Worse to come? An ash plume (inset) from a smallish eruption of Eyjafjallajökull (above) has snarled air traffic, but the volcano is proving hard to read.

(main image)Lucas Jackson/Reuters; (inset) ESA

Iceland’s Volcano Proving Tough to Predict

Volcano prediction can be tough going, but volcanologists really have their hands full with the ongoing eruption at Iceland’s Eyjafjallajökull. When will it end? Will there be even more ash? And will Eyjafjallajökull's bigger and badder neighbor Katla join in? It seems that the very character of Icelandic volcanoes is working against reliable forecasting. If anything, the long-term outlook is bad.

Eyjafjallajökull’s orneriness became obvious 17 April, when scientists at the University of Iceland and the Icelandic Meteorological Office announced that the volcano’s chemistry had changed. At the extremes, volcanoes behave one of two ways: quietly like Hawaii’s Mauna Loa, which almost always just oozes lava for years on end; or boisterously like Mount St. Helens, which quickly rose to an explosive climax and then retired.

The latest chemical analyses of ash explain how Eyjafjallajökull switched without warning from quiet lava to plane-grounding ash plume. The key was a boost in the silica content of the magma arriving at the surface. Silica-rich magma makes for more viscous—and thus more explosive—lavas and can be produced as some minerals crystallize out of subterranean magma. The mountain, which was regularly monitored, gave no chemical warning that the switch was on the way.

Eyjafjallajökull is not divulging its longer-term intentions either. According to geophysicist Páll Einarsson of the University of Iceland, the magma feeding the current eruption seems to be coming from down deep rather than a shallow chamber. So it is impossible to gauge just how much magma could emerge during this episode of activity.

The best clues may come from the historical record. That isn’t encouraging, according to volcanologist Lee Siebert, director of the Global Volcanism Program at the Smithsonian Institution in Washington, D.C. Ominously, three of the previous four eruptions at Eyjafjallajökull have been accompanied by eruptions at its neighbor Katla, according to Siebert’s reading of the literature.

Eyjafjallajökull’s most recent eruption, in 1821, rated a moderate two on the logarithmic volcanic explosivity index. This index runs from a nonexplosive zero to an eight; Mount St. Helens was a five. But Katla has produced 208 ash layers in the past 8400 years, some eruptions rating a four or even a five. Icelanders have a close eye on Katla, Einarsson says, but no sign of stirrings yet.