Unexpected observations of seismic activity and magma movements before and during the 2021 Fagradalsfjall volcanic eruption in Iceland are presented in a pair of papers published in Nature. The insights have implications for understanding the processes that drove the eruption and for future monitoring of volcanic activity.
Fagradalsfjall volcano is located on the Reykjanes Peninsula, around 40 kilometres from Reykjavík, Iceland. 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. The 2021 eruption began on 19 March, after around 800 years of dormancy, and had been preceded by a number of weeks of elevated seismic activity and surface deformation that unusually declined in the days leading up to the eruption. The eruption initially had a low magma flow rate and minimal lava flows, but towards the end of April the magma flow rate increased and high lava fountaining was observed. Understanding the precursors of volcanic eruptions and the process occurring during them is important to be able to provide warnings to prevent loss of life and damage to infrastructure.
Freysteinn Sigmundsson, Michelle Parks and colleagues investigated the precursors to the eruption. Rates of ground displacement and number of earthquakes escalate prior to many eruptions, as magma forces its way towards the surface. Although the 2021 eruption in Iceland was initially preceded by an increase in seismic activity and surface deformation between 24 February and mid-March, a decline in deformation and seismicity was observed over several days just before the eruption. The authors propose that forces are stored in the crust of the Earth prior to eruptions, due to movements of the plates covering the surface of the Earth. Prior to eruptions, these forces may be released as magma enters the crust of the Earth, and the subsequent decline in seismic activity and ground deformation may signify that this process is coming temporarily to an end and magma will erupt. The findings demonstrate that the interaction between volcanic processes, tectonic stress and crust composition need to be considered when forecasting eruptions, the authors conclude.
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). The authors note that the erupted lavas changed over time; during the initial phases of the eruption the lava was predominately from near the crust–mantle interface, but over the following weeks the composition changed, indicating that it was sourced by magmas generated at greater depths. These findings demonstrate that the near-Moho magma storage zone is an extremely dynamic environment, with mixing of magmas occurring on incredibly short timescales (days to weeks). This shows us how fast magma bodies can be formed in real time. The authors indicate that these are some of the first direct observations of basaltic magma systems of this depth, and suggest they may aid our understanding of these types of volcanoes.
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