GMPV9.1

The basaltic effusive eruption at Mt. Fagradalsfjall began on March 19, 2021, ending a 781-year hiatus on Reykjanes Peninsula, Iceland. At the time of writing (January 7, 2022), no eruptive activity has been observed since September 18, 2021. To monitor key eruption parameters (i.e., effusion rate and volume), near-real time photogrammetric monitoring was performed using a combination of satellite and airborne stereo images.

By late September 2021, 32 near real-time photogrammetric surveys were completed, usually processed within 3–6 hours. The results are a significant achievement in full-scale monitoring of a lava flow-field providing temporal data sets of lava volume, thickness, and effusion rate. This enabled rapid assessment of eruption evolution and hazards to populated areas, important infrastructure, and tourist centers.

The lava pathways and lava advancement were very complex and changeable as the lava filled and spilled from one valley into another and short-term prediction of the timing of overflow from one valley to another proved challenging. Analysis of thickness maps and thickness change maps show that the lava transport into different valleys varied up to 10 m³/s between surveys as lava transport rapidly switched between one valley to another.

By late September 2021, the mean lava thickness exceeded 30 m, covered 4.8 km² and has a bulk volume of 150 ± 3 × 10⁶ m³. Around the vent the thickness is up to 122 m. The March–September mean effusion rate is 9.5 ± 0.2 m³/s, ranging between 1–8 m³/s in March–April and increasing to 9–13 m³/s in May–September. This is uncommon for recent Icelandic eruptions, where the highest discharge usually occurs in the opening phase. This behavior may have been due to widening of the conduit by thermo-mechanical erosion with time, and not controlled by magma chamber pressure as is most common in the volcanic zones of Iceland.

How to cite: Pedersen, G., Belart, J. M. C., Óskarsson, B. V., Guðmundsson, M. T., Gies, N., Högnadóttir, T., Hjartardóttir, Á. R., Pinel, V., Berthier, E., Dürig, T., Reynolds, H. I., Hamilton, C. W., Valsson, G., Einarsson, P., Ben-Yehoshua, D., Gunnarsson, A., and Oddsson, B.: Volume, effusion rate, and lava transport during the 2021 Fagradalsfjall eruption: Results from near real-time photogrammetric monitoring, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9207, https://doi.org/10.5194/egusphere-egu22-9207, 2022.

The recent eruption of the Fagradalsfjall complex in the Reykjanes Peninsula of Iceland represents incompletely mixed basaltic magma directly erupted from a sub-crustal storage region. The eruption comprises olivine tholeiite lava with whole rock MgO between 8.7 and 10.1 wt%. The macrocryst cargo comprises olivine up to Fo₉₀, plagioclase up to An₈₉, and Cr-rich clinopyroxene up to Mg# 89. Gabbro and anorthosite xenoliths are rare. Olivine-plagioclase-augite-melt (OPAM) barometry of the groundmass glass from tephra collected from 28^th April to 6^th May yield high equilibration pressures and suggest that this eruption is originally sourced from a deep (0.48±0.06 GPa) storage zone at the crust-mantle boundary.

Over the course of the eruption, Fagradalsfjall lavas have changed significantly in source signature. The first erupted lavas (mid-March) were more depleted (K₂O/TiO₂ ­= 0.14, La/Sm = 2.1, ⁸⁷Sr/⁸⁶Sr = 0.703108, ¹⁴³Nd/¹⁴⁴Nd = 0.513017, ²⁰⁶Pb/²⁰⁴Pb = 18.730) and similar in composition to basalts previously erupted on the Reykjanes Peninsula. As the eruption continued, the lavas became increasingly enriched and were most enriched in early May (K₂O/TiO₂ = 0.27, La/Sm = 3.1, ⁸⁷Sr/⁸⁶Sr = 0.703183, ¹⁴³Nd/¹⁴⁴Nd = 0.512949, ²⁰⁶Pb/²⁰⁴Pb = 18.839), having unusual compositions for Reykjanes Peninsula lavas and similar only to enriched Reykjanes melt inclusions. From early May until the end of the eruption (18^th September), the lava K₂O/TiO₂ and La/Sm compositions displayed a sinuous wobble through time at lower amplitude than observed in the early part of the eruption. The enriched lavas produced later in the eruption are more enriched than lavas from Stapafell, a Reykjanes eruption thought to represent the enriched endmember on the Reykjanes. The full range of compositional variation observed in the eruption is large – about 2.5 times the combined variation of all other historic Reykjanes lavas.

The major, trace, and radiogenic isotope compositions indicate that binary mixing controls the erupted basalt compositions. The mixing endmembers appear to be depleted Reykjanes melts, and enriched melts with compositions similar to enriched Reykjanes melt inclusions or Snaefellsnes alkali basalts. The physical mechanism of mixing and the structure of the crust-mantle boundary magmatic system is a task for future study.

In contrast to the geochemical variations described above, the oxygen isotope composition (δ¹⁸O) of the groundmass glass (5.1±0.1‰) has little variation and is lower than MORB (~5.5‰). Olivine phenocrysts δ¹⁸O  values range from typical mantle peridotite values (5.1‰) to lower values (4.6‰), with the lower values in close equilibrium with the host melt. Given the crust-mantle boundary source of the eruption, these low δ¹⁸O values are unlikely to represent crustal contamination, and are more likely to represent an intrinsically low δ¹⁸O mantle beneath the Reykjanes Peninsula.

How to cite: Marshall, E., Rasmussen, M., Halldorsson, S., Matthews, S., Ranta, E., Sigmarsson, O., Robin, J., Barnes, J., Bali, E., Caracciolo, A., Guðfinnsson, G., and Mibei, G.: An overview of the geochemistry and petrology of the mantle-sourced Fagradalsfjall eruption, Iceland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8304, https://doi.org/10.5194/egusphere-egu22-8304, 2022.

The 2021 effusive eruption at Mt. Fagradalsfjall, on the Reykjanes Peninsula oblique rift in Iceland, was preceded by a 14-month long period of volcano-tectonic unrest (comprising both significant ground deformation and intense seismicity). A seismic swarm was initially detected in the Fagradalsfjall region between the 15^th to 20^th December 2019. Following a short quiescence, activity re-commenced on the 21^st January 2020, with a small cluster of earthquakes near Grindavík (~ 10 km west of Fagradalsfjall). Concurrent deformation was detected on two GNSS stations in this area and on Sentinel-1 interferograms. Geodetic modelling of these observations indicated the deformation most likely resulted from the intrusion of a magmatic sill, directly west of Mt. Thorbjörn, at a depth of about 4 km. This was followed by two additional sill-type intrusions in a similar location, between 6^th March - 17^th April and 15^th May - 22^nd July 2020 respectively. The three intrusions comprised a total volume change of about 9 million cubic meters. In mid-July 2020, inflation was again detected on the Reykjanes Peninsula, this time in the Kýsuvík volcanic system to the east of Fagradalsfjall. This episode of inflation lasted several weeks and geodetic inversions indicated the observed signal was produced by the combination of a deflating sill-like source at a depth of ~16 km and inflation of a body at a depth of ~6 km. The latter, corresponding to a volume change of about 5 million cubic meters. During this period of intrusive activity, seismicity shifted along various regions across the Peninsula, in relation to a combination of processes – magma migration, triggered seismicity and tectonic earthquakes.

Intense seismic swarms commenced on the 24^th February 2021, concentrated at both Fagradalsfjall and also extending across a 20 km segment along the plate boundary – including triggered strike-slip earthquakes up to Mw5.64. At the same time, deformation was detected on local GNSS stations, and subsequent Interferometric Sythethic Aperture Radar Analysis (InSAR) of Sentinel-1 data confirmed the observed deformation was primarily the result of a dike intrusion and slip along the plate boundary. Geodetic inversions indicated a ~9 km long dike with a total intruded volume of around 34 million cubic meters (Sigmundsson et al., in review). During this period, stored tectonic stress was systematically released, resulting in a decline in deformation and seismicity over several days preceding the eruption onset, on 19^th March 2021 in Geldingadalir at Mt. Fagradalsfjall. The eruption continued until the 18^th September 2021 and produced a lava field covering an area of 4.8 km² with an extruded bulk volume of 150 ± 3 × 10⁶ m³ (Pedersen et al., in review).

References

Sigmundsson et al. (in review). Deformation and seismicity decline preceding a rift zone eruption at Fagradalsfjall, Iceland.

Pedersen et al. (in review). Volume, effusion rate, and lava transport during the 2021 Fagradalsfjall eruption: Results from near real-time photogrammetric monitoring. DOI:10.1002/essoar.10509177.1.

How to cite: Parks, M., Vogfjörd, K. S., Sigmundsson, F., Hooper, A., Geirsson, H., Drouin, V., Ófeigsson, B. G., Hreinsdóttir, S., Hjaltadóttir, S., Jónsdóttir, K., Einarsson, P., Barsotti, S., Horálek, J., and Ágústsdóttir, T.: Evolution of deformation and seismicity on the Reykjanes Peninsula, preceding the 2021 Fagradalsfjall eruption, Iceland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13461, https://doi.org/10.5194/egusphere-egu22-13461, 2022.

A period of intense seismicity started more than a year prior to the 2021 Fagradalsfjall eruption in Iceland. During the same period, repeated cycles of surface uplift and subsidence were observed in the Svartsengi and Krýsuvík high-temperature (HT) fields, about 8-10 km west and east of the eruption site in Fagradalsfjall, respectively. Such an uplift has never been observed during 40 years of surface deformation monitoring of the exploited Svartsengi HT field. However, cycles of uplift followed by subsidence have been observed earlier at the unexploited Krýsuvík HT field.

Shortly after the start of the unrest, a group of scientists from GFZ-Potsdam and ÍSOR installed additional seismometers, used an optical telecommunication cable to monitor the seismicity and performed gravity measurements in the unrest area.

The data was used for multidisciplinary modelling of the pre-eruption processes (see Flóvenz et al, 2022. Cyclical geothermal unrest as a precursor to Iceland's 2021 Fagradalsfjall eruption. Nature Geoscience (in revision)). It included a poroelastic model that explains the repeated uplift and subsidence cycles at the Svartsengi HT field, by cyclic fluid intrusions into a permeable aquifer at 4 km depth at the observed brittle-ductile transition (BDT). The model gives a total injected volume of 0.11±0.05 km³. Constraining the intruded material jointly by the deformation and gravity data results in a density of 850±350 kg/m³. A high-resolution seismic catalogue of 39,500 events using the optical cable recordings was created, and the poroelastic model explains very well the observed spatiotemporal seismicity.

The geodetic, gravity, and seismic data are explained by ingression of magmatic CO₂ into the aquifer. To explain the behaviour of cyclic fluid injections, a physical feeder-channel model is proposed.

The poroelastic model and the feeder-channel model are combined into a conceptual model that is consistent with the geochemical signature of the erupted magma. It explains the pre-eruption processes and gives estimates of the amount of magma involved.

The conceptual model incorporates a magmatic reservoir at 15-20 km depth, fed by slowly upwelling currents of mantle derived magma. Volatiles released from inflowing enriched magma into the sub-Moho reservoir migrated upwards. The volatiles were possibly trapped for weeks or months at the BDT at ~7 km depth beneath Fagradalsfjall, generating overpressure, but not high enough to lift the overburden (~220 MPa) and cause surface deformation. After reaching a certain limiting overpressure, or when a certain volume had accumulated, the magmatic volatiles were diverted upwards, just below the BDT towards the hydrostatic pressurized aquifer (~ 40 MPa) at 4 km depth at the bottom of the convective HT fields. They passed through the BDT and increased the pressure sufficiently (>110 MPa) to cause the uplift.

The lessons learned enlighten the most important factors to help detect precursory volcanic processes on the Reykjanes Peninsula; including detailed monitoring of seismicity, surface deformation, gravity changes and gas content in geothermal fluids. Furthermore, geophysical exploration of the deeper crust by seismic and resistivity measurements are crucial to map possible melt and possible pathways towards the surface.

How to cite: Flóvenz, Ó., Wang, R., Hersir, G. P., Dahm, T., Hainzl, S., Vassileva, M., Drouin, V., Heimann, S., Isken, M. P., Gudnason, E. Á., Ágústsson, K., Ágústsdóttir, T., Horálek, J., Motagh, M., Walter, T. R., Rivalta, E., Jousset, P., Krawczyk, C. M., and Milkereit, C.: A comprehensive model of the precursors leading to the 2021 Fagradalsfjall eruption, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10219, https://doi.org/10.5194/egusphere-egu22-10219, 2022.

Geodetic observations during volcanic eruptions are important to constrain from where the eruptive products originate in the sub-surface. Some eruptions are sourced from magma reservoirs shallow in the crust, whereas others may tap magma directly from the mantle. The 2021 Fagradalsfjall eruption took place on the Reykjanes Peninsula, Iceland, during March 19 to September 18, resulting in approximately 0.15 km³ of erupted basaltic lava. A wide-spread crustal subsidence and inward horizontal motion, centered on the eruptive site, was observed during the eruption. Nearest to the emplaced lava flows, additional localized subsidence is observed due to the loading of the lavas. The regional subsidence rate varied during the eruption: it was low in the beginning and then increased, in broad agreement with changes in the bulk effusive rate. In this study we use GNSS and InSAR data to model the deformation source(s) during different periods of the eruption, primarily aiming to resolve the depth and volume change of the magma source. We furthermore calculate crustal stress changes during the eruption and compare to the regional seismicity.

How to cite: Geirsson, H., Parks, M., Sigmundsson, F., Ófeigsson, B. G., Drouin, V., Ducrocq, C., Friðriksdóttir, H. M., Hreinsdóttir, S., and Hooper, A.: Co-eruptive subsidence during the 2021 Fagradalsfjall eruption: geodetic constraints on magma source depths and stress changes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12435, https://doi.org/10.5194/egusphere-egu22-12435, 2022.

How to cite: Greenfield, T., Winder, T., Rawlinson, N., Southern, E., Bacon, C., Ágústsdóttir, T., White, R. S., Brandsdottir, B., Maclennan, J., Horalek, J., Gudnason, E. Á., and Hersir, G. P.: Deep seismicity preceding and during the 2021 Fagradalsfjall eruption, Reykjanes Peninsula, Iceland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5649, https://doi.org/10.5194/egusphere-egu22-5649, 2022.

Since late 2019, the Reykjanes Peninsula in Iceland has experienced elevated seismic activity, which culminated in a dyke intrusion beneath Fagradalsfjall on 24^th February 2021, and an eruption on 19^th March. Seismic anisotropy – the directional dependence of seismic wave speed – can be used to study structural properties of the crust, which may be controlled by the state of stress through preferential closure of micro-cracks. This provides an opportunity to investigate changes in crustal stress regime caused by a dyke intrusion, with potential applications in eruption monitoring and forecasting.

A dense seismic network spanning Fagradalsfjall recorded more than 130,000 earthquakes between June 2020 and August 2021; detected and located using QuakeMigrate¹. From this dataset, we calculate the seismic anisotropy of the upper crust through shear-wave splitting analysis. Exceptional ray-path coverage allows for imaging at high spatial and temporal resolution. We present these results in relation to the regional stress regime and tectonic structure, and search for changes in anisotropy before, during, and after the dyke intrusion and eruption.

1: Winder, T., Bacon, C., Smith, J., Hudson, T., Greenfield, T. and White, R., 2020. QuakeMigrate: a Modular, Open-Source Python Package for Automatic Earthquake Detection and Location. https://doi.org/10.1002/essoar.10505850.1

How to cite: Parsons, A., Bacon, C., Greenfield, T., Winder, T., Ágústsdóttir, T., Brandsdóttir, B., Fischer, T., Doubravová, J., Rawlinson, N., White, R., Gudnason, E. Á., Hersir, G. P., and Hrubcova, P.: Imaging the anisotropic structure of the Reykjanes Peninsula across the 2021 Fagradalsfjall dyke intrusion through local shear-wave splitting analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13504, https://doi.org/10.5194/egusphere-egu22-13504, 2022.

How to cite: Blanck, H., Halldórsson, B., and Vogfjord, K.: Array observations of an oscillating seismic sequence in the Reykjanes Peninsula, SW-Iceland, in December 2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12548, https://doi.org/10.5194/egusphere-egu22-12548, 2022.

The Fagradalsfjall eruption started on the 19^th of March 2021 on a ~180 m long eruptive fissure, following a dike intrusion which had been ongoing for approximately three weeks. The eruption focused shortly thereafter on two eruptive vents. In April, new fissure openings occurred northeast of the initial eruption on the 5^th, 6/7^th, 10^th, and 13^th of April. The northernmost eruption occurred on the 5^th of April, approximately 1 km northeast of the initial fissure, whereas the other fissure openings occurred between this and the initial eruptive vents. Stills from web cameras and time-lapse cameras are available for five of the fissure openings. These data show that the eruptions were preceded by steam emitted from cracks in the exact locations where the eruptions started. The time between the first steam observations and the visual appearance of glowing lava ranged between 15 seconds and 1.5 minutes during night observations and 9 to 23 minutes during daytime observations, the difference is likely explained by different lighting conditions. The eruptive vents are located where the north-easterly oriented dike intersected pre-existing north-south oriented strike-slip faults. These strike-slip faults could be identified on both pre-existing aerial photographs and digital elevation models. A high resolution ICEYE interferogram spanning the first day of the eruption in March reveals deformation where the later vent openings occurred in April. This indicates how Interferometric Synthetic Aperture Radar Analysis (InSAR) could be used to predict where subsequent vent openings are likely. This is of great importance for hazard assessment and defining exclusion zones during fissure eruptions.

How to cite: Hjartardóttir, Á. R., Dürig, T., Parks, M., Drouin, V., Eyjólfsson, V., Reynolds, H., Jensen, E. H., Óskarsson, B. V., Belart, J. M. C., Ruch, J., Gies, N., Pedersen, G. B. M., and Einarsson, P.: Eruptive vent openings during the 2021 Fagradalsfjall eruption, Iceland, and their relationship with pre-existing fractures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10330, https://doi.org/10.5194/egusphere-egu22-10330, 2022.

Using ground deformation measurements of high spatial and temporal resolution SAR, the understanding of new vents created during volcanic eruptions can be improved with 3D mapping of the activated shallow magma plumbing system. Interferometric analysis of radar data from ICEYE X-band satellites with daily coherent ground track repeat (GTR) provides unprecedented time series of deformation in relation to the opening of 6 eruptive vents over 26 days in 2021, at Fagradalsfjall, Iceland. Unrest started in this location at the end of February and tens of thousands of earthquakes were recorded during the following four weeks. The seismicity was linked to gradual formation of a magma-filled dike in the crust and triggered seismicity along the plate boundary. On 19 March, an eruptive fissure opened near the center of the dyke. New vents and eruptive fissures opened on the 5th, 7th, 10th, and 13th April. The daily acquisition rate of the ICEYE satellite facilitated the observation of the ground openings associated with each new vents. Each event can be observed individually and with minimal loss of signal caused by new lava emplacement, which would occur if images were acquired at a slower rate. Being able to retrieve deformation near the edge of the fissure ensures that we have the optimal constraints needed for modelling the subsurface magma path. The ICEYE dataset consists of Stripmap acquisitions (30x50km) in the period 3-21 March, and Spotlight acquisitions (5x5 km) from 22 March and onward. Images have a resolution of about 2 m x 3 m, and 0.5 m x 0.25 m, respectively. The descending 1-day interferogram covering each individual event is used to invert for the distributed opening along the dike plane. We find that each fissure was associated with opening of up to 0.5 meters in the topmost 200 m of crust. The conduits propagated vertically at least 50–80 m/h. The new fissure locations were influenced by local conditions and induced stress changes within the shallow crust.

How to cite: Drouin, V., Tolpekin, V., Parks, M., Sigmundsson, F., Leeb, D., Strong, S., Hjartardóttir, Á. R., Geirsson, H., Einarsson, P., and Ófeigsson, B. G.: Conduits feeding new eruptive vents at Fagradajsfjall, Iceland, mapped by high-resolution ICEYE SAR satellite in a daily repeat orbit, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8679, https://doi.org/10.5194/egusphere-egu22-8679, 2022.

The Reykjanes Peninsula has recently been subject to a seismo-tectonic unrest triggering widespread ground cracks. This started with a strong seismic swarm from 24 February to 17 March 2021 and culminated in a volcanic eruption on March 19, terminating an 800 years quiescence period in the region. The Peninsula hosts four overlapping and highly oblique rift zones. The structural relations between the plate boundary (N070), the rift zones (N030 to N040) and the barely visible fault zones oriented N175 are challenging to assess, as most structures, beside the rifts, are poorly preserved or absent in the landscape. 

To get the full picture of the fracture field generated by the 2021 Reykjanes rifting event, we collected an unprecedented amount of structural data, mapping almost the entire fresh fracture field. Field observations show widespread ground cracks in up to ~7 km distance from the intrusion area with en-echelon metrical segments with a right-lateral sense of shear. Most of these structures are not visible anymore, either covered by lava flows or eroded due to weathering. They are unique testimony of the strong seismicity preceding the eruption and would have remained unnoticed if not caught up by our fixed-wing drone, surveying an area of ~30 km². We used the resulting high-resolution (<5 cm) orthomosaics and DEMs to study three main NS-oriented fracture zones of 3 to 4 kilometers long, mostly generated by ten earthquakes ranging from M5 to M5.6. Results show metric to decametric en-echelon structures with cracks of very limited extension, even in the vicinity of the eruption site. Two of the three main fracture zones clearly show fault reactivation, suggesting episodicity in the rifting processes. Apart from local sinkholes, the third area has probably also been reactivated, but the loose ground composition did not preserve previous structures.

We further used high-resolution optical image correlation technique to analyze aerial photos and drone imagery acquired before and after the large earthquakes sequence in the three fracture zones. Results show clear NS-oriented shear structures with a right-lateral sense of motion of up to 50 cm. This is in good agreement with moment tensors we computed from waveform data at seismic stations up to 1000 km distance. We observe consistent non-double-couple mechanisms, with tension-crack components oriented northwest-southeast. The orientations suggest strike-slip faulting with nodal planes oriented in the same direction as the main fault traces. We also found that the three fracture zones have sigmoid shapes and their overall extension bounds the near-field deformation of the plate boundary. These sigmoids may suggest a local high geothermal gradient and elasto-plastic deformation affecting the Reykjanes Peninsula, that further decreases toward the South Icelandic Seismic Zone.

How to cite: Ruch, J., Bufféral, S., Panza, E., Mannini, S., Oskarsson, B., Gies, N., Alvizuri, C., and Hjartardóttir, Á. R.: Widespread ground cracks generated during the 2021 Reykjanes oblique rifting event (SW Iceland), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11995, https://doi.org/10.5194/egusphere-egu22-11995, 2022.

The dynamics of fault slip in the upper hundreds of meters of Earth’s crust has long been an open question, as their behavior differs from classical elastic dislocation models and their observation still raises challenges. Here, we analyze centimeter-scale ground resolution aerial optical images of the surface ruptures associated with the 8 Mw ≥ 5.0 sub-surface earthquakes that stroke during the Reykjanes seismo-tectonic unrest, starting on February 24, 2021, and ending with the start of an eruption at Fagradasfjall on March 19, 2021. For four major earthquakes, we apply a sub-pixel correlation technique of pre-, syn- and post-crisis aerial and drone orthomosaics to describe the displacement field on surface blocks. We find that surface offsets reached up to 50 cm, with almost pure dextral strike-slip in a NS direction. These orientations contrast with the overall NE-SW-oriented extensional structures originating from magmatic intrusions and appear as a bookshelf faulting system conjugated to the left-lateral strike-slip plate boundary, oriented ~N070.

On hard grounds (e.g.: lava flows), shallow ruptures reached the surface, reactivating pre-existing structures and displaying an en-échelon succession of hectometric-sized fractures. We believe these ruptures are representative of medium-sized faults behavior in the last few hundred meters of the crust. On soft grounds, however, the rupture was only betrayed by meter-sized en-échelon systems, evidenced by thousands of discrete sub-metric surface fractures we were able to observe in the field and map from the orthomosaics. The sharp deformation gradient we imaged indicates that the dislocation drastically decreased above ten to a few tens of meters below the surface. In this layer, diffuse deformation takes on most of the slip deficit, mainly through inelastic processes. As a result, evidence of the February 2021 earthquake did not endure erosion for more than a few months. Except for an isolated sinkhole which allowed us to assume that one fault pre-existed, there were no markers of its presence before the earthquake. We emphasize that this issue must frequently lead to an underestimation of the seismic hazard when performed from surface traces.

How to cite: Bufféral, S., Panza, E., Mannini, S., and Ruch, J.: Sub-surface fault slip dynamics during the 2021 Reykjanes unrest (Iceland), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10343, https://doi.org/10.5194/egusphere-egu22-10343, 2022.

During the Fagradalsfjall volcanic eruption in Iceland in 2021, the atmospheric flow was simulated at high-spatial and temporal resolutions with the numerical system WRF, including the WRF-Chem for the simulation of trace gases and aerosols.  The output of the real-time simulations of SO2 has been compared to observations, showing that on time-scales of 12-24 hours, the numerical system has considerable skill, but moving to temporal scales shorter than 6 hours leads to substantial drop in the model performance.  The data and the model output suggest that there may be strong long-lasting horizontal gradients in the trace gases and limited horizontal mixing at times, calling for a more dense network of monitoring of gases from the volcano.  Wind variability on the time scale of minutes up to few hours remains a challenge.

How to cite: Einarsson, P., Rögnvaldsson, Ó., and Ólafsson, H.: Real-time prediction trace gases from the Fagradalsfjall volcanic eruption, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11386, https://doi.org/10.5194/egusphere-egu22-11386, 2022.

Effusive eruptions are a significant source of volcanic volatile species, injecting various reactive and climate altering products into the atmosphere, while low-level emissions can be hazardous to human health due to the degradation of local or regional air quality. Quantification of the flux and composition of these emissions also offers an insight into the magmatic processes driving the eruption. These factors mean that gas flux measurements are a key monitoring tool for managing the response to such eruptions. The usual target species for gas flux measurements is sulphur dioxide (SO₂) due to its high concentration in volcanic emissions but low ambient concentration, and its ability to be measured with UV and IR spectroscopy from both ground and space.

Fagradalsfjall volcano, Iceland, underwent an effusive eruption between March – September 2021, emitting over 100 million m³ of lava and producing significant SO₂ emissions. The eruption progressed through several distinct phases in eruptive style, with different surface activity and gas emission behaviour for each. Satellite instruments have not traditionally been used for monitoring emissions from effusive eruptions such as this, as they often lack the spatial or temporal resolution to detect and quantify low-level effusive emissions. However, the launch of ESA’s Sentinel-5P, carrying the TROPOMI instrument, in October 2017 opened the door for such measurements, offering a step change in sensitivity to tropospheric emissions over previous missions.

Here, we will present measurements of altitude- and time-resolved SO₂ fluxes from Fagradalsfjall by combining TROPOMI observations with a back-trajectory analysis toolkit called PlumeTraj. We compare the emissions with other geophysical monitoring streams throughout the eruption and explore changes across the different phases of the eruption. This will demonstrate the ability of TROPOMI and PlumeTraj for quantifying intra-day, low-level SO₂ emissions and highlight the potential insight these measurements can provide for future effusive eruptions.

How to cite: Esse, B., Burton, M., Hayer, C., Barsotti, S., and Pfeffer, M.: Quantifying SO2 emissions from the 2021 eruption of Fagradalsfjall, Iceland, with TROPOMI and PlumeTraj, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11537, https://doi.org/10.5194/egusphere-egu22-11537, 2022.

Lavas from the 2021 Fagradalsfjall eruption, Iceland, show remarkable, day to month scale temporal variations in trace element and radiogenic isotopic compositions. Changes have been attributed to variation in the depth and degree of melting and/or source lithology, with progressive melting of a deeper, more enriched source as the eruption proceeded [1]. Distinguishing melting processes from source composition can be difficult to untangle using trace elements alone. Radiogenic isotopes are unaffected by the melting processes, but pinpointing lithological variations requires that the radiogenic isotopic compositions of the (unknown) endmembers are distinct and fairly restricted to be able to calculate relative contribution(s) to a lava.

Stable isotopic composition may provide another perspective on the cause of the clear temporal chemical trends in the eruption. For example, it has been proposed that Fe stable isotopes may detect the contribution of distinct mantle lithologies to a lava, due to the contrasting bonding environment of Fe in mantle minerals. Both empirical and theoretical studies show that at equilibrium, pyroxenite should be enriched in heavy Fe isotopes compared to typical mantle peridotite [e.g. 2]. Due to limited (<0.1‰) isotopic fractionation during mantle melting, unevolved basalts should capture this lithological variation. However, more recent theoretical work has argued that unrealistically high proportions of pyroxenite are needed to cause resolvable variations in basalt Fe isotopic composition [3]. Zinc stable isotopes provide a complementary system, with variation in Zn isotopic composition detected between garnet and spinel bearing lithologies [4], and without the added complexities of redox-driven fractionation that may affect Fe isotopes. The basaltic Fagradalsfjall eruption thus provides a unique time series to test whether the changes in trace element chemistry of the erupted lavas is mirrored by Fe-Zn isotopic variation. Variation in degree of melting alone is not expected to cause significant Fe-Zn isotopic fractionation, whereas a change in contribution to the lavas from pyroxene and/or garnet bearing lithologies may be reflected in the Fe-Zn isotopic composition. By combining redox sensitive (Fe) and redox insensitive (Zn) isotope systems we can potentially investigate magmatic processes in terms of the redox evolution of the source. We will present the Fe and Zn isotopic compositions of 15 fresh, glassy basaltic lavas collected during the first 4 months of the eruption. We will discuss the possible cause(s) of isotopic variations and how this adds to our understanding of the Fagradalsfjall eruption, specifically. Finally, this timeseries allows us to re-visit and evaluate the efficacy of using Fe-Zn isotopes to determine variations in mantle lithology.

[1] Marshall et al. (2021), AGU FM Abstract [2] Williams and Bizimis (2014), EPSL, 404, 396-407 [3] Soderman et al. (2021), GCA, 318, 388-414 [4] Wang et al. (2017), GCA, 198, 151-167

How to cite: Stow, M., Prytulak, J., Burton, K., Nowell, G., Marshall, E., Rasmussen, M., Matthews, S., Ranta, E., and Caracciolo, A.: Temporal Fe-Zn isotopic variations in the chemically heterogeneous Fagradalsfjall eruption, 2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9846, https://doi.org/10.5194/egusphere-egu22-9846, 2022.

Mantle melting processes and the characteristics of the source lithologies are mostly derived from basalt compositions of the mid-ocean ridge system and from oceanic islands. However, these basalts are in most cases the products of crustal processes resulting from magma storage, mixing, differentiation and crustal interaction. In Iceland, magma mixing and homogenization in thoroughly stirred magma reservoirs appear to be the norm, leading to restricted variations of Sr and Nd isotope ratio for a given volcanic system. In contrast, more primitive basalts were erupted during the 2021 Fagradalsfjall eruption on the Reykjanes Peninsula with a large spread in isotope ratios. A strong negative correlation between Sr and Nd isotopes is observed from ratios that span a range from a depleted mantle composition to values akin to the Icelandic mantle such as that of the basalts of the Grímsvötn volcanic system. The isotope ratios are also correlated with the measured discharge rate during the eruption, with a depleted Sr isotope ratio appearing during the period of low discharge (around 5 m³/s) for the first month and a half of the eruption. In early May, the magma flux doubled and basalts with more radiogenic Sr isotope composition were produced. During the summer 2021, the Sr isotope ratios declined, due to lower proportions of melts from undepleted mantle source in the basalt mixture erupted. Whether the eruption ended when melts from the enriched mantle was exhausted or not remains to be elucidated, but clearly the highest eruption discharge rate resulted from melts of a more fertile mantle source.

The variable proportions of depleted versus enriched melts in the eruption products demonstrate the absence of a magma reservoir in which homogenization could take place, and from which decreasing discharge rate with time would be expected.  Instead, the initially low and steady and then increasing magma extrusion rate measured, strongly indicate direct mantle melt ascent to surface, which is also supported by the primitive mineralogy of the high-MgO basalt produced. Leaky-transform faults on the mid-ocean ridge system are characterized by eruptions of primitive basalts on intra-transform spreading centres (e.g. Garrett and Siqueiros fracture zones in the East Pacific). The Fagradalsfjall complex appears to be of similar nature, and the primitive magma and the important compositional and temporal variations demonstrate the effect of mantle source composition and associated processes on the eruption behaviour, as reflected in the magma discharge rate.

How to cite: Sigmarsson, O., Marshall, E. W., Bosq, C., Auclair, D., Rasmussen, M. B., Kleine, B. I., Ranta, E. J., Matthews, S., Halldórsson, S. A., Jackson, M. G., Gudfinnsson, G. H., Bali, E., Stefánsson, A., and Gudmundsson, M. T.: Basalt production controlled by mantle source fertility at Fagradalsfjall, Iceland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8479, https://doi.org/10.5194/egusphere-egu22-8479, 2022.