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TUFF (Ital. tufo)
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See also:TUFF (Ital. tufo) , a See also:rock consisting of volcanic ashes, the ejectamenta of craters in a See also:state of eruption. The products of a volcanic eruption may be classified into three See also:groups: (a) See also:steam and other gases, (b) lavas, (c) ashes. The ashes have not been burnt in any way though they resemble cinders in See also:appearance: they are merely porous, slaggy pieces of See also:lava which have been tossed into the See also:air by outbursts of steam and have become vesicular by the expansion of the gases within them while they were still plastic. Among the loose beds of ash which See also:cover the slopes of many volcanoes, three classes of materials are represented. In addition to true ashes (a) of -the See also:kind above described, there are lumps of the old lavas and tuffs (b) forming the walls of the See also:crater, &c., and which have been torn away by the violent outbursts of steam, pieces of sedimentary rocks (c) from the deeper parts of the See also:volcano, which were dislodged by the rising lava, and are often intensely baked and recrystallized by the See also:heat to which theyhave been subjected. In some See also:great volcanic explosions nothing but materials of the second kind were emitted, as at Bandaisan in See also: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 See also:Eifel and the Devonian tuffs, known as " Schalsteins," in See also:Germany. In the Scotch coalfields some old volcanoes are plugged with masses consisting entirely of sedimentary debris: in such a See also: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 See also:accessory or See also:adventitious materials, however, as distinguished from the true ashes, tend to occur in angular fragments; and when they See also:form a large See also:part of the See also:mass the rock is more properly a " volcanic See also:breccia " than a tuff. The ashes vary in See also:size from large blocks twenty feet or more in See also:diameter to the minutest impalpable dust. The large masses are called " bombs "; they have mostly a rounded, elliptical or See also: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 Pele) they have a crust intersected by many cracks like the See also:surface of a See also:loaf of See also:bread. Any ash in which they are very abundant is called an See also:agglomerate (q.v.). In those layers and beds of tuff which have been spread out over considerable tracts of See also: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 See also:recent origin is generally loose and incoherent, but the older tuffs have been, in most cases, cemented together by pressure and the See also:action of infiltrating See also:water, making rocks which, while not very hard, are strong enough to be extensively used for See also:building purposes (e.g. in the neighbourhood of See also:Rome). If they have accumulated sub-aerially, like the ash beds found on See also:Etna or See also:Vesuvius at the See also:present See also:day, tuffs consist almost wholly of volcanic materials of different degrees of fineness with pieces of See also:wood and See also:vegetable See also:matter, See also:land shells, &c. But many volcanoes stand near the See also:sea, and the ashes See also:cast out by them are mingled with the sediments that are gathering at the bottom of the See also: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 See also:clay, See also:sand, and sometimes fossil shells, 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 See also:district, but such beds thin out rapidly as the distance from the crater increases, and ash deposits covering many square See also: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 See also:character. The coarsest materials or agglomerates show this least distinctly; in the See also:fine beds it is often See also:developed in great perfection. Apart from adventitious material, such as fragments of the older rocks, pieces of trees, &c., the contents of an ash See also:deposit may be described as consisting of more or less crystalline igneous rocks. 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 See also:grey, rounded See also:leucite crystals found on the sides of Vesuvius. Many of these are very perfect and See also:rich in faces, because they See also:grew in a See also:medium which was liquid and not very viscous. See also:Good crystals of See also:augite and See also:olivine are also to be obtained in the ash beds of Vesuvius and of many other volcanoes, See also:ancient and See also:modern. Blocks of these crystalline minerals (See also:anorthite, olivine, augite and See also:hornblende) are See also:common See also:objects in the tuffs of many of the See also:West See also:Indian volcanoes. Where crystals are very abundant the ashes are called " crystal tuffs." In St See also:Vincent and See also:Martinique in 1902 much of the dust was composed of See also:minute crystals enclosed in thin films of See also:glass, because the lava at the moment of eruption had very nearly solidified as a crystalline mass. Some basaltic volcanoes, on the other See also:hand, have ejected great quantities of See also:black glassy See also:scoria which, after consolidation, See also:weather to a red soft rock known as palagonite; tuffs of this kind occur in See also:Iceland and See also:Sicily. In the Lipari Islands and See also:Hungary there are See also:acid (rhyolitic) tuffs, of See also:pale grey or yellow See also:colour, largely composed of lumps and fragments of See also: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 See also: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 See also:float on sea-water for more than a See also:year. The deep sea-deposit known as the " red clay " is largely of volcanic origin and might be suitably described as a " submarine tuff-See also: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 See also:change in the kind of lava which is poured out, the tuffs also indicate this equally clearly. See also:Rhyolite tuffs contain pumiceous, glassy fragments and small scoriae with See also:quartz, See also:alkali See also:felspar, See also:biotite, &c. In Iceland, Lipari, Hungary, See also:Nevada, New See also: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 See also:Wales, Charnwood, the Pentland Hills, &c., similar tuffs are known, but in all cases they are greatly changed by silicification (which has filled them with See also:opal, See also:chalcedony and quartz) and by devitrification. The frequent presence of rounded corroded quartz crystals, such as occur in rhyolitic lavas, See also:helps to demonstrate their real nature. See also:Trachyte tuffs contain little or no quartz but much See also:orthoclase and See also:oligoclase felspar with often biotite, augite and hornblende. In weathering they often change to soft red or yellow " clay-stones, " rich in See also:kaolin with secondary quartz. Recent trachyte tuffs are found on the See also:Rhine (at See also:Siebengebirge), in See also:Ischia, near See also:Naples, Hungary, &c. Andesitic tuffs are exceedingly common. They occur along the whole See also:chain of the Cordilleras and See also:Andes, in the West Indies, New Zealand, Japan, &c. In the See also:Lake district, See also:North Wales, Lorne, the Pentland Hills, the Cheviots and many other districts of See also:Britain, ancient rocks of exactly similar nature are abundant. In colour they are red or See also: In the See also:British Isles they are found in See also:Skye, See also:Mull, See also:Antrim and other places, where there are See also:Tertiary volcanic rocks; in See also:Scotland, See also:Derby-See also:shire, See also:Ireland among the carboniferous strata; and among the still older rocks of the lake district, See also:southern uplands of Scotland and Wales. They are black, dark See also:green or red in colour; vary greatly in coarseness, some being full of See also:round spongy bombs a See also:foot or more in diameter, and, being often submarine, may contain shale, sand-See also: Many of the green slates of the lake district in See also:Cumberland are fine cleaved ashes. In Charnwood See also:forest also the tuffs are slaty and cleaved. The green colour is due to the large development of chlorite. Among the crystalline See also:schists of manyregions green beds or green schists occur, which consist of quartz, hornblende, chlorite or biotite, See also:iron oxides, felspar, &c., and are probably recrystallized or metamorphosed tuffs. They often accompany masses of See also: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 See also:Devon and Germany include many cleaved and partly recrystallized ash-beds, some of which still retain their fragmental structure though their See also:lapilli are flattened and See also: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 See also: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 See also: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 See also:trass (q.v.) has been extensively worked as a See also:hydraulic See also:mortar. (J. S. Additional information and CommentsThere are no comments yet for this article.
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