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