Tuff Rock Type, Composition, Formation, Occurrence & Uses

Tuff Rock
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Tuff is an ingenious rock made from ash and sediments eruptions in volcanic explosions. The explosive volcano eruptions eject rocks, ash, magma, and other materials from its vent. The ejected material, known as ejecta (material that is forced or thrown out as a result of a volcanic eruption) travels upwards and falls back around the volcano. When compacted and solidified, the resulting rock is known as tuff.

Welded tuff from Bandelier National Monument, New Mexico
Welded tuff from Bandelier National Monument, New Mexico

Tuff is derived from the Italian word “Tuffo,” also known as volcanic tuff. Most tuff rocks are light to dark brown.

Tuff deposits can have a thickness of up to 100 feet with an eruptive volume of many cubic feet. This enormous and humongous size is usually a result of successive surges of single eruptions or volcanic eruptions over the years. Sometimes a single volcanic eruption can also make large deposits of tuff.

Most tuff deposits have more thickness near the volcanic vent. The thickness decreases as the distance from the vent increases. Most tuff deposits are lens-shaped instead of layers.

Etruscan tuff blocks from a tomb at Banditaccia
Etruscan tuff blocks from a tomb at Banditaccia

Tuff Composition

Tuff is usually made up of ash and sediment. Rocks that contain more than 60-75% ash are considered tuff, while rock containing 25-75% ash is described as tuffaceous. Tuff made up of sandy volcanic material is known as volcanic sandstone.

The chemical composition of tuff is felsic, and the mineral composition is mostly glass.

Tuff Classification

Tuff is usually classified into the following categories based on the nature of the volcanic rock it consists of.

Welded Tuff

Welded tuff is a rock that is welded together because it was extremely hot at the time of formation. Rocks containing scattered, pea-sized fragments are generally called welded lapilli-tuff.

Rhyolitic Tuff

Tuff containing pumice, glassy fragments, scoria, quartz alkali feldspar, and biotite is known as rhyolitic tuff.

Trachyte Tuff

It is made up of mostly sanidine or anorthoclase. Some deposits contain oligoclase feldspar, with occasional biotite, augite, and hornblende. Trachyte tuff contains little or no quartz.

Andesitic Tuff

Andesitic tuff is mostly red or brown. The deposits can be in the shape of minute granular dust or made up of huge blocks. The rocks contain cavities filled with minerals like calcite, chlorite, quartz, epidote, or chalcedony.

Basaltic Tuff

Basaltic tuff is found at active volcano sites and in lands where eruptions have long ended. Basaltic tuff is usually black, dark green, or red. Basaltic tuff varies in coarseness and can occasionally contain fossil fuels.

Ultramafic Tuff 

Tuff containing abundant olivine or serpentine and feldspar and quartz in trace amounts is known as ultramafic tuff. Ultramafic tuff is rarely found and is reported to be found in a few areas in Africa.

Where is Tuff found?

  • Tuff deposits are found at almost all explosive volcanic sites. Hence, they have a wide distribution in location and age.
  • Rhyolitic tuff is found at high-silica volcanic sites, and its deposits are located in Iceland, Lipari, Hungary, the Basin and Range in the American southwest, and New Zealand.
  • Tuff created due to alkaline volcanism (trachyte tuff) is found in Ischia and Naples. It is also located in East African Rift and Rio de Janerio.
  • Andesitic tuffs (intermediate volcanism) are the most common and found along the chain of Cordilleras and Andes, West Indies, New Zealand, and Japan. They are also located in many districts of Great Britain.
A tuff house in Germany
A tuff house in Germany
  • Basaltic tuffs (mafic volcanism) deposits are located in Hawaii and Scotland.

Tuff Uses

Tuff is mostly used in the construction industry. Its primary economic value is as a building material. Tuff has been used in the ancient world wherever it was available. Romans in Italy used it for many buildings and bridges.

Layers of fallout tuff in Japan
Layers of fallout tuff in Japan

A fine example of Romans using tuff for construction is Ventotene port. Most of the building stone used in Rome and Naples is trachyte tuff, known as peperino. Tuff is also used in Frankfurt, Hamburg, and other cities in Germany to make railway stations.

In the United States, Yucca Mountain nuclear waste repository in the Basin and Range Province in Nevada is made from tuff and ignimbrite.

Tuff is frequently used in Armenia for construction. Many historic and new buildings are made from tuff in Yerevan, the Armenian capital.

Armenia's Government House in Yerevan's Republic Square, built of yellow tuff
Armenia's Government House in Yerevan's Republic Square, built of yellow tuff


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