VHP Photo Glossary: volcanic ash

Photo: hands holding volcanic ash erupted by Mount St. Helens on May 18,1980

1. Photograph by D.E. Wieprecht. Volcanic ash collected in Randle, Washington, located about 40 km NNE of Mount St. Helens. The north edge of the eruption cloud of May 18, 1980, passed over Randle and deposited between 1 and 2 cm of ash on the community. At the same distance along the axis of the eruption cloud, however, about 7 cm of ash and larger-sized tepra fell to the ground.

Photo: SEM image of a single ash grain erupted by Mount St. Helens on May 18,1980

2. SEM image provided by A.M. Sarna-Wojcicki. Close view of a single ash particle from the eruption of Mount St. Helens; image is from a scanning electron microscope (SEM). The tiny voids or "holes" are called vesicles and were created by expanding gas bubbles during the eruption of magma.

Volcanic ash

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.

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