Where does volcanic ash come from?

Ash comes from the explosive fragmentation of lava. Some magmas are gas rich. When they approach the surface the gas begins to expand because the pressure is lower. The gas bubbles continue to grow until they blow the surrounding lava into small pieces. A reasonable analogy is shaking up a bottle of soda and quickly removing the cap. The pressure of the gas inside the bottle is high and forces the soda out. As it does, it also blows the liquid into small droplets.

Steve Mattox, University of North Dakota

ash {ash close up}

Volcanic ash consists of rock, mineral, and volcanic glass fragments smaller than 2 mm (0.1 inch) in diameter, which is slightly larger than the size of a pinhead. Volcanic ash is not the same as 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 0.025 mm (1/1,000th of an inch) in diameter are common. Ash is 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 violent separation of magma (molten rock) into tiny pieces. Explosive eruptions are generated when ground water is heated by magma and abruptly converted to steam and also when magma reaches the surface so that volcanic gases dissolved in the molten rock expand and escape (explode) into the air extremely rapidly. After being blasted into the air by expanding steam and other volcanic gases, the hot ash and gas rise quickly to form a towering eruption column directly above the volcano.

U.S. Department of the Interior, U.S. Geological Survey, Menlo Park, California, USA URL http://volcanoes.usgs.gov/Products/Pglossary/ash.html