Tuff is a rock made chiefly from compacted volcanic ash.
Volcanic ash consists of fragments smaller than 2 millimetres that are blasted from a volcano or formed when magma shatters during an explosive eruption. When ash settles, layers can accumulate on land or beneath water. Burial, compaction, and cementation turn the deposit into tuff.
Tuff can contain glass shards, crystals, and rock fragments. Its color and texture vary according to the magma, eruption chemistry, and degree of welding. Very hot ash deposits may partially fuse while still moving, producing welded tuff, which can be strong and densely layered.
Tuff is a type of pyroclastic rock, not an ordinary sedimentary rock, even though ash can settle like sediment. Tuffaceous deposits are important records of past eruptions and are useful for dating surrounding rocks when the eruption age is known.