The last great eruption of Fagradalsfjall reveals its secrets


The volcanic eruption of Fagradalsfjall 2021 in Iceland has provided new evidence to help volcanologists understand the processes that led to the eruption and for future monitoring of volcanic activity.

The Fagradalsfjall volcano is located on the Reykjanes peninsula, about 40 kilometers from Reykjavík, Iceland. Previous volcanic activity on the Reykjanes Peninsula over the past 3,000 years has been characterized by eruptive periods of 200–300 years, typically separated by 800–1,000 years of dormancy.

The 2021 eruption began on March 19, after about 800 years of quiescence, and was preceded by several weeks of increased seismic activity and surface deformation that declined uncharacteristically in the days leading up to the eruption. Initially the eruption had low magma flow and minimal lava flows, but towards the end of April the magma flow increased and a large lava fountain was observed.


Eruption precursors

The details of this new eruption are now analyzed in a pair of articles published this Wednesday in the journal Nature. In the first, Freysteinn Sigmundsson, Michelle Parks and their colleagues investigated eruption precursors. Ground movement rates and the number of earthquakes increase before many eruptions as magma rises to the surface.

Although the 2021 eruption in Iceland was initially preceded by an increase in seismic activity and surface deformation between February 24 and mid-March, a decrease in deformation and seismicity has been observed for several days just before the rash. The authors propose that the forces are stored in the earth’s crust before eruptions, due to the movements of the plates that cover the surface of the earth.

Prior to eruptions, these forces may be released as magma enters the Earth’s crust, and the subsequent decrease in seismic activity and ground deformation may mean that this process is temporarily coming to an end and magma will erupt. . The results demonstrate that the interplay between volcanic processes, tectonic stress and crustal composition must be considered when predicting eruptions, the authors conclude.

The source of the Fagradalsfjall

In the second paper, Sæmundur Halldórsson and colleagues examined lava ejected during the first 50 days of the eruption. These analyzes revealed a direct source of magma from the boundary between the Earth’s crust and the mantle (the area near Moho). The authors note that the erupting lavas have changed over time; during the initial phases of the eruption the lava came mainly from near the crust-mantle interface, but over the following weeks the composition changed, indicating that it came from magmas generated at greater depths.

These results demonstrate that the magma storage area near Moho is an extremely dynamic environment, with magma mixing occurring on incredibly short time scales (days to weeks). This shows us how quickly magma bodies can form in real time.

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