A major eruption in Grimsvotn - bigger than Eyjafjallajokull

The eruption that started in Grimsvotn under the Vatnajokull glacier last Saturday evening is categorized as a major eruption at this very active eruption site. It is many times bigger than the last eruption in Grimsvotn or the eruption in Eyjafjallajokull last year.

For the first twelve hours the force of the eruption was ten times that of the last eruption in Grimsvotn in 2004, and the Eyjfjallajokull eruption last year, according to Magnus Tumi Gudmundsson, professor of geophysics at the University of Iceland.

He says it is difficult to predict how long the eruption will last. Considering the history of eruptions in this volcano, however, the major force of the eruptions should have dwindled after a few days, says Magnus. The volcano could continue to seethe for a longer time, but the impact should not be great outside the Vatnajokull glacier. "This is simply the nature of these eruptions. Most eruptions are most powerful in the beginning and then they dwindle until they stop," says Gudmundsson.

"In all likelihood the worst will be over in a few days and the volcanic activity that remains will only affect the glacier," he says.

The eruption in Grimsvotn is from the same crater as the 2004 eruption. Gudmundsson says that the erupting fissure is not long, no more than a few hundred metres.

The eruption did not come as a surprise to geophysicists, because the seismic activity in the area has been very much like the one before the eruption of 2004, said Steinunn Jakobsdottir, geophysicist at the Icelandic Meteorological Office, at an informational meeting with ambassadors from foreign countries yesterday. "You could say that we have been expecting this eruption to start at some point during the next few months," said Jakobsdottir.

The eruptions in Grimsvotn are always phreatic or explosive eruptions that can hurl volcanic materials to high altitudes. This happens when alkaline magma makes contact with water under the glacier, causing an explosion.

Eruptions in Grimsvotn are usually relatively small, compared with other eruptions. They are often, however, accompanied by substantial ash fall. The ash fall is expected to be great in the beginning, until the rim of the crater has risen sufficiently to prevent water from getting in contact with the magma, says Jakobsdottir. After that the lava is expected to flow without much ash fall, as was the case in the eruption on Fimmvorduhals last year.

"Usually most of the ash falls on the Vatnajokull glacier. It is only in larger eruptions that the ash fall is substantial outside the glacier," says Magnus Tumi Gudmundsson. "The current eruption is categorized as a major eruption in Grimsvotn, much larger than those of recent years."