Over the last seven years, seismic activity at Bárðarbunga, the second highest mountain in Iceland, has been gradually increasing. On August 16, a swarm of small earthquakes signaled the movement of molten rock underground, Nature reports, and visitors were blocked from the area. Less than two weeks later on August 28, researchers flying over Vatnajökull—Europe’s largest ice cap—spotted several depressions up to 15 meters deep in the side of the volcano, BBC reports. Called cauldrons, they’re the result of melt occurring at the base of the ice.
Finally, just after midnight local time on August 29, a fissure near Bárðarbunga erupted, sending gases and steam from small lava fountains into the air above the glacial ice north of the caldera in the volcano’s crown.
The eruption occurred on an old volcanic fissure on the Holuhraun lava field, about five kilometers north of the Dyngjujökull ice margin, according to a joint report from the University of Iceland and the Icelandic Met Office, which oversees volcanoes in the country. The active fissure was about 600 meters long.
Seismic data and images from a webcam called Mila, located northeast of the site, show that the eruption peaked about 40 minutes in. It ended about four hours later. No volcanic ash was observed, and no plume was detected by radar. The threat to aviation has since been reduced to code orange. To the right is a 1973 satellite image of the Vatnajökull ice cap from NASA, with Bárðarbunga at its northwestern edge (top left).
After the swarm of small earthquakes, 0.4 cubic kilometers of magma formed a sheet of freshly cooled rock (an intrusion called a dike) that stretched for 45 kilometers north of Bárðarbunga, Nature reports. The dike interacted with cracks leading toward a volcano called Askja 20 kilometers away. If the dike had managed to make it all the way to Askja, the stress and supply of fresh magma could have caused it to erupt.
Furthermore, the sheer volume of magma involved suggests that it’s coming from the Earth’s mantle. The source is likely hundreds of kilometers below the surface of the crust, says British Geological Survey’s Evgenia Ilyinskaya, and not the shallow magma chamber beneath the volcano.
Check here for updates from the Icelandic Met Office.
Images: Icelandic Coast Guard (top) & NASA (middle) via Icelandic Met Office