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Ash Cloud Reminds Us That We Should All Be Afraid of Volcanoes

Every so often the Earth chooses to remind us that we really aren’t in control of this planet. The volcanic eruption in Iceland, which began on Wednesday, is just such a reminder. As ash spews out across northern Europe, grounding all flights across Scandinavia and the UK, we begin to realise how powerless we humans are.

But as volcanic eruptions go, the current fireworks on Iceland are small fry. Scientists rank volcanoes according to how explosive they are, using the volcanic explosivity index (VEI), which goes from zero to eight. The measurement is based on how much material is thrown out of the volcano, how high the eruption goes and how long it lasts. Like the scale used to measure earthquake size, the VEI is logarithmic – meaning that a volcano with a VEI of five is 10 times more powerful than one with a VEI of four.

As yet, scientists haven’t managed to gather enough data to calculate the VEI of Eyjafjallajökull, but Thorvaldur Thordarson, an expert on Icelandic volcanism at the University of Edinburgh, estimates that this one is probably a two or three – similar to the eruptions seen on Mount Etna on Sicily in 2002 and 2003, and the kind of eruption we expect to see somewhere on Earth at least once every year.

By contrast, the eruption of Mount St Helens, in the north-west of the US in May 1980, was a one-in-10-year event, with a VEI of around four. Meanwhile, Pinatubo’s boom in the Philippines in 1991 was a one-in-50 to 100-year spectacle, with a VEI of about five or six.

Bigger still was the eruption of Tambora in 1815, on the island of Sumbawa, Indonesia, whose ash was responsible for some of the spectacular sunsets painted by J M W Turner. Rated as a seven on the VEI scale (a one-in-1,000-year event), it was the most deadly eruption in recorded history, killing over 70,000 people.

But as the Eyjafjallajökull event is showing, even baby eruptions can cause quite a nuisance. The last time Iceland experienced an eruption of this size was in 2004, when the Grímsvötn volcano blew. "On that occasion the ash cloud went to the north, but this time the jet stream has carried it south-east, towards the UK," Thordarson says.

Having its ash carried into some of Europe’s busiest flight paths has made Eyjafjallajökull big news. No aircraft can risk flying through the cloud – the chances of choking the engines and stalling the plane are high. And so we have to twiddle our thumbs until Eyjafjallajökull decides she has let off enough steam.

If Eyjafjallajökull is anything like Grímsvötn, the eruption will peter out in a day or two, but there is a chance that things could go on for a lot longer. "We suspect it will end today or tomorrow, but it could last for weeks, months or even years," Thordarson says.

And even if Eyjafjallajökull goes quiet again soon, that doesn’t mean she’s finished yet. "The last time Eyjafjallajökull erupted [in 1823], it lasted for more than a year, so we could see more of the same disruption over the coming months," says Bill McGuire, director of the Aon Benfield UCL Hazard Research Centre at University College London. Thordarson agrees, saying that we may well get a number of intermittent explosive events at Eyjafjallajökull over the next couple of years.

For volcanologists, this most recent eruption is no surprise. "There has been lots of unrest under this particular volcano for the past 10 years, which picked up in intensity at the beginning of this year," says Thordarson. In fact, many volcanologists are rubbing their hands with glee. "It is a nice surprise for us, as this one hasn’t erupted for a while," says Dougal Jerram, from the University of Durham.

To volcanologists, Iceland is heaven. There are 30 active volcanic systems on the island and very frequent firework displays. Geologists believe that the reason for Iceland’s explosive nature is because it sits over a "mantle plume" – a rising column of abnormally hot molten rock, originating at the edge of the Earth’s core.

To make matters even more spectacular, this particular mantle plume has positioned itself under the mid-Atlantic ridge: the crack that runs down the middle of the Atlantic, where the ocean floor divides and continuously spews out fresh lava.

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April 17, 2010

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