Tuff
Classification (And reason for classification) + Etymology: Tuff is a type of rock made of volcanic ash ejected from a vent during a
volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is
considered tuff, while rock containing 25% to 75% ash is described as tuffaceous (for example, tuffaceous sandstone).
Tuff is a relatively soft rock, so it has been used for construction since ancient times. Because it is common in Italy, the Romans used
it often for construction. The Rapa Nui people used it to make most of the moai statues on Easter Island.
Tuff can be classified as either igneous or sedimentary rock. It is usually studied in the context of igneous petrology, although it is
sometimes described using sedimentological terms.
The word tuff is derived from the Italian tufo.
Composition + Petrology + Textures: The material that is expelled in a volcanic eruption can be classified into three types:
Volcanic gases, a mixture made mostly of steam, carbon dioxide, and a sulfur compound (either sulfur dioxide, SO2, or hydrogen
sulfide, H2S, 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
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 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 mm in diameter (sand-sized
or smaller) are called volcanic ash.
Volcanic ash is further divided into fine ash, with particle sizes smaller than 0.0625 mm in diameter, and coarse ash, with particle sizes
between 0.0625 mm and 2 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 mm to 64 mm in diameter) or
agglomerate or pyroclastic breccia (particles over 64 mm in diameter) rather than tuff.
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, and tuff formed predominantly
from glass shards is described as vitric tuff. 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.
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.
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
Subtypes (if applicable):
Properties (if applicable):
Fine-grained, uneven fracture, white streak, highly porous, vitreous to dull luster, 243.80 N/mm2 compressive strength, specific gravity
2.73, density 1-1.8 g/cm3, specific heat capacity 0.20 kJ/Kg K
Rock Cycle/Formation: 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 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 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. Volcanic ash already deposited on the surface can be
transported as mud flows (lahars) when mingled with water from rainfall or through eruption into a body of water or ice.
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 600 °C (1,100 °F). 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
ignimbrites. During welding, the glass shards and pumice fragments adhere together (necking at point contacts), deform, and compact
together, resulting in a eutaxitic fabric. Welded tuff is commonly rhyolitic in composition, but examples of all compositions are
known.
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.
Cooler pyroclastic flows are unwelded and the ash sheets deposited by them are relatively unconsolidated. 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 metals and calcium from the glass. New minerals, such as
zeolites, clays, and calcite, crystallize from the dissolved substances and cement the tuff.
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.
Economic Importance: The primary economic value of tuff is as a building material. In the ancient world, tuff's relative softness
meant that it was commonly used for construction where it was available. Tuff is common in Italy, and the Romans used it for many
buildings and bridges For example, the whole port of the island of Ventotene (still in use), was carved from tuff. The Servian Wall,
built to defend the city of Rome in the fourth century BC, is also built almost entirely from tuff. The Romans also cut tuff into small,
rectangular stones that they used to create walls in a pattern known as opus reticulatum.
The peperino, much used at Rome and Naples as a building stone, is a trachyte tuff. Pozzolana also is a decomposed tuff, but of basic
character, originally obtained near Naples and used as a cement, but this name is now applied to a number of substances not always of
identical character. In the Eifel region of Germany, a trachytic, pumiceous tuff called trass has been extensively worked as a hydraulic
mortar.
Tuff of the Eifel region of Germany has been widely used for construction of railroad stations and other buildings in Frankfurt,
Hamburg, and other large cities. Construction using the Rochlitz Porphyr, can be seen in the Mannerist-style sculpted portal outside
the chapel entrance in Colditz Castle. The trade name Rochlitz Porphyr is the traditional designation for a dimension stone of Saxony
with an architectural history over 1,000 years in Germany. The quarries are located near Rochlitz.
Yucca Mountain nuclear waste repository, a U.S. Department of Energy terminal storage facility for spent nuclear reactor and other
radioactive waste, is in tuff and ignimbrite in the Basin and Range Province in Nevada. In Napa Valley and Sonoma Valley, California,
areas made of tuff are routinely excavated for storage of wine barrels.
Tuff from Rano Raraku was used by the Rapa Nui people of Easter Island to make the vast majority of their famous moai statues.
Occurrence: 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.
- Welded ignimbrites can be highly voluminous, such as the Lava Creek Tuff erupted from Yellowstone Caldera in Wyoming
631,000 years ago. This tuff had an original volume of at least 1,000 cubic kilometers (240 cu mi). Lava Creek tuff is known
to be at least 1000 times as large as the deposits of the May 18, 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 7,000 square
kilometers (2,700 sq mi) of the North Island of New Zealand and about 100,000 square kilometers (39,000 sq mi) 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 40 meters (130 ft).
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. Recent trachyte tuffs are found on the Rhine (at Siebengebirge), 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.
Mafic volcanism
- Mafic volcanism typically takes the form of Hawaiian eruptions that are nonexplosive and produce little ash. However,
interaction between basaltic magma and groundwater or sea water 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 Ka'ula. 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 15 meters (49 ft). These deposits also
rapidly alter to palagonite, and eventually weather to laterite.
- Basaltic tuffs are also found in Skye, Mull, Antrim, 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.
Ultramafic volcanism
- Ultramafic tuffs are extremely rare; their characteristic is the abundance of olivine or serpentine and the scarcity or absence
of feldspar and quartz.
- 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"). These breccias were emplaced as gas–solid mixtures and are typically preserved and mined in
diatremes that form intrusive pipe-like structures. At depth, some kimberlite breccias grade into root zones of dikes
made of unfragmented rock. At the surface, ultramafic tuffs may occur in maar deposits. Because kimberlites are the
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most common igneous source of diamonds, the transitions from maar to diatreme to root-zone dikes have been
studied in detail. Diatreme-facies kimberlite is more properly called an ultramafic breccia rather than a tuff.
Komatites
- Komatiite tuffs are found, for example, in the greenstone belts of Canada and South Africa.[
Folding and metamorphism
- 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.
Additional Facts:
- Tuffs are deposited geologically instantaneously and often over a large region. This makes them highly useful as
time-stratigraphic markers. The use of tuffs and other tephra deposits in this manner is known as tephrochronology and is
particularly useful for Quaternary chronostratigraphy. Individual tuff beds can be "fingerprinted" by their chemical
composition and phenocryst assemblages.Absolute ages for tuff beds can be determined by K-Ar, Ar-Ar, or carbon-14 dating.
Zircon grains found in many tuffs are highly durable and can survive even metamorphism of the host tuff to schist, allowing
absolute ages to be assigned to ancient metamorphic rocks. For example, dating of zircons in a metamorphosed tuff bed in the
Pilar Formation provided some of the first evidence for the Picuris orogeny
- For some reason used a ton in Armenian architechture