Volcanic ash consists of rock, mineral, and volcanic glass fragments smaller
than a tenth of an inch in diameter—or slightly larger than a pinhead.
Volcanic ash is quite different from the soft, fluffy ash that results from
burning wood, leaves, or paper. It is hard, does not dissolve in water, and can
be extremely small—ash particles less than 1/1,000th of an inch in diameter
are common. It is also extremely abrasive (similar to finely crushed window
glass), mildly corrosive, and electrically conductive, especially when wet.
Volcanic ash is created during explosive eruptions by the shattering of solid
rocks and the violent separation of magma into tiny pieces. Explosive
eruptions result when groundwater heated by magma abruptly converts to steam
and also when magma reaches the surface so that volcanic gases dissolved in the
molten rock expand and escape into the air extremely rapidly. Hot ash and gas
rise quickly to form a towering eruption column directly above the volcano.
Above left: Ash from the 1993 eruption of Unzen volcano in Kyushu,
Japan rises halfway up the walls of a house.