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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.
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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.
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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.
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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.
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Tuff may contain different sizes of particles, from dust-sized to boulder-sized particles and be composed of many different types of materials.
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A large number of tuff deposits may contain fragments unrelated to volcanic activity. The involvement comes from volcanic explosions occurring below the ground.
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Numerous tuff deposits form from magma with a rhyolitic composition, but basaltic, andesitic, and other types of magma might contribute to the tuff.
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Tuffs 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.
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Tuff 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.
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Tuff has been used as a building material since ancient times because it is easy to work with and relatively soft.
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In the past large carvings were made from tuff, and there are famous statues on Easter Island made from tuff.
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Tuff is common in Italy and the Romans used it for buildings and bridges. Romans also thought bees nested in tuff.
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When tuff forms from a hot ash flow it can create shells around object or people.
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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.
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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.
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