Tuff

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Tuff is relatively soft and porous rock that made of ash and other sediments from volcanic vents that has solidified into the rock. After following ejection and deposition, the ash is compacted into a solid rock in a process called consolidation. Rock that contains greater that 50% percent tuff is considered tuffaceous. It can be classified as either sedimentary or igneous rock. They are usually studied in the context of igneous petrology, although they are sometimes described using sedimentological terms.

Name origin: The name of Tuff driven from the Italian tufo, also known as volcanic tuff

Texture: Pyroclastic

Origin: Extrusive/Volcanic

Chemical Composition: Felsic

Color: Light to dark brown

Mineral Composition: Predominantly Glass

Miscellaneous: Light gray pumice fragments in white ash matrix

Tectonic Environment: Convergent Boundary – Andean-type subduction zones, intracontinental hot spots and rifts

Tuff Classification and Composition

Welded tuff

Welded tuff is a pyroclastic rock that was sufficiently hot at the time of deposition to weld together. If the rock contains scattered, pea-sized fragments or fiamme in it, it is generally called a welded lapilli-tuff. During welding, the glass shards and pumice fragments stick together, deform and compact.

Rhyolitic tuff

Tuff is generally classified according to nature of the volcanic rock of which it consists. Rhyolite tuffs contain pumiceus, glassy fragments and small scoriae with quartz, alkali feldspar, biotite, etc. 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.

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.

Andesitic tuff

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.

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 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.

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 (blue-ground) which when thoroughly oxidized and weathered becomes a friable brown or yellow mass (the “yellow-ground”).

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. 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.

Tuff Formation

Most tuff formations include a range of fragment sizes and varieties. These range from fine-grained dust and ash (ash tuffs) to medium-sized fragments called lapilli (lapilli tuffs) to large volcanic blocks and bombs (bomb tuffs). Tuffs originate when foaming magma wells to the surface as a mixture of hot gases and incandescent particles and is ejected from a volcano. The conditions under which the ejected ash solidifies determine the final nature of the tuff. It can vary both in texture and in chemical and mineralogical composition because of variations in the conditions of their formation and the composition of the ejected material. If the pyroclastic material is hot enough to fuse, a welded tuff (called ignimbrite) forms at once. Other tuffs lithify slowly through compaction and cementation, and can stratify when they accumulate under water.

Tuff Uses

It 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.

It’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. It is common in Italy, and the Romans used it for many buildings and bridges.

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 of They 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.

Conclusion

• The deposits of tuffs can be hundreds of miles thick and the total volume of eruption can be many cubic miles. That thickness can be from a single or multiple eruptions over a long period of time.
• A small volcanic cone of low relief that surrounds a shallow crater is called a tuff ring. The craters are known as maars and are formed by explosions caused by the contact between cold groundwater and hot magma. The ring forms as the materials fall back to the Earth.
• Welded tuff is rock formed when ejecta is hot enough when it lands and the particles are soft and sticky. The ejected particles are welded together. The deposits may be near the vent and “unwelded” tuff at a distance where colder, smaller particles fell to the ground.
• A broad range of materials is often referred to as tuff. Often the only requirement is that the materials were produced by a volcanic eruption.
• It may contain different sizes of particles, from dust-sized to boulder-sized particles and be composed of many different types of materials.
• A large number of tuff deposits may contain fragments unrelated to volcanic activity. The involvement comes from volcanic explosions occurring below the ground.
• Numerous tuff deposits form from magma with a rhyolitic composition, but basaltic, andesitic, and other types of magma might contribute.
• They are usually significantly altered in composition and texture after deposition. The alteration may begin with the stewing of a hot ash layer in its own gases and condensed fluids, or outside water added to hot ash.
• This rocks may be found in the Northwest of the United States, much of Washington and Oregon as a result of the Mount St. Helens explosion. Other areas include New Zealand, Easter Island, Greece, and Peru.
• It has been used as a building material since ancient times because it is easy to work with and relatively soft.
• In the past large carvings were made from tuff, and there are famous statues on Easter Island made from tuff.
• It is common in Italy and the Romans used it for buildings and bridges. Romans also thought bees nested in tuff.
• When tuff forms from a hot ash flow it can create shells around object or people.
• The Mt. Vesuvius eruption and tuff resulted in the preserved shapes and postures of the people that were trapped by the eruption and covered ash.
• The nuclear waste repository located in the Yucca Mountain, a terminal storage facility for spent nuclear reactor and radioactive waste, is in tuff and ignimbrite.

References

• Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
• Wikipedia contributors. (2019, February 5). Tuff. In Wikipedia, The Free Encyclopedia. Retrieved 01:32, May 13, 2019, from https://en.wikipedia.org/w/index.php?title=Tuff&oldid=881913191