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Tuff (from the Italian "tufo" and pronounced "toof") is a rock type consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. The products of a volcanic eruption are (a) volcanic gases, (b) magmass, and (c) steam from water heated by the magma. Where all three interact violently, especially where there is an abundance of water, the magma is blown apart. If the resulting pieces of ejecta are small enough, the material is called volcanic ash, consisting of sand-sized or smaller particules. These particles are small, slaggy piece of lava which have been tossed into the air by outbursts of steam and other gases and have become vesicular by the expansion of the gases within them while they were still plastic.

Among the loose beds of ash which cover the slopes of many volcanoes, three classes of materials are represented. In addition to true ashes (a) of the kind above described, there are lumps of the old lavas and tuffs (b) forming the walls of the crater, etc., and which have been torn away by the violent outbursts of steam, pieces of sedimentary rocks (c) from the deeper parts of the volcano, which were dislodged by the rising lava, and are often intensely baked and 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 Bandaisan in Japan in 1888. There have been many eruptions also at which the quantity of broken sedimentary rocks mingled with the ashes is very great; as instances we may cite the volcanoes of the Eifel and the Devonian tuffs, known as "Schalsteins," in Germany. In the 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 bombs"; 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 (at Volcano and Mont Pelé) 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 which 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 neighbourhood of Rome). If they have accumulated subaerially, like the ash beds found on 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 muds or sands or even in some cases ashy limestones 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 shellss, 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 stratified character. The coarsest materials or agglomerates show this least distinctly; in the fine beds it is often developed in great perfection.

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 crystalline igneous rocks. If the lava within the crater has beert 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 which 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 Indian 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 basaltic 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 (rhyolitic) tuffs, of pale grey or yellow colour, 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."

For petrographical purposes tuffs are generally classified according to the nature of the volcanic rock of which they consist; this is the same as the accompanying lavas if any of these 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 felspar, biotite, etc. In Iceland, Lipari, Hungary, Nevada, New Zealand, recent tuffs of this kind occur. The broken pumice is clear and isotropic, and when the particles are very small they have often crescentic, sickle-shaped, or biconcave outlines, showing that they are produced by the shattering of a vesicular glass; this is sometimes described as ash-structure. In the ancient rocks of Wales, 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 contain little or no quartz but much orthoclase and oligoclase felspar with often 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 are exceedingly common. They occur along the whole chain of the 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 Britain, ancient rocks of exactly similar nature are abundant. In colour 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, 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 are also of wide spread occurrence both in districts where volcanoes are now active and in lands where eruptions have long since ended. In the British Isles they are found in Skye, Mull, Antrim and other places, where there are Tertiary volcanic rocks; in Scotland, Derbyshire, 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 Faeroes, Jan Mayen, Sicily, Vesuvius, 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 zeolites, such as analcite, prehnite, natrolite, scolecite, chabazite, heulandite, etc. Ultra-basic tuffs are by no means frequent; their characteristic is the abundance of olivine or serpentine and the scarcity or absence of felspar. In this class the peridotite, breccias or kimberlites of the diamond-fields of South Africa may perhaps be placed. The principal rock is a dark bluish green serpentine (blue-ground) which when thoroughly oxidized and weathered becomes a friable brown or yellow mass (the "yellow-ground"). Besides olivine and augite (chrome diopside) there occur crystals of hypersthene, brown mica, garnet (Cape ruby), magnetite, ilmenite and kyanite, together with crystalline blocks of garnet, augite and olivine (which some petrographers have called eclogites). Many lumps of shale are embedded in the breccia, and some have supposed that the diamonds are due to the ultra-basic magma dissolving carbon, which subsequently crystallized as the rock cooled down. Many of the crystals are broken, and as the rock fragments also are angular, rather than rounded, the kimberlite is more properly an ultra-basic breccia than a tuff.

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 slates of the lake district in Cumberland are fine cleaved ashes. In Charnwood forest also the tuffs are slaty and cleaved. The green colour is due to the large develooment of chlorite. Among the crystalline schists of many regions green beds or green schists occur, which consist of quartz, hornblende, chlorite or biotite, iron oxides, felspar, etc., and are probably recrystallized or metamorphosed tuffs. They often accompany masses of epidiorite and hornblende-schists which are the corresponding lavas and sills. 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.

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. Puzzuolana 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 a trachytic, pumiceous tuff called trass has been extensively worked as a hydraulic mortar.

See also: List of minerals