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 compacted
into a solid rock in a process called consolidation. Tuff is sometimes called
tufa, particularly when used as construction material, although tufa
also refers to a quite different rock. Rock that contains greater than 50% tuff
is considered tuffaceous. Tuff is a relatively soft rock, so it
has been used for construction since ancient times. Since 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 in Easter Island.
Tuff can be classified as either sedimentary or igneous rocks. They are
usually studied in the context of igneous petrology, although they are
sometimes described using sedimentological terms.
Volcanic ash The material that is expelled in a
volcanic eruption can be classified into three types:
Volcanic gases, a mixture made mostly of water vapour, carbon dioxide, and a
sulfur compound. lava, the name of magma when it comes
out to the surface. and tephra, chunks of solid material of
all shapes and sizes that come flying out of the volcano.
Tephra is made when magma inside the volcano is blown apart by coming into
contact with the hot volcanic gases. It is common for magma to explode as the
gas dissolved in it comes out of solution as the pressure decreases when
it flows to the surface. These violent explosions produce solid chunks of
material that then fly away from the volcano. When these chunks are smaller
than 2 mm in diameter they are called volcanic ash. It is made of small,
slaggy pieces of magma and rock that have been tossed into the air by
outbursts of steam and other gases. Breccias
Among the loose beds of ash that cover the slopes of many volcanoes, three
classes of materials are represented. In addition to true ashes of the kind
described above, there are lumps of the old lavas and tuffs forming the walls of
the crater, etc., which have been torn away by the violent outbursts of steam,
and pieces of sedimentary rocks from the deeper parts of the volcano that were
dislodged by the rising lava and are often intensely baked and recrystallized
by the heat to which they have been subjected.
In some great volcanic explosions nothing but materials of the second kind
were emitted, as at Mount Bandai in Japan in 1888. There have been many
eruptions also in which the quantity of broken sedimentary rocks that mingled
with the ash is very great; as instances we may cite the volcanoes of the Eifel
and the Devonian tuffs, known as "Schalsteins," in Germany. In the
Scottish coalfields some old volcanoes are plugged with masses consisting
entirely of sedimentary debris: in such a case it is supposed that no lava was
ejected, but the cause of the eruption was the sudden liberation and expansion
of a large quantity of steam. These accessory or adventitious materials,
however, as distinguished from the true ashes, tend to occur in angular
fragments; and when they form a large part of the mass the rock is more
properly a "volcanic breccia" than a tuff. The ashes vary in size from large
blocks twenty feet or more in diameter to the minutest impalpable dust. The
large masses are called "volcanic bombs"; they have mostly a rounded,
elliptical or pear-shaped form owing to rotation in the air before they
solidified. Many of them have ribbed or nodular surfaces, and sometimes they
have a crust intersected by many cracks like the surface of a loaf of bread. Any
ash in which they are very abundant is called an agglomerate.
In those layers and beds of tuff that have been spread out over considerable
tracts of country and which are most frequently encountered among the
sedimentary rocks, smaller fragments preponderate greatly and bombs more than
a few inches in diameter may be absent altogether. A tuff of recent origin is
generally loose and incoherent, but the older tuffs have been, in most cases,
cemented together by pressure and the action of infiltrating water, making
rocks which, while not very hard, are strong enough to be extensively used for
building purposes. If they have accumulated subaerially, like the ash
beds found on Mt. Etna or Vesuvius at the present day, tuffs consist almost
wholly of volcanic materials of different degrees of fineness with
pieces of wood and vegetable matter, land shells, etc. But many volcanoes
stand near the sea, and the ashes cast out by them are mingled with the
sediments that are gathering at the bottom of the waters. In this way ashy
muds or sands or even in some cases ashy limestones are being formed. As a matter
of fact most of the tuffs found in the older formations contain admixtures of
clay, sand, and sometimes fossil shells, which prove that they were beds spread
out under water. During some volcanic eruptions a layer
of ashes several feet in thickness is deposited over a considerable district,
but such beds thin out rapidly as the distance from the crater increases, and
ash deposits covering many square miles are usually very thin. The showers of
ashes often follow one another after longer or shorter intervals, and hence
thick masses of tuff, whether of subaerial or of marine origin, have
mostly a stratified character. The coarsest materials or agglomerates show
this least distinctly; in the fine beds it is often developed in great
perfection. Igneous rock
Apart from adventitious material, such as fragments of the older rocks, pieces
of trees, etc., the contents of an ash deposit may be described as consisting
of more or less crystalline igneous rocks. If the lava within the crater has
been at such a temperature that solidification has commenced, crystals
are usually present. They may be of considerable size like the grey, rounded
leucite crystals found on the sides of Vesuvius. Many of these are very perfect
and rich in faces because they grew in a medium that was liquid and not very
viscous. Good crystals of augite and olivine are also to be obtained in the
ash beds of Vesuvius and of many other volcanoes, ancient and modern. Blocks of
these crystalline minerals are common objects in the tuffs of many of the West
Indian volcanoes. Where crystals are very abundant the ashes are called
"crystal tuffs." In St. Vincent and Martinique in 1902, much of the dust was
composed of minute crystals enclosed in thin films of glass because the lava at
the moment of eruption had very nearly solidified as a crystalline mass. Some
basaltic volcanoes, on the other hand, have ejected great quantities of black
glassy scoria, which, after consolidation, weather to a red soft
rock known as palagonite; tuffs of this kind occur in Iceland and Sicily. In the
Lipari Islands and Hungary there are acid tuffs, of pale grey or yellow
color, largely composed of lumps and fragments of pumice. Over a large
portion of the sea bottom the beds of fine mud contain small, water-worn,
rounded pebbles of very spongy volcanic glass; these have been floated from the
shore or cast out by submarine volcanoes, and may have travelled for
hundreds of miles before sinking; it has been proved by experiment that some
kinds of pumice will float on sea-water for more than a year. The deep
sea-deposit known as the "red clay" is largely of volcanic origin and might be
suitably described as a "submarine tuff-bed."
Welded tuff Welded tuff is a pyroclastic rock, of
any origin, that was sufficiently hot at the time of deposition to weld together.
Strictly speaking, if the rock contains scattered pea-sized fragments or fiamme
in it, it is called a welded lapilli-tuff. Welded tuffs can be of
fallout origin, or deposited from pyroclastic density currents, as in the
case of ignimbrites. During welding, the glass shards and pumice fragments adhere
together, deform, and compact together, resulting in a eutaxitic fabric.
Welded ignimbrites can be highly voluminous, such as the Lava Creek Tuff
erupted from Yellowstone Caldera in Wyoming 640,000 years ago. 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 of
8—greater than any eruption known in the last 10,000 years. The intensity of
welding may decrease towards the upper margin of a deposit, towards areas in
which the deposit is thinner, and with distance from source. Welded tuff is
commonly rhyolitic in composition, but examples of all compositions are known.
Rhyolitic tuff For petrographical purposes, tuff is
generally classified according to the nature of the volcanic rock of which it
consists; this may be the same as the accompanying lavas if any were emitted
during an eruption, and if there is a change in the kind of lava which is
poured out, the tuffs also indicate this equally clearly. 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.
The broken pumice is clear and isotropic, and very small particles
commonly have crescentic, sickle-shaped, or biconcave outlines, showing that they
are produced by the shattering of a vesicular glass, sometimes described as
ash-structure. The tiny glass fragments derived from broken pumice are called
shards; the glass shards readily deform and flow when the deposits are
sufficiently hot, as shown in the accompanying image of welded tuff.
In the ancient rocks of Wales, Charnwood, the Pentland Hills, etc.,
similar tuffs are known, but in all cases they are greatly changed by
silicification and by devitrification. The frequent presence of rounded
corroded quartz crystals, such as occur in rhyolitic lavas, helps to demonstrate
their real nature. A example of this tuff 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.
Trachyte tuff 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 clay-stones, rich in kaolin with
secondary quartz. Recent trachyte tuffs are found on the Rhine, in Ischia, near
Naples, Hungary, etc. Andesitic tuff
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 up with many secondary minerals, such as calcite, chlorite, quartz,
epidote, chalcedony; but in microscopic sections, 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.
Basaltic tuff Basaltic tuffs are also of widespread
occurrence both in districts where volcanoes are now active and in lands
where eruptions have long since ended. They are found in Skye, Mull, Antrim and
other places, where there are Paleogene volcanic rocks; in Scotland, Derbyshire
and Ireland among the carboniferous strata; and among the still older rocks
of the Lake District, 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, 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 tuff Ultramafic tuffs are extremely rare;
their characteristic is the abundance of olivine or serpentine and the scarcity
or absence of feldspar and quartz. Rare occurrences may include unusual surface
deposits of maars of kimberlites of the diamond-fields of southern Africa and
other regions. The principal rock of kimberlite is a dark bluish-green
serpentine-rich breccia which when thoroughly oxidized and weathered
becomes a friable brown or yellow mass. 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
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.
Folding and metamorphism In course of time, other changes than
weathering may overtake tuff deposits. Sometimes they are involved in folding
and become sheared and cleaved. Many of the green slates of the lake district in
Cumberland are fine 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.
Economic importance Tuff's primary economic value 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, was carved out from tuff. The Servian
Wall, built to defend the city of Rome in the 4th 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 Romans thought bees nested in tuff. The substance is mentioned in the
Aeneid. 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. 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 out 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.
Tuff is important in Armenian architecture.
See also Sillar
Eutaxitic texture Tuffite
References This article incorporates text from a
publication now in the public domain: Chisholm, Hugh, ed.. Encyclopædia
Britannica. Cambridge University Press.