Fine powdery material emitted during a volcanic eruption. the ash is often so fine (the particles may be less than 0.25 mm in diameter) that it can be carried by the wind over vast distances. During the catastrophic Mt St Helens eruption of 18 May 1980, a violent explosion blew a large cloud of ash to a height of some 18 km; the eruption continued for 9 hours, producing further quantities of ash that fell to blanket parts of Washington, N Idaho, and W and central Montana. It is estimated that several km3 of ash were expelled, and the ash cloud eventually crossed to the east coast of the USA. Ash cones, or ash volcanoes, are formed where the ash emitted accumulates around the vent; such cones are usually concave in profile and relatively gentle-sided.

Ash represents a major volcanic hazard, causing buildings to collapse as the ash piles up on their roofs. It can combine with water to form damaging mudflows called lahars. The eruption of Mt Vesuvius in AD79 blanketed the town of Pompeii with 3 m of ash and over 2000 people were killed, mostly by ash asphyxiation.