A subterranean magma chase
On August 16, 2014, a monitoring network detected an intense swarm of earthquakes beneath the Bárðarbunga volcano, which is buried under the vast Vatnajökull ice cap. This was the start of a remarkable geological event. Instead of rising vertically, a blade-like intrusion of magma, called a dyke, began to travel horizontally underground. For two weeks, scientists tracked this subterranean magma flow in near real-time. Networks of seismometers and GPS stations followed the magma's progress as it fractured rock, generating tens of thousands of earthquakes.
The dyke propagated over 45 kilometers to the northeast, extending from beneath the glacier to the Holuhraun lava field at a depth of 5 to 8 kilometers. The GPS data showed ground displacement of up to 14 centimeters, confirming the massive volume of magma being injected into the crust. This event provided a rare opportunity to observe the mechanics of a rifting episode—the process by which tectonic plates pull apart and create new crust. While the dyke was advancing, several small subglacial eruptions occurred along its path, creating depressions in the ice cap called cauldrons.
Eruption and collapse
The magma finally breached the surface on August 29, 2014, starting a fissure eruption in the Holuhraun lava field, just north of the glacier's edge. This eruption lasted for six months, ending on February 27, 2015. It produced a spectacular display of lava fountains and flows, ultimately creating a new lava field covering more than 85 square kilometers. The total volume of erupted lava was about 1.4 cubic kilometers, making it Iceland's largest eruption since 1783.
As magma drained from the reservoir deep beneath Bárðarbunga, the volcano's 110-square-kilometer ice-filled caldera began to collapse. The collapse was not a violent explosion, but a gradual subsidence. Over the six months of the eruption, the caldera floor sank by as much as 65 meters. The event was one of the largest and best-monitored caldera collapses in modern history. The eruption also released enormous quantities of gas, primarily sulfur dioxide (SO2). At its peak, the eruption emitted up to 120,000 tons of SO2 per day, more than all of Europe's industrial emissions combined, causing air quality alerts across Iceland and parts of Europe.