Slumbering volcanoes keep scientists on their toes, as questions about if and when they will erupt never cease. Eyjafjallajökull, the Icelandic volcano that threw European aviation into a tailspin last spring, is a prime example of how volcanologists were aware that something was brewing inside the mountain that had lain dormant for two centuries. New research suggests that the culprit responsible for waking Eyjafjallajökull from its sleep was the magma flowing beneath it.

Writing in the journal Nature, the volcanologists from Iceland, the Netherlands, Sweden and the US say that the eruptions are the culmination of 18 years of intermittent volcanic unrest. Using seismic monitoring and GPS (global positioning system) data, as well as satellite, radar and surface measurements, they assessed geophysical changes in Eyjafjallajökull, particularly as the volcano's edifice began to deform. According to the team, the volcano ballooned for almost three months before it started to erupt in March from one flank causing one of the biggest crisis in European transport ever.

Led by Dr Freysteinn Sigmundsson from the Nordic Volcanological Centre at the University of Iceland, the team began a vigilant watch on the mountain in late summer 2009 after a subtle shift emerged at a GSP station on Eyjafjallajökull's flank. By early 2010, the researchers noted a hike in the rate of deformation and the number of earthquakes. As the deformation and shocks failed to abate, the scientists installed more GPS stations near the mountain. Flash-forward a few weeks, and more rapid inflation surfaced. The researchers understood that magma was moving upwards through the 'plumbing' inside the volcano.

When Eyjafjallajökull started to erupt in late March, its flanks had swelled by more than around 15 centimetres as magma flowed from deep within the Earth into shallow chambers underneath the mountain, according to the researchers.

Once the eruption began, the deformation ceased. But Eyjafjallajökull differed in that instead of deflating as the magma flowed - which is the norm for volcanoes - it remained inflated until mid April, when the first eruption ended.

The volcano erupted again on April 22, and the lava flowed through a new conduit under the ice on the summit of the mountain. Water flashed to steam and gas escaped from bubbles in the magma, creating an 'ash plume' that climbed high up in the sky and triggering huge headaches for all travellers across Europe.

While the results helped shed light on the Icelandic phenomenon, the researchers say more testing is needed to determine why volcanoes erupt when they do because questions remain over the geological processes that kick-start an actual eruption.