It left the skies over Britain clear of aircraft trails for the first time in decades and led to travel misery. For several days in April, flights in northern Europe were grounded as a cloud of ash and dust spewed out of the Eyjafjallajökull volcano in Iceland and were blown around the continent.

Now, the geological events leading up to the eruption have been documented, and they show that the volcano had been rumbling for many months before it blew its top. In a paper published today in Nature, Freysteinn Sigmundsson, of the Nordic Volcanological Centre at the University of Iceland, describes how he led an international team to use a combination of GPS, seismic monitoring and satellite radar interferometry to track years of deformations and volcanic activity on and around Eyjafjallajökull.

Sigmundsson found that for 11 weeks before the volcano began erupting in March, one flank was swollen by more than 15cm (6in). Magma had been flowing from deep underground into shallower compartments under the mountain.

Kurt Feigl, a professor of geosciences at the University of Wisconsin-Madison and a co-author of the study, said the volcano had been showing signs of restlessness. "Several months of unrest preceded the eruptions, with magma moving around downstairs in the plumbing and making noise in the form of earthquakes," he said. "By monitoring volcanoes, we can understand the processes that drive them to erupt."

The deformation of the Earth's crust around Eyjafjallajökull, and the resulting small earthquakes, began to increase in January. A few weeks later, sensors and GPS stations began detecting rapid expansion of the mountain.

The first eruption, caused by magma flowing into the mountain from underneath, began on 20 March. It continued for three weeks before pausing for two days and then resuming on 22 April.

The second time around, the erupting lava punched through the ice at the top of the mountain. The water exploded into steam and rapidly cooled the magma, which is a mixture of molten rock and various solid impurities, and normally circulates under the Earth's crust. The magma turned into a fine-grained dust cloud that rose high into the atmosphere and was blown around the whole of northern Europe. Sustained, highly variable activity continued until 22 May, with an average of 30,000-60,000 litres (6,600-13,200 gallons) of magma coming out every second.

Normally, when volcanoes erupt, they deflate as the magma drains out. But for some reason, Eyjafjallajökull kept its shape after the first eruption.

The researchers suggested that this could be because of a limited supply of magma in the first place, and the position of the volcano. The volcanoes of Iceland are the surface peaks of the Mid-Atlantic Ridge, but Eyjafjallajökull lies at some distance from the main rift zone. This means less heat from magma reaches it than reaches volcanoes nearer the rift zone.

The eruption was probably started by an intrusion of magma deep inside the volcano, though this is something that needs to be confirmed at other volcanoes.

The researchers stress that study of the events leading up to the eruption will not necessarily help to predict future events. "We're still trying to figure out what wakes up a volcano," said Feigl.

"The explosiveness of the eruption depends on the type of magma, and the type of magma depends on the depth of its source. We're a long way from being able to predict eruptions. But if we can visualise the magma as it moves upward inside the volcano, then we will improve our understanding of the processes driving volcanic activity."