by Hai Shunxi

Created on: April 16, 2010

When they hear the phrase “volcanic ash,” many people take it for granted that the stuff is something like the ashes left behind by a campfire or in a fireplace. Volcanic ash isn’t the leftovers of a fire, though, it’s something altogether different. It’s tiny shards of rock the size of a sand grain or smaller, usually with a texture quite similar to glass.

Unlike the volcanoes heroes encounter in movie thrillers; few real volcanoes produce rivers of glowing, red-hot lava. In many eruptions, instead of running out on the ground, rising magma explodes from the volcano’s vent, launching billions of droplets of molten rock into the air. There, the smallest droplets almost instantly solidify. Such violent eruptions occur when there is a lot of water, carbon dioxide, and gases in the molten magma.

At the same time as the magma is ejected, solid rock around the vent is pulverized by the force of the escaping magma. This adds to the volume of ash created by an eruption. Large pieces, up to several meters in size, fall to earth near the volcanic vent, but the smallest pieces – those less than 2mm (about 1/12th of an inch) in size – can rise on currents of heated air to create a massive cloud high above the volcano.

The very rapid cooling of the smallest drops of ejected magma usually produces tiny, jagged bits of volcanic glass. Volcanic glass behaves much like ordinary window glass. A key feature is that (like window glass) these small pieces tend to have irregular, sharp edges. This trait is responsible for much of the damage that clouds of volcanic ash can and do cause.

Volcanic ash produced by major eruptions can spread over tens of thousands of square miles. Ash from the Mt. St. Helens (Washington, USA) eruption on 18 May, 1980, reached 80,000 feet (24,500m) into the atmosphere. An estimated 540 million tons (490 million metric tons) of ash spread over 22,000 square miles (60,000 km²) of North America. The 1991 eruption of Mt. Pinatubo in the Philippines spread ash over an area slightly more than twice as large, and has been estimated to have been an order of magnitude larger than the Mt. St. Helens eruption.

Though larger particles settle out to rain on roofs, lawns, and streets within a few hours, the finest ash particles – about the texture of bread flour – can persist in the atmosphere for weeks or even years, circling the globe on the jet streams. Atmospheric ash after the 1883 eruption of Krakatoa in Indonesia, said to be the most violent eruption in recorded history, was so thick that global weather patterns did not return to normal for several years.

The composition and shape of ash particles represent a danger to both man and machine. When inhaled, the particles' knife-like edges can cause lung damage and subsequent scarring. If ash particles are sucked into the intake of a running engine, they can abrade and corrode working metal and non-metallic parts. If the operating temperature of the engine is sufficiently hot, the ash particles can re-melt and deposit on engine parts as they cool. There have been several cases of jet engine failure blamed on encounters with clouds of volcanic ash, including an Alaska Air jet (1989) and a British Airways flight (1982).

For more information, see SKYbrary and US Geological Survey

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