In August 2014, a record wave of seismic activity started at the subglacial volcano Bardarbunga, one of Iceland's largest caldera volcanoes. By the end of the month, an eruption had started, but not at the caldera itself, but at Holuhraun, a nearby ice-free lava field.
By late September, lava was still flowing at Holuhraun, and seismic activity at Bardarbunga was still intense. Numerous earthquakes greater than M5.0 had struck, and the volcano's caldera had subsided by almost 30 meters, as magma migrated from underneath the volcano towards the eruption at Holuhraun. Scientists still did not rule out that another eruption might start at the caldera itself, with ensuing glacial melting, flooding and ashfall. This video explains the powers at play beneath the surface.
4500 million years ago, soon after our solar system was created, earth was a ball of fire, a melting pot of molten rock.
Little by little, the surface cooled and the earth's crust was formed. Deep underground, even today, the temperature is still extremely high. Matter migrates towards the surface. As it gets closer to the surface, it starts to melt, forming plumes of magma which keeps migrating upwards, either where the temperature underground is abnormally high (called hotspots), or where earth's tectonic plates meet. Iceland is on such a plate boundary, but it's also located on top of a hotspot. Therefore, eruptions happen more frequently in Iceland than in most other places.
The center of Iceland's hotspot is located underneath the area around Bardarbunga volcano. The subglacial volcano seems relatively innocent when seen covered by ice. But take the icecap away, and we see Iceland's largest volcano and one of its most active. The distance between the caldera's ridges is ca. 7 km (4 mi), equal to the length of over 60 soccer fields.
Under the mountain, molten rock flows up deep within the earth, gathering in a huge chamber thought to be under the volcano.
In recent years, pressure has been increasing underneath. The land has risen, but eventually something has got to give. Like water, magma seeks the easiest route, and instead of breaking up through the chamber, it flows to the side, into a fissure. This flow of magma, also called a dike intrusion, has been growing longer and longer, and now reaches 40 km (25 mi) north from the volcano. Underneath the 200-year-old lava field Holuhraun, the flow stopped, and the magma broke up to the surface, starting a lava eruption, with no ashfall.
What happens next is uncertain. Many large earthquakes have struck Bardarbunga, and scientists say they cannot rule out an eruption at the volcano. If that happens, the eruption would have to melt through ice up to 850 m (0.5 mi) thick, resulting in flooding and ash being produced. As of yet however, nothing indicates an imminent eruption at Bardarbunga.