Tuff

Tuff is a rock formed when volcanic ash and other pyroclastic material compact and cement together. Its texture and composition preserve evidence of explosive eruptions and later alteration.

Volcanic ash: Fine fragments of rock, minerals, and volcanic glass ejected during an eruption. Compacted volcanic ash supplies the defining material in most tuff.
Pyroclastic flow: A fast-moving, ground-hugging current of hot gas and volcanic fragments. Deposits from these currents commonly compact into welded or unwelded tuff.
Pozzolan: A siliceous or aluminous material that reacts with lime in water to form cementitious compounds. Some tuffs act as natural pozzolans and have been used in lime-based construction.
Volcanic breccia: A volcanic rock made predominantly of angular fragments larger than 64 millimeters. Its coarse clasts contrast with the ash-sized material that defines tuff.
Pyroclastic material: Fragmented material produced and expelled by explosive volcanic eruptions. Tuff forms from this broader mixture of ash and larger fragments.
Welding (geology): The fusing of hot volcanic glass shards under pressure after deposition. Welding can consolidate hot ash deposits into dense, strongly joined tuff.
Tuffeau: A soft, pale limestone-rich building stone quarried in parts of western France. Despite its name, tuffeau is a sedimentary stone, often confused with volcanic tuff.
Agglomerate (geology): A pyroclastic rock dominated by rounded or partly rounded volcanic blocks and bombs. Agglomerate is coarser than tuff and consists mainly of large erupted clasts.
Pumice: A porous volcanic rock formed when gas-rich magma cools rapidly. Pumice fragments can be incorporated into coarse, distinctive tuffs.
Lithification: The processes that turn loose sediment into solid rock through compaction and cementation. Tuff becomes rock through lithification, though heat-driven welding may also bind it.

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