Rapid magma flow beneath Grindavik

Rapid magma flow beneath Grindavik

Lava eruption on the Reykjanes Peninsula in Iceland. © Meghan McCollum/ iStock

From December 18, 2023, numerous lava eruptions occurred in the immediate vicinity of the small Icelandic town of Grindavik. Thanks to early warnings, the residents were able to be evacuated a month earlier. However, the destruction of the place could only be partially prevented. Researchers have now examined the geological and physical processes leading up to the eruptions. In a roughly 15-kilometer-long magma tunnel below Grindavik, the hot rock flowed at a speed of 7,400 cubic meters per second – the highest magma flow rate recorded to date.

Iceland lies on the boundary between the North American and Eurasian tectonic plates and is known for its volcanic activity. From October 24, 2023, there were increased earthquakes in the region around the small town of Grindavik, around 50 kilometers southwest of the capital Reykjavik. The cause was a 15 kilometer long magma tunnel beneath the city, which quickly filled with magma, expanded and came closer to the surface. As a precautionary measure, the city’s population was evacuated on November 10th. On December 18th, the molten rock made its way to the surface and a violent fissure eruption occurred. The eruptions only stopped again three days later. However, on January 14, 2024, renewed eruptions destroyed parts of Grindavik. It is still unclear whether people will be able to return to their city.

Ongoing volcanic activity

A team led by Freysteinn Sigmundsson from the Nordic Volcanological Center at the University of Iceland in Reykjavik has now analyzed the geological and physical processes from the start of the earthquakes in October 2023 in more detail. To do this, the researchers evaluated seismic measurements and satellite data on the deformation of the ground. They also modeled how fractures and tectonic stresses caused the magma tunnel to form and fill in the eruption area.

“The events are part of an ongoing period of volcanic-tectonic activity on the Reykjanes peninsula, where Grindavik is located,” explain Sigmundsson and his team. “Seismic activity increased dramatically on October 25, 2023, followed by a period of subsurface magma accumulation. Modeling suggests a magma deposit at a depth of around five kilometers, which expanded by an average of 7.5 cubic meters per second.” This resulted in a 15-kilometer-long magma tunnel that reached under the city of Grindavik.

How the magma made its way

“According to our modeling, the maximum flow rate at which the magma pushed into the tunnel was 7,400 cubic meters per second and was reached on November 10th,” reports the team. This means that the maximum flow velocity is up to a thousand times higher than in previous eruptions in the region. “Such high flow velocities provide insight into the formation of large eruptions and indicate a serious potential risk that the underground rock flows will spread to the surface and turn into eruptions.” Up until now, the pressure in the underground magma reservoir was considered to be the determining factor for such events. If it becomes too large, the common assumption is that the liquid rock will make its way through the surrounding rock layers, creating tunnels and passageways.

However, in the magma body that feeds the magma tunnel under Grindavik, the overpressure was relatively small in relation to the forces that the magma developed. The current study now provides an explanation as to why such large eruptions could still form. Accordingly, tectonic tensions had built up in the surrounding rock over a long period of time. When a path opened up at the boundary of the magma body, the rock layers quickly developed large cracks through which the magma could spread further. “Our results can also help to better understand magmatic activity in other parts of the world,” say the researchers.

Source: Freysteinn Sigmundsson (University of Iceland, Reykjavik) et al., Science, doi: 10.1126/science.adn2838

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