Volcanic ash consists of fragments 2 mm or smaller in diameter and is a type of tephra (any fragment forcibly launched through the air from a volcano). Often the term 'volcanic ash' is loosely used to include larger tephra too and the terms ash and tephra are commonly used interchangeably. Fragments comprise rocks, minerals, and volcanic glass.
The average grainsize of rock fragments, including volcanic ash, emitted from a single exploding volcanic vent varies greatly among different eruptions, even within a single explosive event lasting hours to days. Heavier, larger rock fragments typically fall back to the ground on or close to the volcano, and progressively lighter, smaller fragments are blown farther from the volcano by wind. Volcanic ash, the smallest particles, can travel tens to thousands of kilometers downwind from a volcano depending on the wind speed, volume of ash erupted, and height of the eruption column.