Two years ago, Europe experienced its largest volcanic explosion for 240 years. Now, a real-time study of the event has provided unprecedented information about the formation of calderas, the poorly understood structures some volcanic eruptions leave behind.
The Bardarbunga eruption in Iceland got a fraction of the global attention of its neighbor Eyjafjallajökull four years earlier, because it did not cause the same disruption to air travel. However, it actually released 10 times as much volcanic material – up to two cubic kilometers (half a cubic mile) – as its more famous predecessor.
Calderas form when a magma reservoir within a volcano empties. Material above it collapses, leaving a depression that in some cases fills to become a lake. Calderas have been seen on other planets and moons, and can be the size of large cities. Only six have formed since 1900, and since it's not exactly safe to watch their formation up close, there is still a lot about them we don't know, including the question of whether the collapse causes the reservoir to empty, or vice versa.
According to Gudmundsson, Bardarbunga's magma reservoir lay 12 kilometers (7.5 miles) beneath the surface. Instead of being forced straight up, the magma breached a wall in its containing dike, flowing beneath the Earth for 45 kilometers (28 miles) until it found a place where it could come to the surface. The subsidence of the rock layer above triggered 77 earthquakes of magnitude 5 and above.
The team's observations indicate that the magma's escape preceded the caldera's formation, rather than being caused by it, but a single example is insufficient to determine if this is always the case.
What Bardarbunga was doing above the surface was nothing compared to what was happening below ground. Peter Hartree via Wikimedia Commons. cc-by-sa-2.0
Over the course of six months, an already deep bowl – 11 kilometers (7 miles) at its widest extent – sunk 65 meters (213 feet). The ground originally sank 1 meter (3.3 feet) a day, slowing through the 181 days the eruption continued. “With an area of 110 square kilometers (42 square miles), this is the largest caldera collapse ever monitored. The results provide the clearest picture yet of the onset and evolution of this enigmatic geological process," said co-author Dr Eoghan Holohan of the GFZ German Research Center for Geosciences in a statement.
The authors tracked the magma's flow by observing distortions in seismic waves passing through the Earth. They observed that the river of escaping magma from the volcano’s chamber was an impressively cohesive system: Shocks either at the chamber or where magma reached the surface propagated all the way to the other end of the lava flow.
Living up to Iceland's reputation as the original land of ice and fire, the Bardarbunga caldera lies beneath the Vatnajokull glacier, Europe's largest, causing the caldera to fill with ice. "The event was a blessing in disguise as the eruption could have happened directly beneath the ice. In that case, we'd have had a water vapor explosion with a volcanic ash cloud even bigger and longer lasting than the one that followed the eruption of Eyjafjallajokull in 2010," said co-author Dr Thomas Walter of the GFZ.
The work's publication was part of a special issue in Science on natural hazards. By the standards of other observed caldera formations, Bardabunga was exceptionally shallow. While covering a larger area than any we have previously witnessed in formation, it is tiny compared to supervolcanoes such as those at Yellowstone.