tuffs

Tuffs are rocks formed from the consolidation of volcanic ash and other ejecta that settle from eruption clouds during explosive volcanic activity. They are part of the broader category of pyroclastic rocks and are typically dominated by fine ash grains, though they may include lapilli and, less commonly, larger fragments. When coarse components are abundant, the rock is described as a lapilli tuff or tuff breccia; if the ash is cemented into a rock with few visible larger fragments, it is simply a tuff. Welded tuffs are a special case where hot ash deposits fuse during emplacement, producing dense, often glassy rock with fiamme, flame-like structures formed by viewable elongated glassy patches.

Tuffs vary in mineralogy from felsic to mafic, reflecting the chemistry of the volcanic source. Common components include volcanic glass shards, plagioclase and quartz in silicic tuffs, and various feldspars, pyroxenes, or amphiboles in more mafic varieties. Silica- or calcite-rich cements bind the particles during diagenesis; secondary minerals such as zeolites may form during alteration.

Tuffs occur worldwide in volcanic arcs and extensional basins and can form extensive ignimbrite sheets as deposits of pyroclastic flows, or as ash deposits that later lithify into tuffs. They can be porous and relatively soft, but welded varieties can be relatively dense and strong.

In human use, volcanic tuff has been used as a building stone in many regions, notably in parts of Italy (tufo), Turkey, and Central Asia, valued for workability and color.