometimes [[fossil]] [[animal shell|shell]]s, which prove that they were beds spread out under water.
During some volcanic eruptions a layer of ashes several feet in thickness is deposited over a considerable district, but such beds thin out rapidly as the distance from the crater increases, and ash deposits covering many square miles are usually very thin. The showers of ashes often follow one another after longer or shorter intervals, and hence thick masses of tuff, whether of subaerial or of marine origin, have mostly a [[Stratigraphy|stratified]] character. The coarsest materials or agglomerates show this least distinctly; in the fine beds it is often developed in great perfection.
==Igneous rock==
Apart from adventitious material, such as fragments of the older rocks, pieces of trees, etc., the contents of an ash deposit may be described as consisting of more or less [[crystal]]line [[igneous rock]]s. If the lava within the crater has been at such a temperature that solidification has commenced, crystals are usually present. They may be of considerable size like the grey, rounded [[leucite]] crystals found on the sides of Vesuvius. Many of these are very perfect and rich in faces because they grew in a medium that was liquid and not very viscous. Good crystals of [[augite]] and [[olivine]] are also to be obtained in the ash beds of Vesuvius and of many other volcanoes, ancient and modern. Blocks of these crystalline minerals ([[anorthite]], olivine, augite and [[hornblende]]) are common objects in the tuffs of many of the West [[India]]n volcanoes. Where crystals are very abundant the ashes are called "crystal tuffs." In St. Vincent and Martinique in [[1902]], much of the dust was composed of minute crystals enclosed in thin films of [[glass]] because the lava at the moment of eruption had very nearly solidified as a crystalline mass. Some [[basalt]]ic volcanoes, on the other hand, have ejected great quantities of black glassy [[scoria]], which, after consolidation, weather to a red soft rock known as [[palagonite]]; tuffs of this kind occur in [[Iceland]] and [[Sicily]]. In the [[Lipari Islands]] and [[Hungary]] there are [[acid]] ([[rhyolite|rhyolitic]]) tuffs, of pale grey or yellow color, largely composed of lumps and fragments of [[pumice]]. Over a large portion of the sea bottom the beds of fine mud contain small, water-worn, rounded pebbles of very spongy volcanic glass; these have been floated from the shore or cast out by submarine volcanoes, and may have travelled for hundreds of miles before sinking; it has been proved by experiment that some kinds of pumice will float on sea-water for more than a year. The deep sea-deposit known as the "red clay" is largely of volcanic origin and might be suitably described as a "submarine tuff-bed."
==Welded tuff==<!-- This section is linked from [[Yellowstone National Park]] -->
Welded tuff is a [[pyroclastic rock]], of any origin, that was sufficiently hot at the time of deposition to weld together. Strictly speaking, if the rock contains scattered pea-sized fragments or fiamme in it, it is called a welded lapilli-tuff. Welded tuffs (and welded lapilli-tuffs) can be of fallout origin, or deposited from pyroclastic density currents, as in the case of ignimbrites. During welding, the glass shards and pumice fragments adhere together (necking at point contacts), deform, and compact together, resulting in a 'eutaxitic fabric' (see image and contrast with the ash shapes in unwelded tuff).
Welded ignimbrites can be highly voluminous, such as the [[Lava Creek Tuff]] erupted from [[Yellowstone Caldera]] in [[Wyoming]] 640,000 years ago. [[Lava Creek Tuff]] is known to be at least 1000 times as large as the deposits of the [[May 18]], [[1980]] eruption of [[Mount St. Helens]], and it had a [[Volcanic Explosivity Index]] (VEI) of 8 -- greater than any eruption known in the last 10,000 years. The intensity of welding may decrease towards the upper margin of a deposit, may decrease towards areas in which the deposit is thinner, and with distance from source. Welded tuff is commonly rhyolitic in composition, but examples of all compositions are known.
==Rhyolite tuffs==
For [[Petrology|petrographical]] purposes tuffs are generally classified according to the nature of the volcanic rock of which they consist; this may be the same as the accompanying lavas if any were emitted during an eruption, and if there is a change in the kind of lava which is poured out, the tuffs also indicate this equally clearly. Rhyolite tuffs contain pumiceous, glassy fragments and small scoriae with [[quartz]], [[alkali]] [[feldspar]], [[biotite]], etc. Iceland, Lipari, Hungary, the [[Basin and Range]] of the American southwest, and [[New Zealand]] are among the areas where such tuffs are prominent. The broken pumice is clear and isotropic, and very small particles commonly have crescentic, sickle-shaped, or biconcave outlines, showing that they are produced by the shattering of a vesicular glass, sometimes described as ash-structure. The tiny glass fragments derived from broken pumice are called shards; the glass shards readily deform and flow when the deposits are sufficiently hot, as shown in the accompanying image of welded tuff. [[Image:Tuff shards.jpg|thumb|right|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.]]
In the ancient rocks of [[Wales]], [[Charnwood Forest|Charnwood]], the [[Pentland Hills]], etc., similar tuffs are known, but in all cases they are greatly changed by silicification (which has filled them with [[opal]], [[chalcedony]] and quartz) and by devitrification. The frequent presence of rounded corroded quartz crystals, such as occur in rhyolitic lavas, helps to demonstrate their real nature.
==Trachyte tuffs==
Trachyte tuffs contain little or no quartz but much [[sanidine]] or [[anorthoclase]] and sometimes oligoclase feldspar, with occasional biotite, augite and hornblende. In weathering they often change to soft red or yellow clay-stones, rich in [[kaolin]] with secondary quartz. Recent trachyte tuffs are found on the [[Rhine]] (at [[Siebengebirge]]), in [[Ischia]], near [[Naples]], [[Hungary]], etc.
==Andesitic tuffs==
Andesitic tuffs are exceedingly common. They occur along the whole chain of the [[American cordillera|Cordilleras]] and [[Andes]], in the [[West Indies]], New Zealand, Japan, etc. In the Lake district, North Wales, Lorne, the Pentland Hills, the Cheviots and many other districts of [[Great 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 up with many secondary minerals, such as [[calcite]], [[Chlorite group|chlorite]], quartz, [[epidote]], chalcedony: but in microscopic sections 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 of [[Cotopaxi]], [[Krakatoa]] and Mont Pelé.
==Basaltic tuffs==
Basaltic tuffs are also of wide spread occurrence both in districts where volcanoes are now active and in lands where eruptions have long since ended. They are found in [[Isle of Skye|Skye]], [[Isle of Mull|Mull]], [[County Antrim|Antrim]] and other places, where there are [[tertiary]] volcanic rocks; in [[Scotland]], [[Derbyshire]] and [[Ireland]] among the [[carboniferous]] strata; and among the still older rocks of the [[Lake District]], 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 in Iceland, the [[Faroe Islands]], [[Jan Mayen]], Sicily, [[Sandwich Islands]], [[Samoa]], etc. When weathered they are filled with calcite, chlorite, [[serpentine]] and, especially where the lavas contain [[nepheline]] or [[leucite]], are often rich in [[zeolite]]s, such as [[analcite]], [[prehnite]], [[natrolite]], [[scolecite]], [[chabazite]], [[heulandite]], etc.
==Ultramafic tuffs==
Ultramafic tuffs are extremely rare; their characteristic is the abundance of [[olivine]] or [[serpentine]] and the scarcity or absence of [[feldspar]] and [[quartz]]. Rare occurrences may include unusual surface deposits of [[maar]]s of [[kimberlite]]s of the [[diamond]]-fields of southern Africa and other regions. The principal rock 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"). These breccias were emplaced as gas-solid mixtures and are typically preserved and mined in [[diatreme]]s 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.
==Folding and metamorphism==
In course of time other changes than weathering may overtake tuff deposits. Sometimes they are involved in folding and become sheared and cleaved. Many of the green [[slate]]s of the lake district in [[Cumberland, England|Cumberland]] are fine cleaved ashes. In [[Charnwood Forest]] also the tuffs are slaty and cleaved. The green color is due to the large development of chlorite. Among the crystalline [[schist]]s of many regions green beds or green schists occur, which consist of quartz, hornblende, chlorite or biotite, [[iron oxide]]s, feldspar, etc., and are probably recrystallized or [[Metamorphism|metamorphosed]] tuffs. They often accompany masses of epidiorite and hornblende-schists which are the corresponding lavas and [[Sill (geology)|sill]]s. Some chlorite-schists also are probably altered beds of volcanic tuff. The "Schalsteins" of [[Devon]] 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.
[[image:Easter-Island.jpg|right|thumb|[[Ahu Tongariki]] with 15 [[Moai]] made of tuff from [[Rano Raraku]]. The second [[Moai]] from the right has a [[Pukao]] which is made of red [[Scoria]].]]
== Practical uses ==
In the ancient world, tuff's relative softness meant that it was commonly used for construction where it was available. Tufa is common in [[Italy]], and the [[Ancient Rome |Romans]] used it for many buildings and bridges. For example, the whole port of the island of [[Ventotene]] (still in use), was carved out from tuff. The [[Servian Wall]], built to defend the city of [[Rome]] in the [[4th century BC]], is also built almost entirely from tuff. The Romans also cut tuff into small rectangular stones that they used to create walls in a pattern known as [[opus reticulatum]].
The Romans thought bees nested in tuff. The substance is mentioned in the ''[[Aeneid]]'' (Book XII, ln 805).
==Economic importance==
[[Yucca Mountain]] Repository, a U.S. Department of Energy terminal storage facility for spent nuclear reactor and other radioactive waste, is in tuff and [[ignimbrite]] in the [[Basin and Range Province]] in [[Nevada]]. In [[Napa valley]] and Sonoma valley, [[California]], areas made out of tuff are routinely excavated for storage of wine barrels.
Tuff from [[Rano Raraku]] was used by the [[Rapa Nui]] people of [[Easter Island]] to make the vast majority of their famous [[Moai]] (statues).
Otherwise, tuffs are not of much importance in an economic sense. The [[peperino]], much used at Rome and Naples as a building stone, is a [[trachyte]] tuff. [[Pozzolana]] also is a decomposed tuff, but of basic character, originally obtained near [[Naples]] and used as a [[cement]], but this name is now applied to a number of substances not always of identical character. In the [[Eifel]] region of Germany a trachytic, pumiceous tuff called [[trass]] has been extensively worked as a hydraulic [[mortar (masonry)|mortar]].
== See also ==
* [[List of minerals]]
* [[Scoria]]
* [[Lava]]
* [[Sillar]]
* [[Lapilli]]
* [[Pyroclastic flow]]
* [[Bishop Tuff]]
== References==
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