What is volcanic ash?
Residents on the Island of Hawai‘i may see volcanic ash fall from Kilauea volcano. Ash fall was recently reported in the areas of Pahala and Na‘alehu in Ka‘u and described by residents there as being “like dust.” In general, the larger particles of ash fall closer to the source of the volcanic emission and fine particles are carried longer distances.
Volcanic ash is formed during explosive volcanic eruptions when gases dissolved in magma (molten rock) expand and escape violently into the air, or when water is heated by magma and abruptly flashes into steam. The force of escaping gas shatters solid rocks. Expanding gas also shreds magma, blasting it into the air, where it solidifies into fragments of volcanic glass. Tephra is the general term for volcanic rock fragments exploded or carried into the air during an eruption. Such fragments can range in size from less than 2 mm (0.08 inches) to more than 1 m (3.2 feet) in diameter. The smallest tephra is called volcanic ash, pieces of pulverized rock and volcanic glass the size of sand or silt. Tiny ash particles can be less than 0.001 mm (0.00004 inches) in diameter. Volcanic ash can be carried great distances by wind, unlike larger tephra, which typically falls back to the ground at or near the vent. Thus, ash can cover a much larger area and disrupt the lives of far more people than
