Months before Eyjafjallajökull blew its top, volcanologists knew something big was going on. So why was the world so unprepared and could it happen again, asks DICK AHLSTROM

THE ICELANDIC volcano that threw European aviation into chaos last April and May had sat quietly for 200 years. Yet volcanologists knew weeks before Eyjafjallajökull actually erupted that something big was going on inside the mountain.

It had experienced 18 years of intermittent volcanic unrest but things really started to change by late summer 2009, according to a comprehensive new analysis of what happened deep under Eyjafjallajökull.

Scientists from Iceland, Sweden and the Netherlands have spent months pouring over older records and also data from 2009 and 2010 prior to the eruption. They believe they can now tell the full story of how Eyjafjallajökull woke from centuries of slumber to cause the biggest disruption to European air transport since the second World War.

The details are found in a letter published in Naturethis morning by lead author Dr Freysteinn Sigmundsson of the Nordic Volcanological Centre at the University of Iceland and colleagues.

They had decades of GPS data, satellite, radar and surface measurements and seismic monitoring data available as they kept an 18-year vigil on Eyjafjallajökull.

Then in the latter half of 2009 a GPS station on the side of the mountain sent back a reading suggesting something bigger was happening. They quickened the pace of observations, adding more recording devices.

“If you watch a volcano for decades you can tell when it is getting restless,” stated co-author Dr Kurt Feigl of the University of Wisconsin-Madison. Parts of the dormant volcano began to swell, a sure sign that liquid rock or magma was percolating into chambers under the mountain.

Its sides swelled by more than five millimetres per day from early March 2010, finally ballooning out 15 centimetres before the first eruption occurred on March 20th.

The scientists were puzzled because Eyjafjallajökull did not then begin to shrink back down as the magma flowed, something most volcanos do, the authors noted.

The routine eruption stopped flowing on April 12th but then just two days later the second eruption hit, the one that would cripple European air travel. “The second eruption occurred within the ice-capped caldera [central crater] of the volcano, explosively amplified by magma-ice interaction,” the authors write.

Water from the snow flashed to steam and gas escaped from bubbles in the magma to blast an ash plume high into the atmosphere. Eyjafjallajökull belched out an estimated 250 million cubic metres of ash and pumice up to nine kilometres high.

The eruption certainly set no records but the unusual weather conditions at the time sent the ash first out across Northern Europe and then back across to cover Central Europe, Britain and Ireland.

The ash cloud ebbed and flowed during the next month, at times reaching as far south as Italy, Spain and Portugal. Much of the European air space was shut down on safety grounds, as airlines and aircraft manufacturers struggled to understand how dangerous flying through the ash might be. Initially it proved safer simply not to fly at all, something that unfortunately also meant stranded passengers right across Europe.

By May 21st it was all over, the eruption ceased and the ash cloud blew away on the wind. Flights returned to normal and for most of us the cloud became a distant memory. But the volcanologists have been left to maintain their ongoing vigil on Eyjafjallajökull.

It is quiet but not silent. A report on June 23rd noted occasional small ash clouds venting from the mountain that last for some minutes before ceasing.

The scientists have also been left to ponder the volcano’s unusual behaviour in not shrinking quickly during its eruption. They believe this could be because only small amounts of magma built up in chambers within the mountain and were replenished from deeper down.

The bigger question from a European perspective is what happens next? Will we see another ash cloud? The authors say they know when something is wrong but knowing when an eruption will begin is another matter.

“Clear signs of volcanic unrest over years may indicate the reawakening of such volcanoes, whereas immediate short-term eruption precursors may be subtle and difficult to detect,” they say.