Tuff is a volcanic rock consisting of consolidated volcanic ash. It is a product of explosive volcanic eruptions. Tuff can be considered to be a volcanic analogue of sandstone.
Both sand grains and volcanic ash grains have a diameter between 1/16…2 mm. However, the term “tuff” is often used for many pyroclastic rocks (pyroclasts are fragments thrown out of volcanoes during explosive eruptions — ash, lapilli, bombs, blocks) that contain fragments a lot larger (or smaller) than 2 mm (1/16 mm). Sometimes the term is used even for consolidated material that has undergone limited posteruption reworking1. This is not only that the general public is using the term too loosely but geologists too use terms like “lapilli tuff” and “tuff breccia” that contain lots of larger pyroclastic material.
Pyroclastic material deposited from pyroclastic density currents (pyroclastic flows) is known as ignimbrite. Ignimbrite can be considered to be a special type of tuff (it is sometimes known as welded tuff although not all ignimbrites are welded).
Classification of polymodal (consisting of differently sized pyroclasts) pyroclastic rocks. Tuff in the strict scientific usage is on the lower right corner with ash tuff (mean diameter of pyroclasts less than 1/16 mm which is analogous to siltstone). At least 75% of the pyroclastic material of tuff and ash tuff has a mean diameter less than 2 mm. Lapillistone is mostly composed of lapilli (more than 75%), pyroclastic breccia (agglomerate) are mostly composed of volcanic blocks (bombs). Tuff breccia and lapilli tuff are more mixed polymodal pyroclastic rocks. Figure drawn by me, data (classification principles) taken from Le Maitre, 20052.
1. Tilling, Robert. (2007). Tuff. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 18. 680-682.
2. Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.
3. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.