Credit: www.fredrikholm.se

Not many had heard of the Icelandic volcano Eyjafjallajökull before its violent eruption in April 2010. But as European air traffic came to a halt for several days, the volcano's name — unfriendly though it is to non-Icelandic tongues — quickly achieved notoriety.

As with most volcanic eruptions, the explosion in April did not come without warning. Signs of volcanic unrest had accumulated over 18 years, following at least two decades of quiescence. And starting in late March, magma had been flowing from the volcano's flanks. In a grand finale, the summit exploded on 14 April and sent fine-grained tephra to altitudes of up to nine kilometres.

However, Freysteinn Sigmundsson and colleagues report that the summit eruption that caused trouble for so many European travellers was not fed by the same magma source as the earlier effusive flank eruption (Nature 468, 426–430; 2010). Nor did the surface of the volcano, as observed by satellites in the run-up to the explosion, follow the usual pattern of gradual inflation before the eruption, and rapid deflation once the lava started to emerge.

Instead, the first, effusive eruption in late March was accompanied by very little change in the shape of the surface, suggesting that most of the magma flowed from large depths. There, it had accumulated over the preceding few months as magma filled the base of the volcano in a complicated geometry. The flank eruption ceased on 12 April. But pressure continued to build up at depth and, within two days, seems to have triggered the explosive summit eruption. How exactly the triggering worked is uncertain: geochemical analyses of the lava as well as the patterns of surface deformation point to different sources for the flank and summit eruptions.

Eyjafjallajökull is a moderately active volcano, offset from the rift between the Eurasian and North American plates. As a result, its subsurface structure is relatively cold and storage of magma at shallow depths is limited. This could be the cause of the unusual deformation patterns observed that made it difficult to foresee the explosive eruption.

If so, short-term precursors of eruptions in less-active, off-ridge volcanoes may be hard to pick up more generally, because in those locations the signs of an imminent explosive eruption can be complex and subtle.