What is Tuff?
[[File:Bandelier-Pockmarked Cliff.jpg|thumb|200px|Welded tuff from Bandelier National Monument, [[New Mexico]].]] '''Tuff''' (from [[Italian language|Italian]] ''tufo'') is a type of [[Rock (geology)|rock]] produced by the eruption of [[volcano]]es. It consists of consolidated <ref>By 'consolidated' is meant: the volcanic ash has been turned into a rock over time, by various geological processes.</ref> [[volcanic ash]] ejected from vents during a volcanic eruption. Rock with more than 50% tuff is called ''tuffaceous''. The chemical and mineral composition of tuff varies according to what was in the molten [[magma]]. == Volcanic ash == In a [[volcanic eruption]], [[Magma]] is blown apart by volcanic gases and steam. The products are volcanic gases, [[lava]], [[steam]], and [[tephra]] (fragments). The solid material produced and thrown into the air is called ''tephra'', regardless of composition or fragment size. If the pieces of ejecta are small, the material is called '''volcanic ash''', defined as such particles less than 2&nbsp;mm in [[diameter]], [[sand|sand-sized]] or smaller. These particles are small, [[slag]]gy pieces of magma and rock that have been tossed into the air by outbursts of [[steam]] and other gases. == Structure == === Breccias === [[File:HoleInTheWallTuff.JPG|thumb|18.5 million-year-old tuff exposed at Hole in the Wall, Mojave National Preserve, [[California]].]] [[File:Ettringer Tuff.jpg|thumb|right|10 by 15 cm sample of tuff containing angular fragments of other rocks ([[Germany]])]] [[File:Agglomerate.jpg|thumb|right|Agglomerate]] Among the loose beds of ash that cover the slopes of many [[volcano]]es, three classes of materials are represented. # True ashes of the kind described above. # Lumps of the old lavas and tuffs forming the walls of the crater, which have been torn away by the violent outbursts of steam. # Pieces of [[sedimentary rock]]s from the deeper parts of the volcano that were dislodged by the rising lava and are often baked and re[[crystal]]lized by the heat. In