Professor Reidar Trønnes, who was a research scientist at the University of Iceland's Nordic Volcanological Institute from 2000 to 2004, says as eruptions go, the Eyjafjallajokull volcano is not that large. Nevertheless, concerns about the effects of volcanic ash on jet engines led to a range of airport closures in northern Europe on Friday.

Volcanic ash, which is made up of tiny glass shards that are carried aloft in a foamy mix of steam, can damage jet engines by melting right inside them and causing them to seize up. "

Residents from a number of central Norwegian cities reported the smell of sulphur in the air, and some residents in northern Norway reported finding volcanic ash on their automobiles. Trønnes says that the ash gets shot high into the air as magma that was once deep in the Earth comes to the surface and is depressurized. Any water that has dissolved in the magma comes boiling out when the magma is no longer under pressure, much the way that CO2 bubbles out of your selzer water when the cap is removed, he says. The plume coming out of the Eyjafjallajokull volcano also contains a good deal of steam because the intensely hot magma is melting the ice cap that blankets the volcano, he adds.

While the Eyjafjallajokull volcano's eruption is highly dramatic, most volcanologists like Trønnes are watching the volcano's much larger neighbour to the east, Katla. This volcano, buried under the Mýrdalsjökull glacier, Iceland's fourth largest ice sheet, usually erupts twice a century, Trønnes says, but has erupted just once in the last 100 years -- in 1918. "Katla has had two large eruptions every century since Iceland was settled 1,100 years ago," he said. "It is long overdue -- or it could mean that Katla has changed its behaviour."

Trønnes said that a number of large volcanic eruptions over the last several decades may have helped drain the vast magma reservoirs that would feed any eruption of Katla. These include eruptions as far back as one that created the island of Surtsey in 1963-1967 and one that took place on nearby Heimaey in 1973. "The fact that we have had these two large eruptions in the 1960s and 1970s may have relieved the pressure in the Katla reservoir, although this is just speculation," he said.

The Eyjafjallajokull volcano now appears to have released enough pressure that Trønnes does not expect any large-scale explosions, but the melting of the glacier caused by lava flows will continue to pose risks of potentially large and devastating floods, such as one that caused Icelandic officials to evacuate 800 people from their homes on Wednesday, April 14, he said.

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