Tuff, Volcanic

a dense volcanic rock formed by the cementation of ash, scoria, lapilli, and other éjecta from an erupting volcano. Tuffs consist of fragments of volcanic glass, pumice, and other substances; tuffs composed of whole crystals or crystal fragments are also encountered.

Tuffs are classified according to composition, nature, and fragment sizes. In terms of composition, we distinguish among rhyo-litic, dacitic, andesitic, basaltic, and other tuffs. The color of tuffs varies; Artik tuff is violet-pink, and Ani tuff is orange or yellow (both types are found in the Armenian SSR). Tuffs are formed either by direct deposition from the air during a volcanic eruption or as a result of the transport of tuffaceous material by water or wind.

Tuff is a valuable building material of excellent ornamental quality. It is used, for example, as a filler in light concrete or as a wall material. In the USSR the main tuff deposits are in Armenia. In other countries the most important deposits are near Rome and Naples in Italy and in Iceland.