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Relatively soft, porous rock that is usually formed by the compaction and cementation of volcanic ash or dust. Tuff may vary greatly not only in texture but also in chemical and mineralogical composition. In some eruptions, foaming magma wells to the surface as an emulsion of hot gases and incandescent particles; the shredded pumice-like material spreads swiftly, even over gentle slopes, as a glowing avalanche (nuée ardente) that may move many miles at speeds of 100 mph (160 kph).
This entry comes from Encyclopædia Britannica Concise. For the full entry on tuff, visit Britannica.com.