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Tuff

Pictures of Igneous Rock Types

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Tuff is technically a sedimentary rock formed by the accumulation of volcanic ash plus pumice or scoria. (more below)
A volcaniclastic rock
Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)
Tuff is so closely associated with volcanism that it is usually discussed along with the truly igneous rocks around it. Tuff tends to form when erupting lavas are stiff and high in silica, which holds the volcanic gases in bubbles rather than letting it escape. The brittle lava is readily shattered into jagged pieces, collectively called tephra (TEFF-ra) or volcanic ash. Fallen tephra may be reworked by rainfall and streams. Tuff is a rock of great variety and tells the geologist a lot about conditions during the eruptions that gave birth to it.

If tuff beds are thick enough or hot enough, they can consolidate into a fairly strong rock. The city of Rome's buildings, both ancient and modern, are commonly made of tuff blocks from the local bedrock. In other places, tuff may be fragile and must be carefully compacted before buildings can be constructed on it. Residential and suburban buildings that short-change this step remain prone to landslides and washouts, whether from heavy rainfall or from the inevitable quakes, like that which struck San Salvador in Central America on 13 January 2001. There many buildings on the local tierra blanca tuff collapsed.

See more closeup pictures of tuff, plus other related rocks, in the gallery of volcanic rocks.

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