Iceland said heightened alert levels for airlines may be in place for months as one of its biggest volcanoes continues to rumble, threatening an eruption and ash cloud that would menace flights across the Atlantic.

“It could be weeks and months” before the threat passes, said Melissa Anne Pfeffer, an atmospheric volcanologist at the Icelandic Met Office, which has an “orange” aviation warning in place, its second-highest level.

The volcano is now in its second week of rumbling as magma has spread to form a dike under the Dyngjujokull glacier, north of the volcano. Bardarbunga, which lies underneath Vatnajokull, Europe’s largest glacier, has been rocked by earthquakes that have led to evacuations and road closures.

The agency on Aug. 23 raised its alert level to “red” after mistakenly issuing an eruption warning. Airlines are on alert on concern they may face a repeat of disruptions seen in 2010 when the Eyjafjallajokull spewed a column of ash 9 kilometers (5.6 miles) into the air. That eruption shut airspace across western Europe for six days, forcing carriers to cancel more than 100,000 flights. Ash is a danger to jets because the glass-like particles can damage engines.

Explosivity Index

“This volcano — most of its past eruptions — have been between 2 and 4 on the volcano explosivity index,” said Pfeffer. “Between this range we can expect anywhere between an infusive lava eruption, where we have ash only reaching a couple of hundred of meters, to at the extreme end it could be up to 15 kilometers.”

Thousands of earthquakes have been measured at the volcano since the Met Office started detecting what it calls “anomalous seismicity” on Aug. 16. Some 600 tremors were detected yesterday, most at a depth of 8 to 12 kilometers, according to the latest Met Office status update.

The dike is now 40 kilometers long. The magma underneath Vatnajokull has been migrating northwards in a dike which extends beyond the borders of the glacier.

An eruption there “would most likely produce an effusive lava eruption with limited explosive, ash-producing activity,” according to the Met Office. That doesn’t prevent an eruption from disturbing airline passengers.

“In the initial phases of an eruption, even if the lava is reaching the surface outside of the glacier, that’s still a very wet area,” said Pfeffer. “It’s a flood plain and it’s very wet so there’s still a lot of water available to make the eruption explosive.”