Editing Tuff (section)

==Volcanic Ash==
The material that is expelled in a [[Types of volcanic eruptions|volcanic eruption]] can be classified into three types:

# [[Volcanic gas]]es, a mixture made mostly of [[steam]], [[carbon dioxide]], and a sulfur compound (either [[sulfur dioxide]], SO<sub>2</sub>, or [[hydrogen sulfide]], H<sub>2</sub>S, depending on the temperature)
# [[Lava]], the name of [[magma]] when it emerges and flows over the surface
# [[Tephra]], particles of solid material of all shapes and sizes ejected and thrown through the air

[[File:Tuff shards.jpg|thumb|right|Light-microscope image of tuff as seen in thin section (long dimension is several mm): The curved shapes of altered glass shards (ash fragments) are well preserved, although the glass is partly altered. The shapes were formed about bubbles of expanding, water-rich gas.]]

Tephra is made when magma inside the volcano is blown apart by the rapid expansion of hot volcanic gases. Magma commonly explodes as the gas dissolved in it comes out of solution as the pressure decreases [[Extrusive rock|when it flows to the surface]]. These violent explosions produce particles of material that can then fly from the volcano. Solid particles smaller than 2&nbsp;mm in diameter ([[sand|sand-sized]] or smaller) are called volcanic ash.<ref name=EB1911/><ref name="schmidt-1981"/>

Volcanic ash is further divided into fine ash, with particle sizes smaller than 0.0625&nbsp;mm in diameter, and coarse ash, with particle sizes between 0.0625&nbsp;mm and 2&nbsp;mm in diameter. Tuff is correspondingly divided into coarse tuff (coarse ash tuff) and fine tuff (fine ash tuff or dust tuff). Consolidated tephra composed mostly of coarser particles is called lapillistone (particles 2&nbsp;mm to 64&nbsp;mm in diameter) or agglomerate or pyroclastic [[breccia]] (particles over 64&nbsp;mm in diameter) rather than tuff.<ref name="schmidt-1981"/>

Volcanic ash can vary greatly in composition, and so tuffs are further classified by the composition of the ash from which they formed. Ash from high-silica volcanism, particularly in ash flows, consists mainly of shards of [[volcanic glass]],{{sfn|Fisher|Schmincke|1984|p=96}}<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology: Igneous, Sedimentary, and Metamorphic |date=1996 |publisher=W. H. Freeman |location=New York |isbn=0-7167-2438-3 |pages=27–29 |edition=2nd}}</ref> and tuff formed predominantly from glass shards is described as vitric tuff.<ref name="obrien-1963">{{cite journal |last1=O'Brien |first1=R. T. |title=Classification of tuffs |journal=Journal of Sedimentary Research |date=1 March 1963 |volume=33 |issue=1 |pages=234–235 |doi=10.1306/74D70E20-2B21-11D7-8648000102C1865D|bibcode=1963JSedR..33..234O }}</ref> The glass shards are typically either irregular in shape or are roughly triangular with convex sides. They are the shattered walls of countless small bubbles that formed in the magma as dissolved gases rapidly came out of solution.{{sfn|Blatt|Tracy|1996|pp=27-29}}

Tuffs formed from ash consisting predominantly of individual crystals are described as crystal tuffs, while those formed from ash consisting predominantly of pulverized rock fragments are described as lithic tuffs.<ref name="obrien-1963"/> 

The chemical composition of volcanic ash reflects the entire range of volcanic rock chemistry, from high-silica [[rhyolitic]] ash to low-silica [[basaltic]] ash, and tuffs are likewise described as rhyolitic, andesitic, basaltic, and so on.{{sfn|Fisher|Schmincke|1984|pp=98-99}}

{{anchor|Welded tuff}}