Exclusive photos: Research reveals details about Iceland's ‘silent’ volcano eruption

Scientists from the University of Iceland and the Icelandic Meteorological Office, Reykjavik, have presented unexpected observations of seismic activity and magma movements before and during the 2021 Fagradalsfjall volcanic eruption, according to a pair of papers published in Nature journal on Wednesday.

The insights could provide a boost in understanding the processes that drove the unusually 'silent' eruption and for future monitoring of volcanic activity. This is critical for creating warnings to prevent loss of life and damage to infrastructure.

Revealing the unusually 'silent' precursors of the Fagradalsfjall volcano

In one paper, Freysteinn Sigmundsson, Michelle Parks, and colleagues investigated the precursors to the eruption. The study is critical because, before many eruptions, volcanoes exhibit an increase in activity, i.e., an increase in the number of earthquakes and an increase in surface deformation. Both of which are related to magma forcing its way up through the shallow crust.

"Prior to the 2021 eruption at Fagradalsfjall, we, in fact, observed the opposite. Both a decline in seismicity and deformation in the few days before the eruption onset," volcanologist Dr. Michelle Parks, lead author of the study, tells Interesting Engineering (IE).

New considerations for forecasting volcanic eruptions

The researchers in this paper propose that forces (stresses) are stored in the crust of the Earth prior to eruptions due to movements of the plates covering the surface of the Earth.

As the stored tectonic stress was released during the magma intrusion and associated earthquakes, there was less magma migration in the lateral (sideways) direction. This resulted in an observed decline in seismicity and deformation.

Essentially, the magma was forced to travel higher in the crust, which reduced the driving pressure and magma inflow rate. "The upper 1 km of the crust here is weak, so the magma was able to move towards the surface in a relatively silent manner, without any further increases in activity prior to the eruption onset," explains Parks.

The findings demonstrate that the interaction between volcanic processes, tectonic stress, and crust composition needs to be considered when forecasting eruptions, the authors conclude.

Still, "precursors will be different for different volcanoes." Dr. Michelle Parks reveals to IE.

"Before evaluating pre-eruptive trends, it is important to realize that the stress field is not static. [It] will evolve based on additional magma intrusions and earthquakes, which may affect the precursory activity and whether or not a dike intrusion culminates in an eruption. Further research is required in these areas," adds Parks.

A unique look into the generally inaccessible depths of the volcanic system

In another paper, Sæmundur Halldórsson and colleagues examined lava expelled during the first 50 days of the eruption. These analyses revealed direct sourcing of magma from the boundary between the Earth’s crust and the mantle (the near-Moho zone), according to the press release published by Nature.

The researchers behind the paper explain that this feature sets the eruption apart from most others globally. Usually, the magma is modified by prolonged storage in shallow levels of the crust, and so this leads to a loss of information about the deepest parts of the volcanic system.

Their work, therefore, "stands out" because Fagradalsfjall provides a high-speed connection to the base of the crust like never before and offers an opportunity to investigate the generally inaccessible depths of such a system.

And that's not all. The researchers note that the composition of the erupted lavas changed over time. The lava was predominately from near the crust-mantle interface during the initial phases of the eruption. While over the subsequent weeks, the composition suggested lava was being sourced by magmas at greater depths.

An extremely dynamic environment in the near-Moho magma storage zone

These findings demonstrate that the near-Moho magma storage zone is an extremely dynamic environment, with the mixing of magmas occurring on incredibly short timescales (days to weeks). This shows us how fast magma bodies can be formed in real-time.

However, the authors point out that this was the first opportunity that researchers have had to make these real-time observations of the deepest parts of a volcanic system.

Therefore, they cannot be sure how representative the processes they observed at Fagradalsfjall are of other systems in Iceland and the rest of the world.

A brief history of the Fagradalsfjall volcano, Reykjavík, Iceland

After around 800 years of dormancy, the Fagradalsfjall volcano, located on the Reykjanes Peninsula around 40 kilometers from Reykjavík, Iceland, erupted on March .19, 2021.

The eruption was characterized by an eruptive fissure opening up in the Geldingadalir valleys. Still, what's most peculiar about the eruption was that after weeks of increased seismic activity, there was an unusual decline in seismic activity and surface deformation.

Previous volcanic activity on the Reykjanes Peninsula in the last 3,000 years has been characterized by eruptive periods of 200–300 years, usually separated by 800–1,000 years of dormancy.