TUFF, a rock consisting of volcanic ash, the ejectamenta of craters in a state of eruption (Ital. tufo). The products of a volcanic eruption may be classified into (a) steam and other gases, (b) lavas, (c) ashes. The ashes have not been burnt in any way, though they resemble cinders in appearance; they are merely porous, slaggy pieces of lava which have been tossed into the air 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, 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 20 ft. or more in diameter to the minutest impalpable dust. The large masses are called "bombs"; they have mostly a rounded, elliptical or pear-shaped form, owing to rotation in the air while still viscous. Many of them have ribbed or nodular surfaces, and sometimes (at Vulcano and Mont Pelee) a crust intersected by many cracks like the sur face of a loaf of bread. Any ash in which they are very abundant is called an agglomerate (q.v.).
In those layers and beds of tuff which have been spread over considerable tracts of country and which are most frequently encountered among the sedimentary rocks, smaller fragments pre ponderate 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 used for building purposes (e.g., in the neighbourhood of Rome). If they have accumulated sub-aerially, 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 vol canoes 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 admix tures of clay, sand and sometimes fossil shells, which prove that there were beds spread out under water.
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 been at such a tempera ture that solidification has commenced, crystals are usually pres ent. 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; blocks of crystalline minerals (an orthite, 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 scoriae, which, after consolidation, weather to a red soft rock known as palagonite; tuffs of this kind occur in Iceland and Sicily.