Rapid shifting of a deep magmatic source at Fagradalsfjall volcano, Iceland

Time-series InSAR measurement using ICOPS and estimation of along-track surface deformation using MAI during the 2021 eruption of Fagradalsfjall Volcano, Iceland

The eruption in Fagradalsfjall Volcano, located in Reykjanes Peninsula, Iceland, from several centuries' dormant states, occurred for the first time on March 19, 2021. Observations of Fagradalsfjall Volcano were conducted in 2021, and the eruption period lasted for six months until 18 September 2021. Six days pair of interferograms were generated from ninety synthetic aperture radar (SAR) data. Thus, the SAR data will be acquired from the Sentinel-1 satellite from January until December 2021. Time-series measurements were conducted using a combination of persistent scatterer (PS) and distributed scatterer (DS) points to produce denser measurement points (MPs) in the study area. The improved combined scatterers interferometry with optimized point scatterers (ICOPS) algorithm is the time-series method that utilizes both PS and DS MPs and optimizes those combined MPs using a deep learning algorithm over different temporal intervals and using a statistical clustering approach to optimize the MPs spatially. Validation was conducted by comparing the ICOPS result with GPS measurement in Reykjavik. The comparison with the GPS measurement was performed to validate the line-of-sight (LOS) deformation from the ICOPS measurement, which resulted in an RMSE value of about 0.58 cm, which is considered a good correlation. Besides the time-series Interferometry SAR (InSAR) measurement, we used the integrated InSAR and multiple aperture interferometry (MAI) methods to estimate both LOS and along-track surface deformation, respectively, during the Fagradalsfjall, Iceland volcanic eruption. A pair of ALOS-2 data was used between 28 February 2021 and 23 May 2021. The result from the MAI method shows a deformation of approximately ± 2 mm in the azimuth direction around Fagradalsfjall Volcano. The deformation around Fagradalsfjall Volcano was suggested to be due to the activity of the magma reservoir beneath the Earth's surface, which was formed by dike intrusion. The analysis of the seismicity in Fagradalsfjall was discussed by visualization of the distribution of earthquakes during the deformation occurrence. Further analysis can be conducted by applying multitrack analysis to find the 3D deformation pattern due to the eruption.

A dynamic mid-crustal magma domain revealed by the 2023 to 2024 Sundhnúksgígar eruptions in Iceland

Mid-crustal magma domains are the source of many basaltic eruptions. Lavas from individual eruptions are often chemically homogeneous, suggesting that they derive from single, well-mixed magma reservoirs. The 2023 to 2024 eruptions at Sundhnúksgígar in the Svartsengi volcanic system in Iceland provide an opportunity to observe the behavior of a mid-crustal magma domain at high spatial and temporal resolution by detailed sampling and geochemical characterization. We observed substantial mantle-derived geochemical variability in the products erupted in the first hours of the December 2023 and January, February, and March to May 2024 eruptions, indicating that the eruptions derived from multiple magma reservoirs, which mineral-melt equilibration pressures place in the mid crust. The unusual presence of geochemical heterogeneity in the mid-crustal magma domain provides insights into how dynamic and complex mid-crustal magma domains can be.

Deep crustal assimilation during the 2021 Fagradalsfjall Fires, Iceland

Active basaltic eruptions enable time-series analysis of geochemical and geophysical properties, providing constraints on mantle composition and eruption processes1-4. The continuing Fagradalsfjall and Sundhnúkur fires on Iceland's Reykjanes Peninsula, beginning in 2021, enable such an approach5,6. Earliest lavas of this volcanic episode have been interpreted to exclusively reflect a change from shallow to deeper mantle source processes7. Here we show using osmium (Os) isotopes that the 2021 Fagradalsfjall lavas are both fractionally crystallized and strongly crustally contaminated, probably by mid-ocean-ridge gabbros and older basalts underlying the Reykjanes Peninsula. Earliest eruptive products (187Os/188Os ≤ 0.188, platinum (Pt)/iridium (Ir) ≤ 76) are highly anomalous for Icelandic lavas or global oceanic basalts and Os isotope ratios remain elevated throughout the 2021 eruption, indicating a continued but diluted presence of contaminants. The 2022 lavas show no evidence for contamination (187Os/188Os = 0.131, Pt/Ir = 30), being typical of Icelandic basalts (0.132 ± 0.007). Initiation of the Fagradalsfjall Fires in 2021 involved pre-eruptive stalling, fractional crystallization and crustal assimilation of earliest lavas. An established magmatic conduit system in 2022 enabled efficient magma transit to the surface without crustal assimilation.

Simultaneous rift-scale inflation of a deep crustal sill network in Afar, East Africa

Decades of studies at divergent plate margins have revealed networks of magmatic sills at the crust-mantle boundary. However, a lack of direct observations of deep magma motion limits our understanding of magma inflow from the mantle into the lower crust and the mechanism of sill formation. Here, satellite geodesy reveals rift-scale deformation caused by magma inflow in the deep crust in the Afar rift (East Africa). Simultaneous inflation of four sills, laterally separated by 10s of km and at depths ranging 9-28 km, caused uplift across a ~ 100-km-wide zone, suggesting the sills are linked to a common mantle source. Our results show the supply of magma into the lower crust is temporally episodic, occurring across a network of sills. This process reflects inherent instability of melt migration through porous mantle flow and may be the fundamental process that builds the thick igneous crust beneath magmatic rifts and rifted margins globally.

Exceptionally low mercury concentrations and fluxes from the 2021 and 2022 eruptions of Fagradalsfjall volcano, Iceland

Mercury (Hg) is naturally released by volcanoes and geothermal systems, but the global flux from these natural sources is highly uncertain due to a lack of direct measurements and uncertainties with upscaling Hg/SO2 mass ratios to estimate Hg fluxes. The 2021 and 2022 eruptions of Fagradalsfjall volcano, southwest Iceland, provided an opportunity to measure Hg concentrations and fluxes from a hotspot/rift system using modern analytical techniques. We measured gaseous Hg and SO2 concentrations in the volcanic plume by near-source drone-based sampling and simultaneous downwind ground-based sampling. Mean Hg/SO2 was an order of magnitude higher at the downwind locations relative to near-source data. This was attributed to the elevated local background Hg at ground level (4.0 ng m-3) likely due to emissions from outgassing lava fields. The background-corrected plume Hg/SO2 mass ratio (5.6 × 10-8) therefore appeared conservative from the near-source to several hundred meters distant, which has important implications for the upscaling of volcanic Hg fluxes based on SO2 measurements. Using this ratio and the total SO2 flux from both eruptions, we estimate the total mass of gaseous Hg released from the 2021 and 2022 Fagradalsfjall eruptions was 46 ± 33 kg, equivalent to a flux of 0.23 ± 0.17 kg d-1. This is the lowest Hg flux estimate in the literature for active open-conduit volcanoes, which range from 0.6 to 12 kg d-1 for other hotspot/rift volcanoes, and 0.5-110 kg d-1 for arc volcanoes. Our results suggest that Icelandic volcanic systems are fed from an especially Hg-poor mantle. Furthermore, we demonstrate that the aerial near-source plume Hg measurement is feasible with a drone-based active sampling configuration that captures all gaseous and particulate Hg species, and recommend this as the preferred method for quantifying volcanic Hg emissions going forward.

Toward a near real-time magma ascent monitoring by combined fluid inclusion barometry and ongoing seismicity

Fluid inclusion microthermometry on olivines, clinopyroxenes, and amphiboles was used during a volcanic eruption, in combination with real-time seismic data and rapid petrographic observations, for petrological monitoring purposes. By applying this approach to the study of 18 volcanic samples collected during the eruption of Tajogaite volcano on La Palma Island (Canary Islands) in 2021, changes in the magma system were identified over time and space. Magma batches with distinct petrographic and geochemical characteristics emerged from source zones whose depth progressively increased from 27 to 31 kilometers. The rise of magma of deeper origin is attested by fluid inclusions made of N2 and CO, markers of mantle outgassing. Magma accumulation occurred over different durations at depths of 22 to 27 and 4 to 16 kilometers. Time-integrated magma ascent velocities (including ponding times) were estimated at between 0.01 and 0.1 meters per second. This method is cost-effective and quickly identifies changes in the magma system during an eruption, enhancing petrological monitoring procedures.

Near-surface magma flow instability drives cyclic lava fountaining at Fagradalsfjall, Iceland

Lava fountains are a common manifestation of basaltic volcanism. While magma degassing plays a clear key role in their generation, the controls on their duration and intermittency are only partially understood, not least due to the challenges of measuring the most abundant gases, H2O and CO2. The 2021 Fagradalsfjall eruption in Iceland included a six-week episode of uncommonly periodic lava fountaining, featuring ~ 100-400 m high fountains lasting a few minutes followed by repose intervals of comparable duration. Exceptional conditions on 5 May 2021 permitted close-range (~300 m), highly time-resolved (every ~ 2 s) spectroscopic measurement of emitted gases during 16 fountain-repose cycles. The observed proportions of major and minor gas molecular species (including H2O, CO2, SO2, HCl, HF and CO) reveal a stage of CO2 degassing in the upper crust during magma ascent, followed by further gas-liquid separation at very shallow depths (~100 m). We explain the pulsatory lava fountaining as the result of pressure cycles within a shallow magma-filled cavity. The degassing at Fagradalsfjall and our explanatory model throw light on the wide spectrum of terrestrial lava fountaining and the subsurface cavities associated with basaltic vents.

Bubble-enhanced basanite-tephrite mixing in the early stages of the Cumbre Vieja 2021 eruption, La Palma, Canary Islands

Syneruptive magma mixing is widespread in volcanic eruptions, affecting explosivity and composition of products, but its evidence in basaltic systems is usually cryptic. Here we report direct evidence of mixing between basanitic and tephritic magmas in the first days of the 2021 Tajogaite eruption of Cumbre Vieja, La Palma. Groundmass glass in tephritic tephra from the fifth day of the eruption is locally inhomogeneous, showing micron-scale filamentary structures of Si-poor and Fe-, Mg-rich melt, forming complex filaments attached to bubbles. Their compositional distribution attests the presence of primitive basanitic magma, with compositions similar to late-erupted melts, interacting with an evolved tephritic melt during the first week of the event. From filament morphology, we suggest their generation by dragging and folding of basanitic melt during bubble migration through melt interfaces. Semi-quantitative diffusion modelling indicates that the filamentary structures are short-lived, dissipating in timescales of tens of seconds. In combination with thermobarometric constraints, we suggest a mixing onset by sub-Moho remobilization of a tephritic reservoir by basanite input, followed by turbulent ascent of a mingled magma. In the shallow conduit or lava fountain, bubble nucleation and migration triggered further mingling of the distinct melt-phases. This phenomenon might have enhanced the explosive behaviour of the eruption in such period, where violent strombolian explosions were common.

Discrete magma injections drive the 2021 La Palma eruption

Understanding the drivers of the onset, evolution, and end of eruptions and their impact on eruption style is critical in eruption forecasting and emergency management. The composition of erupted liquids is a key piece of the volcano puzzle, but untangling subtle melt variations remains an analytical challenge. Here, we apply rapid, high-resolution matrix geochemical analysis on samples of known eruption date spanning the entire 2021 La Palma eruption. Sr isotope signatures reveal distinct pulses of basanite melt driving the onset, restart, and evolution of the eruption. Elemental variations in matrix and microcrysts track progressive invasion, and draining, of a subcrustal crystal mush. Associated variations in lava flow rate, vent development, seismicity, and SO₂ emission demonstrate that volcanic matrix resolves eruption patterns that could be expected in future basaltic eruptions globally.

Deformation and seismicity decline before the 2021 Fagradalsfjall eruption

Increased rates of deformation and seismicity are well-established precursors to volcanic eruptions, and their interpretation forms the basis for eruption warnings worldwide. Rates of ground displacement and the number of earthquakes escalate before many eruptions1-3, as magma forces its way towards the surface. However, the pre-eruptive patterns of deformation and seismicity vary widely. Here we show how an eruption beginning on 19 March 2021 at Fagradalsfjall, Iceland, was preceded by a period of tectonic stress release ending with a decline in deformation and seismicity over several days preceding the eruption onset. High rates of deformation and seismicity occurred from 24 February to mid-March in relation to gradual emplacement of an approximately 9-km-long magma-filled dyke, between the surface and 8 km depth (volume approximately 34 × 106 m3), as well as the triggering of strike-slip earthquakes up to magnitude MW 5.64. As stored tectonic stress was systematically released, there was less lateral migration of magma and a reduction in both the deformation rates and seismicity. Weaker crust near the surface may also have contributed to reduced seismicity, as the depth of active magma emplacement progressively shallowed. This demonstrates that the interaction between volcanoes and tectonic stress as well as crustal layering need to be fully considered when forecasting eruptions.