Editing Tuff (section)

==Transport and lithification==
The most straightforward way for volcanic ash to move away from the vent is as ash clouds that are part of an [[eruption column]]. These fall to the surface as ''fallout'' deposits that are characteristically [[Sorting (geology) |well-sorted]] and tend to form a blanket of uniform thickness across terrain. [[Column collapse]] results in a more spectacular and destructive form of transport, which takes the form of [[pyroclastic flows]] and [[pyroclastic surge|surges]] that characteristically are poorly sorted and pool in low terrain. Surge deposits sometimes show [[sedimentary structures]] typical of high-velocity flow, such as [[dunes]] and [[antidunes]].<ref name="philpotts-ague-2009-77">{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=978-0-521-88006-0 |page=73 |edition=2nd}}</ref> Volcanic ash already deposited on the surface can be transported as mud flows ([[lahar]]s) when mingled with water from rainfall or through eruption into a body of water or ice.{{sfn|Schmincke|2003|pp=138-157}}

Particles of volcanic ash that are sufficiently hot will weld together after settling to the surface, producing a '''welded tuff'''. Welding requires temperatures in excess of {{convert|600|C|F|sigfig=2|sp=us}}. If the rock contains scattered, pea-sized fragments or [[fiamme]] in it, it is called a welded [[lapilli tuff]]. Welded tuffs (and welded lapilli tuffs) can be of fallout origin, or deposited from ash flows, as in the case of [[ignimbrite]]s.{{sfn|Fisher|Schmincke|1984|p=215}} During welding, the glass shards and pumice fragments adhere together (necking at point contacts), deform, and compact together, resulting in a [[Eutaxitic texture|eutaxitic fabric]].{{sfn|Schmincke|2003|pp=186-187}} Welded tuff is commonly rhyolitic in composition, but examples of all compositions are known.{{sfn|Fisher|Schmincke|1984|p=209}}{{sfn|Blatt|Tracy|1996|p=29}}

A sequence of ash flows may consist of multiple ''cooling units''. These can be distinguished by the degree of welding. The base of a cooling unit is typically unwelded due to chilling from the underlying cold surface, and the degree of welding and of secondary reactions from fluids in the flow increases upwards towards the center of the flow. Welding decreases towards the top of the cooling unit, where the unit cools more rapidly. The intensity of welding may also decrease towards areas in which the deposit is thinner, and with distance from source.<ref>{{cite journal |last1=Ross |first1=Clarence S. |last2=Smith |first2=Robert L. |title=Ash-flow tuffs: Their origin, geologic relations, and identification |journal=USGS Profession Paper Series |series=Professional Paper |date=1961 |issue=366 |doi=10.3133/pp366|doi-access=free |page=19|hdl=2027/ucbk.ark:/28722/h26b1t |hdl-access=free }}</ref>

Cooler pyroclastic flows are unwelded and the ash sheets deposited by them are relatively unconsolidated.{{sfn|Schmincke|2003|pp=186-187}} However, cooled volcanic ash can quickly become lithified because it usually has a high content of volcanic glass. This is a thermodynamically unstable material that reacts rapidly with ground water or sea water, which leaches [[alkali metal]]s and [[calcium]] from the glass. New minerals, such as [[zeolite]]s, [[clay]]s, and [[calcite]], crystallize from the dissolved substances and cement the tuff.{{sfn|Schmincke|2003|p=138}}

Tuffs are further classified by their depositional environment, such as lacustrine tuff, subaerial tuff, or submarine tuff, or by the mechanism by which the ash was transported, such as fallout tuff or ash flow tuff. Reworked tuffs, formed by erosion and redeposition of ash deposits, are usually described by the transport agent, such as aeolian tuff or fluvial tuff.{{sfn|Fisher|Schmincke|1984|pp=89-90}}

<gallery widths="200px" heights="200px">
File:Volcanic Ash Fall Layers Izu Oshima Japan 1.jpg|Layers of fallout tuff in Japan
File:BishopTuff.jpg|Rocks from the [[Bishop tuff]] in [[California]], unwelded with [[pumice]] on left, welded with [[fiamme]] on right
File:Bandelier Tuff San Diego Canyon.jpg|Bandelier Tuff at San Diego Canyon, New Mexico, USA. The lower Otowi Member is a single massive cooling unit, while the upper Tshirege Member is composed of multiple cooling units.
</gallery>