Tuffs have the potential to be deposited wherever explosive volcanism takes place, and so have a wide distribution in location and age.
High-silica volcanism Rhyolite tuffs contain pumiceous, glassy fragments and small scoriae with
quartz,
alkali feldspar,
biotite, etc. Iceland, Lipari, Hungary, the
Basin and Range of the American southwest, and
New Zealand are among the areas where such tuffs are prominent. In the ancient rocks of
Wales,
Charnwood, etc., similar tuffs are known, but in all cases, they are greatly changed by silicification (which has filled them with
opal,
chalcedony, and quartz) and by devitrification. The frequent presence of rounded corroded quartz crystals, such as occur in rhyolitic lavas, helps to demonstrate their real nature. Lava Creek tuff is known to be at least 1,000 times as large as the deposits of the
1980 eruption of Mount St. Helens, and it had a
Volcanic Explosivity Index (VEI) of 8, greater than any eruption known in the last 10,000 years. Ash flow tuffs cover of the
North Island of
New Zealand and about of
Nevada. Ash flow tuffs are the only volcanic product with volumes rivaling those of
flood basalts. The Tioga Bentonite of the northeastern United States varies in composition from crystal tuff to tuffaceous shale. It was deposited as ash carried by wind that fell out over the sea and settled to the bottom. It is
Devonian in age and likely came from a vent in central
Virginia, where the tuff reaches its maximum thickness of about .
Alkaline volcanism Trachyte tuffs contain little or no quartz, but much
sanidine or
anorthoclase and sometimes oligoclase feldspar, with occasional biotite, augite, and hornblende. In weathering, they often change to soft red or yellow
claystones, rich in
kaolin with secondary quartz. in
Ischia and near
Naples. Trachyte-carbonatite tuffs have been identified in the
East African Rift. Alkaline crystal tuffs have been reported from
Rio de Janeiro.
Intermediate volcanism Andesitic tuffs are exceedingly common. They occur along the whole chain of the
Cordilleras and
Andes, in the
West Indies, New Zealand, Japan, etc. In the
Lake District, North Wales,
Lorne, the
Pentland Hills, the
Cheviots, and many other districts of
Great Britain, ancient rocks of exactly similar nature are abundant. In color, they are red or brown; their scoriae fragments are of all sizes from huge blocks down to minute granular dust. The cavities are filled with many secondary minerals, such as
calcite,
chlorite, quartz,
epidote, or chalcedony; in microscopic sections, though, the nature of the original lava can nearly always be made out from the shapes and properties of the little crystals which occur in the decomposed glassy base. Even in the smallest details, these ancient tuffs have a complete resemblance to the modern ash beds of
Cotopaxi,
Krakatoa, and Mont Pelé. However, interaction between basaltic magma and groundwater or seawater results in hydromagmatic explosions that produce abundant ash. These deposit ash cones that subsequently can become cemented into tuff cones.
Diamond Head, Hawaii, is an example of a tuff cone, as is the island of
Kaʻula. The glassy basaltic ash produced in such eruptions rapidly alters to
palagonite as part of the process of lithification. Although conventional mafic volcanism produce little ash, such ash as is formed may accumulate locally as significant deposits. An example is the Pahala ash of
Hawaii island, which locally is as thick as . These deposits also rapidly alter to palagonite, and eventually weather to
laterite. Basaltic tuffs are also found in
County Antrim,
Skye,
Mull, and other places, where
Paleogene volcanic rocks are found; in Scotland,
Derbyshire, and Ireland among the
Carboniferous strata, and among the still older rocks of the Lake District, the southern uplands of Scotland, and Wales. They are black, dark green, or red in colour; vary greatly in coarseness, some being full of round spongy bombs a foot or more in diameter; and being often submarine, may contain shale, sandstone, grit, and other sedimentary material, and are occasionally fossiliferous. Recent basaltic tuffs are found in
Iceland, the
Faroe Islands,
Jan Mayen, Sicily, the
Hawaiian Islands,
Samoa, etc. When weathered, they are filled with calcite, chlorite,
serpentine, and especially where the lavas contain
nepheline or
leucite, are often rich in
zeolites, such as
analcite,
prehnite,
natrolite,
scolecite,
chabazite,
heulandite, etc.
Kimberlites Occurrences of ultramafic tuff include surface deposits of
kimberlite at
maars in the
diamond-fields of southern Africa and other regions. The principal variety of kimberlite is a dark bluish-green, serpentine-rich breccia (blue-ground) which, when thoroughly oxidized and weathered, becomes a friable brown or yellow mass (the "yellow-ground").
Folding and metamorphism in Rome, made of tuff blocks ,
City of Brisbane In course of time, changes other than weathering may overtake tuff deposits. Sometimes, they are involved in folding and become
sheared and
cleaved. Many of the green
slates of the English
Lake District are finely cleaved ashes. In
Charnwood Forest also, the tuffs are slaty and cleaved. The green color is due to the large development of chlorite. Among the crystalline
schists of many regions, green beds or green schists occur, which consist of quartz, hornblende, chlorite or biotite,
iron oxides, feldspar, etc., and are probably recrystallized or
metamorphosed tuffs. They often accompany masses of epidiorite and hornblende – schists which are the corresponding lavas and
sills. Some chlorite-schists also are probably altered beds of volcanic tuff. The "Schalsteins" of
Devon and Germany include many cleaved and partly recrystallized ash-beds, some of which still retain their fragmental structure, though their lapilli are flattened and drawn out. Their steam cavities are usually filled with calcite, but sometimes with quartz. The more completely altered forms of these rocks are platy, green chloritic schists; in these, however, structures indicating their original volcanic nature only sparingly occur. These are intermediate stages between cleaved tuffs and crystalline schists. ==Importance==