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FLYING SHARDSA scanning electron microscope image of explosive ash from the Eyjafjallajökull volcano reveals the particle's sharp edges, which would have abraded the surfaces of any airplanes flying through it.S. Gislason et al/PNAS 2011

The most detailed visual study yet of volcanic ash from last year’s Icelandic eruption reveals just how sharp, abrasive and potentially dangerous the particles were.

After Eyjafjallajökull erupted in April 2010, sending volcanic plumes high into the atmosphere, officials closed Europe’s airspace for days because of the risk of ash scouring planes or being sucked into jet engines and shutting them down. “Aviation authorities made the right decision,” says team leader Susan Stipp, a geoscientist at the University of Copenhagen.

Hours after the volcano began erupting, University of Iceland volcanologists Sigurdur Gíslason and Helgi Alfredsson raced toward it to collect ash. They were the last ones to cross a bridge to safety before meltwater floods from atop Eyjafjallajökull washed the road away.

Gíslason sent some of the fresh ash, along with another batch collected 12 days later, to Stipp, whose lab studies how natural particles flow in the environment. The scientists put the ash through a barrage of tests, like attaching a single particle to the tip of a tiny beam to measure changes in mass. Their results appear the week of April 25 in the Proceedings of the National Academy of Sciences.

As ash particles exit the volcano, volatile gases condense on them and coat them with salts including the elements chlorine, fluorine and arsenic. Stipp and her colleagues dunked the ash particles in water, as might happen in a flood, and watched as tiny bits of salt washed away. In one case, 35 millionths of a millionth of a gram vanished within 15 seconds. Knowing how fast these salts dissolve, Stipp says, can help scientists understand whether the ash is dangerous to drinking water.

The researchers kept washing the ash, but even after being stirred around in water for two weeks it kept its sharp edges, Stipp says. “The particles remain extremely sharp even after they’ve been grinding against each other.”

Ash that was produced right after Eyjafjallajökull exploded on April 14 was more abrasive than the sample collected 12 days later, and was also smaller and more powdery, the team found. Many of the explosive ash bits glommed onto larger particles — suggesting that scientists may have underestimated the fraction made of particles less than 10 micrometers across, a limit often used to mark a breathing hazard.

Other labs could follow the same tests to see how dangerous a particular eruption is, says Stipp.

Another upcoming study supports the idea that Eyjafjallajökull’s ash clumps together. In a paper to appear in Geology, Jacopo Taddeucci of Italy’s National Institute of Geophysics and Volcanology and colleagues describe ash from the final days of the eruption in May 2010. Even then, Eyjafjallajökull was spitting out both sharp, dense fragments and more fragile, irregularly shaped ones, says team member Daniele Andronico, also at the Italian institute.

Ash sometimes clumped together in aggregates, the team found. On hitting the ground, these aggregates broke apart into a cloud of smaller particles, dropping more particles than expected. The research shows that volcanoes don’t always play by the rule book, says Andronico.

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