Volcanic ash consists of rock, mineral, and volcanic glass fragments smaller than 2 mm in diameter. It is hard (and, therefore, abrasive), does not dissolve in water, and can be extremely small - ash particles less than 0.025 mm in diameter are common. Ash is created during explosive eruptions by the shattering of solid rocks and violent separation of magma (molten rock) into tiny pieces. After being blasted into the air, the hot ash rises quickly to form a towering eruption column above the volcano.
Fine ash from the 1980 eruption of Mount St. Helens, collected in Randle, Washington, USA. Photo: © D.E. Wieprecht