Studies provide new insights into unusual 2021 Fagradalsfjall eruption – the first eruption on the Reykjanes Peninsula after 800 years of dormancy

Scientists from the University of Iceland, the Icelandic Met Office (IMO) and their colleagues have published two papers in the latest issue of Nature, presenting new findings from the 2021 eruption at Fagradalsfjall—the first eruption on the Reykjanes Peninsula after 800 years of dormancy.

• The studies show that the precursors to the eruption were unusual compared to many other eruptions across the world and that the composition of the lava changed as the eruption continued.

As reported by the IMO¹, researchers closely observed the seismic activity on Reykjanes Peninsula in SW-Iceland, which began in December 2019, culminated with the eruption on March 19, 2021 and continued for around half a year. An extensive network of measuring stations and the proximity with towns in southwest Iceland enabled scientists to follow developments very closely.

One of the papers – titled “Deformation and seismicity decline before the 2021 Fagradalsfjall eruption” -discusses the precursors to the eruption and how they differ from the precursors of many other eruptions around the world.

There was a significant earthquake activity on the Reykjanes Peninsula in the weeks leading up to the 2021 eruption, marked by tectonic stress release in the crust.

However, for several days before the eruption, deformation and seismic activity declined in the area around the eruption site.

This precursory pattern is different compared to precursors to many other eruptions around the world, which often show escalating rates of ground displacement and seismicity shortly before the eruption onset, as the magma forces its way to the surface.

The scientists behind the paper explain that the behaviour at Fagradalsfjall was caused by the interplay between magma flow and plate tectonic stress.

As magma forces its way through the crust before an eruption, tectonic stress may be released, causing earthquakes and ground deformation in the early stages. A decline in seismicity and deformation may indicate that this process is coming to an end and that the magma may erupt.

During the roughly three-week period preceding the eruption at Fagradalsfjall, there was both considerable surface deformation and a large number of earthquakes. This was caused by the emplacement of a vertical magma-filled dyke between the surface and a depth of 8 km (5 miles). At the same time, tectonic stress in the crust was released. Earthquakes occurred in nearby areas, up to magnitude 5.6.

The scientists also suggest that the decline in seismicity in the days before the eruption could be explained by the fact that the magma had then almost reached the surface, where the crust is weakest and there is therefore less resistance.

It also shows that consideration must be given to the relationship between volcanoes and tectonic stress in eruption forecasting. A release of tectonic stress followed by a decline in deformation and seismicity rate may be a characteristic precursory activity anticipated for a certain class of eruption.

The second paper – “Rapid shifting of a deep magmatic source at Fagradalsfjall volcano, Iceland” – discusses the changes to the composition of the lava that flowed through Geldingadalir and the surrounding area as the eruption continued.

Scientists sampled the lava frequently during the first 50 days of the eruption and measured the volcanic gases around the eruption site and revealed that the lava at Fagradalsfjall was directly sourced from a magma reservoir at great depth, at the boundary between the crust and the mantle – the near-Moho zone.

Eruption directly from the near-Moho zone has not been observed in other eruptions that have been the subject of focused investigation in real time. In these previous cases, the magma has come from shallower levels in the crust. Until now, there has therefore been a lack of information about the deepest parts of magmatic systems, but the eruption at Fagradalsfjall has provided the scientific community with new knowledge of the processes involved.

At the start of the eruption, the lava was relatively rich in magnesium in comparison with lava from other historical eruptions in Iceland, indicating an unusually hot magma supply.

There was also a lot of carbon dioxide in the volcanic gases emitted from the eruption vent, indicating an unusually deep magma supply.

The scientists explain that this suggests that the magma underwent little cooling on its way up through the crust to the surface.

The use of different geobarometers, that measure the equilibration pressure and temperature of the magma and its crystals, suggested that the eruption was sourced from a magma reservoir close to the crust-mantle boundary, around 15 km (9 miles) from the surface.

This means that the eruption was fed by something like a high-speed link direct to the mantle.

The research also revealed that the composition of the lava at Fagradalsfjall radically changed as the eruption progressed, changing in composition from shallowly derived compositions to deeply derived compositions. This suggests that during the eruption, different magma flowed into the reservoir during the eruption that was generated at greater depths than the magma already present in the reservoir.

The scientists point out that it has long been argued that different kinds of magma can mix deep in magmatic systems before an eruption, but this study presents real-time evidence that these processes do occur.

Furthermore, changes to the composition of volcanic products show that new magma can flow into a deep reservoir rapidly, in a timescale of around 20 days, mixing with the magma already in the reservoir and potentially triggering the eruption.

These findings may aid our understanding of volcanoes and the geochemistry of the mantle and could support the development of models of magmatic systems all over the world.

References:

¹ Fagradalsfjall eruption unusual in many ways compared to other eruptions – IMO – September 14, 2022

² Deformation and seismicity decline before the 2021 Fagradalsfjall eruption – Freysteinn Sigmundsson et al. – Nature 2022 – DOI https://doi.org/10.1038/s41586-022-05083-4 – OPEN ACCESS

³ Rapid shifting of a deep magmatic source at Fagradalsfjall volcano, Iceland – Sæmundur A. Halldórsson et al. – Nature 2022 – DOI https://doi.org/10.1038/s41586-022-04981-x – OPEN ACCESS

Featured image credit: IMO