Volcanic ash is not the product of combustion, like the soft fluffy material created by burning wood, lees, or paper, but rather consists of fragments of rocks, minerals, and volcanic glass ranging in size from sand to clay-like (from 2 mm (1/12 in) to less than 0.004 mm (1/256th in) in diameter). Ash is hard, abrasive, mildly corrosive, conducts electricity when wet, and does not dissolve in water. Ash is spread over broad areas by wind.
Volcanic ash is formed during explosive volcanic eruptions. Explosive eruptions occur when gases dissolved in molten rock (magma) expand as the magma rises, and then escape violently into the air, or when water is heated by magma and abruptly flashes into steam. The force of the escaping, expanding gas violently shatters solid rocks and shreds the magma blasting it into the air. Once airborne, the magma solidifies into fragments of volcanic rock and glass. Wind can then blow the tiny ash particles tens to thousands of kilometers away from the volcano.