Iceland today raised its aviation alert for the restless Bardarbunga volcano to red, which is the highest warning on the country's five-point scale, indicating that some type of an eruption was either imminent or in progress.
Thousands of small earthquakes have rattled the volcano deep beneath Iceland's Vatnajokull glacier in the last week, with activity picking up today after a lull the day before.
Scientists had planned to fly over the glacier later today to look for changes on the surface but it was not clear if that would still take place.
Earlier this week authorities had evacuated several hundred people from highlands north of the Vatnajokull glacier as a precaution. The area is uninhabited but popular with hikers.
A 2010 eruption of the Eyjafjallajokul volcano produced an ash cloud that caused a week of international aviation chaos, with more than 100,000 flights cancelled. Aviation regulators since have reformed policies about flying through ash, so a new eruption would be unlikely to cause that much disruption.
Seismologists say magma is moving under the glacier but so far has travelled horizontally at a depth of 5 to 10 kilometers (3 to 6 miles). The volcano will erupt if the magma rises and melts the ice above.
Met Office vulcanologist Melissa Pfeffer said the amount of ash produced would depend on the thickness of the ice.
"The thicker the ice, the more water there is, the more explosive it will be and the more ash-rich the eruption will be," she said.
Iceland sits on a volcanic hot spot in the Atlantic's mid-oceanic ridge and eruptions have occurred frequently, triggered when the Earth's plates move and when magma from deep underground pushes its way to the surface.
