Tuff is a volcanic rock made mostly of compacted and cemented ash and other tephra.
It forms when explosive eruptions eject fragments of volcanic material into the atmosphere or across the ground. Fine ash can settle in layers, while hotter ash-rich flows may move rapidly as pyroclastic density currents. Burial, compaction, and chemical cementing eventually turn the loose deposit into rock.
Tuff can contain volcanic glass shards, crystals, and fragments of older rock. Its composition depends on the magma that erupted, so tuffs may be rhyolitic, dacitic, andesitic, or basaltic. Some tuffs are soft enough to quarry and carve, while welded tuffs become dense and strong when hot ash particles fuse together.
Tuff is not the same as ordinary sedimentary shale, even though both can occur in thin layers. Shale forms mainly from compacted mud, whereas tuff is made principally from volcanic ejecta. The name “tuff” is related to the older term “tufo,” used for porous volcanic stone in parts of Italy.