How the Earth’s crust accommodates magma emplacement influences the signals that can be detected by monitoring volcano seismicity and surface deformation, which are routinely used to forecast volcanic eruptions. However, we lack direct observational links between deformation caused by magma emplacement and monitoring signals. Here we use field mapping and photogrammetry to quantify deformation caused by the emplacement of at least 2.5 km³ of silicic magma in the Reyðarártindur pluton, Southeast Iceland. Our results show that magma emplacement triggered minor and local roof uplift, and that magma reservoir growth was largely aseismic by piecemeal floor subsidence. The occurrence and arrangement of fractures and faults in the reservoir roof can be explained by magmatic overpressure, suggesting that magma influx was not fully accommodated by floor subsidence. The tensile and shear fracturing would have caused detectable seismicity. Overpressure eventually culminated in eruption, as evidenced by exposed conduits that are associated with pronounced local subsidence of the roof rocks, corresponding to the formation of an asymmetric graben at the volcano surface. Hence, the field observations highlight processes that may take place within silicic volcanoes, not accounted for in widely used models to interpret volcanic unrest.

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In August 2021, Askja volcano in Iceland returned to the spotlight after a sudden onset of rapid uplift followed decades of continuous subsidence. In this study the extended record of microgravity data from Askja between 1988 to 2017 is revisited, and new microgravity data from 2021 and 2022 are introduced, which were collected after the uplift had started. Askja caldera had been steadily subsiding since at least 1984 and was characterised by a net decrease in microgravity, potentially signalling the contraction of its magma chamber or eviction of magma either laterally or to deeper levels. The microgravity data indicate that despite ongoing subsidence between 2017 and early 2021, a significant gravity increase can be detected in the center of the caldera between 2017 and August 2021. This increase may be introduced during – or leading up to – the period of uplift. The new microgravity data also indicate that during the period of 40 cm uplift after August 2021 to fall 2022, gravity changes approach the free-air gradient, suggesting subsurface density decreases as a driving process. This process may relate to the vesiculation of magma previously emplaced in the volcano roots, a change in the hydrothermal system, or replacement of dense basaltic magma with less dense rhyolitic magma, or a combination of these processes. However, uncertainties for this period are elevated and may obscure a gravity signal expected from additional mass accumulation. The timing and high uncertainties of some campaigns make it challenging to be conclusive on the driving process behind the uplift, but future microgravity campaigns could help solve the ambiguity. The study also provides a description of potential pitfalls in microgravity campaigns and recommendations on how the reliability of microgravity data can be improved.

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In August 2021 Askja caldera in Iceland started to show uplift after decades of subsidence. The uplift signal is centered at the northwestern edge of lake Ӧskjuvatn and an order of magnitude larger than the subsidence in the last decade. In September 2021 a geodesy campaign was carried out at Askja, including relative microgravity measurements acquired with the use of two Scintrex CG-5 instruments. Relative microgravity campaigns at Askja are not straightforward due to the long walking distances between sites, which makes a “double loop” procedure impossible. We revisit existing Scintrex relative microgravity data sets (2015 onward) and analyse data using the same joint weighted least squares inversion routine. We define recommendations for future relative microgravity campaigns at Askja which will be important to establish the cause of the ongoing uplift. The density of subsurface magma is only identifiable with microgravity data. Knowledge of the type of magma accumulating under Askja is vital to assess possible hazard implications.@en

The Krafla volcanic system has geothermal areas within the Krafla caldera and at Bjarnarflag in the Krafla fissure swarm, 9-km south of the Krafla caldera. Arrays of boreholes extract geothermal fluids for power plants in both areas. We collected and analyzed InSAR, GPS, and leveling data spanning 1993-2015 in order to investigate crustal deformation in these areas. The volcanic zone hosting the geothermal areas is also subject to large scale regional deformation processes, including plate spreading and deflation of the Krafla volcanic system. These deformation processes have to be taken into account in order to isolate the geothermal deformation signal. Plate spreading produces the largest horizontal displacements, but the regional deformation pattern also suggests readjustment of the Krafla system at depth after the 1975-1984 Krafla rifting episode. Observed deformation can be fit by an inflation source at about 20km depth north of Krafla and a deflation source at similar depth directly below the Krafla caldera. Deflation signal along the fissure swarm can be reproduced by a 1-km wide sill at 4km depth closing by 2-4cm per year. These sources are considered to approximate the combined effects of vertical deformation associated with plate spreading and post-rifting response. Local deformation at the geothermal areas is well resolved in addition to these signals. InSAR shows that deformation at Bjarnarflag is elongated along the direction of the Krafla fissure swarm (∼ 4km by ∼ 2km) while it is circular at Krafla (∼ 5km diameter). Rates of deflation at Krafla and Bjarnarflag geothermal areas have been relatively steady. Average volume decrease of about 6.6×10⁵ m³/yr for Krafla and 3.9×10⁵ m³/yr for Bjanarflag are found at sources located at ∼ 1.5km depth, when interpreted by a spherical point source of pressure. This volume change represents about 8×10 ^-3 m³/ton of the mass of geothermal fluid extracted per year, indicating important renewal of the geothermal reservoir by water flow.

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After a period of quiescence since a sill intrusion in 1999-2000, a subtle deformation signal was again detected at Eyjafjallajökull, beginning in the summer of 2009, at a continuous GPS station on the southern flank. At our request the German Space Centre (DLR) immediately began tasking the TerraSAR-X satellite to acquire three SAR images every 11 days, giving a time series of SAR images prior to the eruption with unprecedented temporal sampling (although interrupted by snow during the winter).
Previously we modelled individual interferograms that showed that there was a large uplift signal. We modelled this as a series of sills and a dike with a total volume of ~0.05 km3. During the flank eruption, beginning on 20 March, no significant deformation is detected, but coinciding with the start of the explosive eruption on April 14, we detected subsidence centred on the caldera. What we modelled showed us that Eyjafjallajökull was an unusual which we modelled …. This deformation does not relate to pressure changes within a single magma chamber.
Here we extend our analysis InSAR time series covering full eruptive period. After correcting for DEM errors and reduction of atmospheric signal, we have found a number of signals that we interpreted in terms of magma movement. These magma movements are separately analysed in 3 phases: pre-eruptive (inflation), co-eruptive (no deformation) and post-eruptive (deflation).
The displacement time series from June 2009 to 4 February 2010 (pre-eruptive-phase) shows line-of-sight shortening on the southwest flank of about 2 cm. The displacement signal is present in a set of interferograms and it has a consistent behaviour in time, implying that it is not due to atmospheric contamination. We performed atmospheric stratification over the entire Interferogram (4 Feb 2010) to check how much of the signal correlated with the topography would disappear when removed. The correlation coefficient over the southwest flank is very small compared with the signal from the entire interferogram. We can say that in this area not much of the atmospheric effects related with the topography are present, suggesting that the signal could be deformation.
For the co/post-eruptive phases we calculated phase difference between nearby points to check their evolution in time. In the southeast flanks we observe deflation through all analyzed period, while in western flanks of the volcano we observe Inflation during effusive eruption, followed by deflation during explosive eruption, and a new inflation pattern between 05 June and 19 July that we cannot explain. In preliminary modelling we fit this post-eruptive phase with a pressure decrease of an ellipsoidal source, equivalent to a volume reduction of ~0.03 km3.
The limitations when analysing this dataset are mainly concerning the phase unwrapping performance through ice- and ash-covered areas. This is caused by decorrelation owing to ash cover where there is almost complete loss of coherence. We applied new methods to overcome these limitations. To improve point density over the scene, we combined PS (Persistent Scatterers) and SB (Small Baselines) methods. By combining highly coherent interferograms, the increase of distributed scatterers is clear and the phase unwrapping performance improved. To detect and correct non-systematic unwrapping errors, we calculated azimuth and range offsets. Additionally, because of the fact that L band has higher penetration, we processed ALOS images trough single interferogram analysis. By these means we were able to extract more of the deformation signal around decorrelated areas.
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After a period of quiescence since a sill intrusion in 1999-2000, a subtle deformation signal was again detectedat Eyjafjallajökull, beginning in the summer of 2009, at a continuous GPS station on the southern flank. At ourrequest the German Space Centre (DLR) immediately began tasking the TerraSAR-X satellite to acquire threeSAR images every 11 days, giving a time series of SAR images prior to the eruption with unprecedented temporalsampling (although interrupted by snow during the winter). Here we present the results of InSAR time seriesanalysis of this data set. After correcting for DEM errors and reduction of atmospheric signal, we find a numberof signals that we interpret in terms of magma movement.The displacement time series from June 2009 to 4 February 2010 (pre-eruptive-phase) shows line-of-sight short-ening on the south-west flank of about 2 cm. The signal shows a largely linear behaviour and is smooth in time,implying that it is not due to atmospheric contamination. The signal seems consistent with the nearby continuousGPS station THEY. We therefore interpret it as due to the intrusion of magma to shallow depths. Superimposed onthis uplift signal are two periods of subsidence, in August and November 2009, perhaps representing redistributionof the intrusion.Between 4 February and 20 March 2010 there is a large uplift signal which we model as a series of sills and a dike,with a total volume of∼0.05 km3 During the flank eruption, beginning on 20 March, no significant deformationis detected, but coinciding with the start of the explosive eruption on April 14, we detect subsidence centredon the caldera. The subsidence proceeds in an approximately steady-state fashion until the end of the eruption.In preliminary modelling we fit this with a pressure decrease of an ellipsoidal source, equivalent to a volumereduction of∼0.03 km3.@en

The 2010 eruption of Eyjafjallajökull volcano and the resulting ash cloud highlights the need for research on Icelandic volcanoes. While most of the interest was sparked by the closure of air space over much of Europe, the potentially life-threatening consequences for the people living in the area directly beneath the volcano alone are incentive enough to better understand volcanic processes. Katla volcano is directly adjacent to Eyjafjallajökull volcano, and historically has been more active and produced larger eruptions. The consequences of an eruption at Katla could therefore be much more severe than those witnessed this spring at Eyjafjallajökull. Timely prediction of an impending eruption would greatly reduce the severity of these consequences, which is one of the ultimate goals of volcanic research. After a period of quiescence since a sill intrusion in 1999-2000, a subtle deformation signal was again detected at Eyjafjallajökull, beginning in the summer of 2009, at a continuous GPS station on the southern flank. We immediately began tasking the TerraSAR-X satellite to acquire SAR images every 11 days, giving a time series of SAR images prior to the eruption with unprecedented temporal sampling (although interrupted by snow during the winter). Here we present the results of InSAR time series analysis of this data set. After correcting for DEM errors and reduction of atmospheric signal we find a number of signals that we tentatively interpret as a combination of magma movement, elastic response to snow melting and landsliding.. The mean velocities from June 2009 to February 2010 show a subsidence pattern in the southeastern part of the volcano flanks and uplift in the southwest. However, such a different deformation signal between two areas so close could also imply atmospheric, topographic or phase unwrapping errors. To assess the contribution to the deformation signal from these possible error sources, we examined time series of displacements during this period for various areas. The results show a largely linear behavior between nearby areas from 18th June 2009 to 04 February 2010, followed by an excursion in the deformation signal during 17th October 2009. Significantly, the signal is smooth in time, implying that it is not due to atmospheric contamination. The deformation seems consistent with the continuous GPS station THEY, and can indeed indicate magma migration. However, further work is required to reliably separate out the deformation signals that are not related to volcanic processes.@en