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Understanding why Eyjafjallajokull has wreaked such havoc on Europe requires a little basic volcanology which volcanologists, thoroughly enjoying their week in the spotlight, are only too happy to provide. Eyjafjallajokull means "island mountain glacier" in Icelandic, and the top of the ice cap covers the volcano's peak. The ice is the thing: the weight of the glacier atop the volcano helps the magma inside build to a higher pressure, so on April 14, when the mountain had its second eruption, it blew with enough force to send volcanic gases and ash miles into the sky. And the cold water from the melted ice quickly chilled the lava, fragmenting it into tiny glass particles that could be carried away in the plume. (That's what ash really is not rock dust but little shreds of silica.) The ash plume reached the troposphere, some 35,000 ft. (almost 11 km) up, high enough to hang at the altitude where passenger planes cruise and high enough to be blown to northern Europe and beyond. "It's a minor eruption in the grand scheme of things," says Jon Davidson, an earth scientist at Durham University in Britain. "But there was a conspiracy of factors that made it worse."
Ice and wind were just two of those factors; our dependence on air travel was the other. If you wanted to turn a $300 million jumbo jet into scrap metal, you couldn't find a faster way to do it than flying it through the heart of a volcanic cloud. Heavy ash can sandblast the windows, leaving them impossible to see through. But the real threat is to the jet engines: ash is sucked into the hot combustion chamber, where it melts into molten glass, clogging the machinery, degrading the fan blades and potentially causing the engine to flame out. That's exactly what happened to a 1982 British Airways flight that ran into an undetected volcanic-ash cloud off Indonesia, losing all four engines before it was able to make an emergency landing.
But even very thin, dispersed ash clouds can badly damage a plane. In 2000 a NASA research jet flew through a high-altitude ash cloud without the pilot's realizing it. The flight continued without incident, but when scientists took apart the engines later, they discovered $3.2 million worth of damage that could have soon crippled the plane. "Ash can definitely do some real damage to your engines," says Thomas Grindle, chief of aircraft maintenance at NASA's Dryden Flight Research Center in Edwards, Calif., who wrote a report on the incident. "And we didn't even know it was happening at the time."
Pilots can fly around or under an ash cloud, but it's difficult to measure the exact boundaries of the plume, and as writer and amateur pilot James Fallows puts it, flying at low altitude is "like driving cross-country in first gear." Airlines have an official zero-tolerance approach to volcanic ash, so when scientific models showed the plume spreading across Britain and much of northern Europe in the hours after the eruption, one airport after another benched its planes where they sat. And as the plume lingered over Europe and airports remained closed for two days, then three and then longer, utter travel chaos hit, with hundreds of thousands of people around the world realizing they were stranded far from home and quite suddenly in the pre-jet era. "These kinds of eruptions happened all the time in the past," says Dougal Jerram, an earth scientist at Durham University. "But the disruption is a product of how we live today."