Ultra-Fast Magma Flow Detected Under Grindavík, Iceland

The 15-kilometer-long magma dike that formed beneath Grindavík, Iceland, in November 2023, which caused widespread damage and evacuation of the local population, reached an unprecedented subsurface magma flow rate of 7400 cubic meters per second, researchers report. The dike formation preceeded the more recent Sundhnúkur eruptions in December 2023 and January 2024. The study, which combined satellite-based geodetic observations and seismic measurements of the Sundhnúkur crater chain and physical modeling, shows how fracturing and tectonic stress can drive massive magma flow into dikes with only modest overpressure in the feeding magma body. According to the authors, the measurement of such high flow rates provides insight into the formation of major dikes and demonstrates a serious hazard potential for this and other dike systems with similar features, which can result in large-volume surface eruptions. Vertical dikes in the Earth's crust can transport magma to the surface and cause an eruption. However, the timescales and processes underlying dike formation aren't fully understood. In November 2023, a massive dike began to rapidly form below the Sundhnúkur crater chain in southwest Iceland, passing under the town of Grindavík. To better understand the Grindavík dike's formation and the physical processes involved, Freysteinn Sigmundsson and colleagues analyzed satellite-based three-dimensional ground deformation measurements and seismic observations to model the dike's evolution and magma transport. The findings show that the subsurface magma flow rate peaked at an unexpectedly fast 7400 cubic meters per second during the formation of the Grindavík dike and demonstrate that tectonic stresses and fracturing are important factors in dike formation beyond pressure from the underlying magma source itself.