Askja Caldera Research Articles

Morphological changes of the south-eastern wall of Askja caldera, Iceland over the past 80 years

Calderas are subcircular depressions with near-vertical walls, which are often gravitationally unstable and prone to mass movements that sequentially widen their basins. However, the details of these erosional changes are difficult to decipher due to short observational periods. Here, we use a photogrammetric dataset of nearly 80 years to study the landslide-prone south-eastern wall of Askja caldera (Iceland). We analyzed aerial data from 1945 and 1987, stereo satellite data from 2013 and 2022, and drone images acquired in 2019, 2022, and 2023. We developed an inventory of geomorphological features and identified types of slope instability. We describe over 700 features, including circa 500 fractures, 200 sinkholes, and four major landslides. We found that morphological changes were persistent over the observation period, accumulating in a sector that collapsed in 2014. We discuss various factors of slope instability at Askja including possible volcano-permafrost interaction, and other processes that could induce mass wasting.

Evidence of a Shallow Magma Reservoir Beneath Askja Caldera, Iceland, From Body Wave Tomography

AbstractIn August 2021, Askja caldera switched to reinflation following ∼40 years of continuous deflation that was first measured some 20 years after its last eruption in 1961. Various lines of evidence, including from geodetic modeling, suggest that both the deflation and reinflation events are related to a shallow magma body. To better understand the subsurface plumbing system, we derive P‐wave velocity (Vp), S‐wave velocity (Vs), and Vp/Vs models of the mid‐upper crust by leveraging a new local earthquake traveltime data set. A cylindrical low‐velocity zone, ∼3 km wide and extending to ∼8 km below sea level (bsl), was imaged beneath the caldera. Within it, two distinct lower velocity and higher Vp/Vs anomalies are illuminated, one centered at ∼0.5 km and the other at ∼6 km bsl. The shallower anomaly lies directly beneath the zone of uplift and is likely associated with the current reinflation event.

Mapping Ice Buried by the 1875 and 1961 Tephra of Askja Volcano, Northern Iceland Using Ground‐Penetrating Radar: Implications for Askja Caldera as a Geophysical Testbed for In Situ Resource Utilization

AbstractEruptions of the Askja Volcano in Northern Iceland in 1875 and 1961 blanketed the caldera with rhyolitic and basaltic tephra deposits, respectively, which preserved layers of seasonal snowpack as massive ice. Askja serves as an operational and geophysical analog to test ground‐penetrating radar field and analysis techniques for in situ resource utilization objectives relevant to the martian and lunar environments. We conducted ground‐penetrating radar surveys at center frequencies of 200, 400, and 900 MHz to map the thickness and extent of tephra deposits and underlying massive ice at three caldera sites. We identified up to 1 m of tephra preserving up to 4.4 m of massive ice. We measured the real dielectric permittivity of the overlying tephra and the total attenuation at each frequency of the tephra and ice. A key objective of our investigation was to determine if attenuation (or loss) could be used as an additional diagnostic signature of massive ice preserved at depth when compared to ice‐free stratigraphy. Loss rates of the ice‐rich subsurface decrease with increasing ice thickness relative to the overburden, which may constitute a possible signature. Attenuation also increased with increasing frequency. The tephra, ice, and other volcanic deposits at each of our three caldera sites and the ice‐free, pumice‐mantled 1961 Vikrahraun lava flow exhibited consistently low loss rates at all frequencies. This result highlights the ambiguity associated with identifying the unique signature of ice within low‐loss stratigraphies, a possible challenge for its identification in the martian or lunar subsurface using radar.

A Machine Learning-Based Thermobarometer for Magmatic Liquids

Abstract Experimentally calibrated models to recover pressures and temperatures of magmas are widely used in igneous petrology. However, large errors, especially in barometry, limit the capacity of these models to resolve the architecture of crustal igneous systems. Here, we apply machine learning to a large experimental database to calibrate new regression models that recover P–T of magmas based on melt composition plus associated phase assemblage. The method is applicable to compositions from basalt to rhyolite, pressures from 0.2 to 15 kbar, and temperatures of 675°C to 1400°C. Testing and optimisation of the model with a filter that removes estimates with standard deviation above the 50th percentile show that pressures can be recovered with root-mean-square-error (RMSE) of 1.1 to 1.3 kbar and errors on temperature estimates of 21°C. Our findings demonstrate that, given constraints on the coexisting mineral assemblage, melt chemistry is a reliable recorder of magmatic variables. This is a consequence of the relatively low thermodynamic variance of natural magma compositions despite their relatively large number of constituent oxide components. We apply our model to two contrasting cases with well-constrained geophysical information: Mount St. Helens volcano (USA), and Askja caldera in Iceland. Dacite whole-rocks from Mount St Helens erupted 1980 to 1986, inferred to represent liquids extracted from cpx–hbl–opx–plag–mt–ilm mush, yield melt extraction source pressures of 5.1 to 6.7 kbar in excellent agreement with geophysical constraints. Melt inclusions and matrix glasses record lower pressures (0.7–3.8 kbar), consistent with magma crystallisation within the upper reaches of the imaged geophysical anomaly and during ascent. Magma reservoir depth estimates for historical eruptions from Askja match the location of seismic wave speed anomalies. Vp/Vs anomalies at 5 to 10 km depth correspond to hot (~990°C) rhyolite source regions, while basaltic magmas (~1120°C) were stored at 7 to 17 km depth under the caldera. These examples illustrate how our model can link petrology and geophysics to better constrain the architecture of volcanic feeding systems. Our model (MagMaTaB) is accessible through a user-friendly web application (https://igdrasil.shinyapps.io/MagmaTaBv4/).

Decades of subsidence followed by rapid uplift: Insights from microgravity data at Askja Volcano, Iceland

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.

Hydrothermally altered deposits of 2014 Askja landslide, Iceland, identified by remote sensing imaging

Volcanic flanks subject to hydrothermal alteration become mechanically weak and gravitationally unstable, which may collapse and develop far-reaching landslides. The dynamics and trajectories of volcanic landslides are hardly preserved and challenging to determine, which is due to the steep slopes and the inherent instability. Here we analyze the proximal deposits of the 21 July 2014, landslide at Askja (Iceland), by combining high-resolution imagery from satellites and Unoccupied Aircraft Systems. We performed a Principal Component Analysis in combination with supervised classification to identify different material classes and altered rocks. We trained a maximum-likelihood classifier and were able to distinguish 7 different material classes and compare these to ground-based hyperspectral measurements that we conducted on different rock types found in the field. Results underline that the Northern part of the landslide source region is a hydrothermally altered material class, which bifurcates halfway downslope and then extends to the lake. We find that a large portion of this material is originating from a lava body at the landslide headwall, which is the persistent site of intense hydrothermal activity. By comparing the classification result to in-situ hyperspectral measurements, we were able to further identify the involved types of rocks and the degree of hydrothermal alteration. We further discuss associated effects of mechanical weakening and the relevance of the heterogeneous materials for the dynamics and processes of the landslide. As the study demonstrates the success of our approach for identification of altered and less altered materials, important implications for hazard assessment in the Askja caldera and elsewhere can be drawn.

Comment on “Dynamics of the Askja caldera July 2014 landslide, Iceland, from seismic signal analysis: precursor, motion and aftermath” by Schöpa et al. (2018)

Comment on “Dynamics of the Askja caldera July 2014 landslide, Iceland, from seismic signal analysis: precursor, motion and aftermath” by Schöpa et al. (2018)

Seismicity of the Askja and Bárðarbunga volcanic systems of Iceland, 2009–2015

A large seismic network deployed in the Icelandic highlands recorded >100,000 earthquakes from 2009 to 2015. We develop a local magnitude scale, appropriate for use in central Iceland, which is similar to the scale used by the Iceland Meteorological Office. Using this large catalogue of earthquakes, we analyze the spatial and temporal changes in seismicity rates and b-values. In microearthquakes recorded from the usually ductile lower crust we find that b-values are high, reflecting the presence of high thermal gradients and low stresses driving seismicity associated with the movement of melt. In contrast, b-values in the upper crust are variable. Low b-values, indicative of a high stress environment, are observed during seismic swarms such as those around Mt. Herðubreið and around Bárðarbunga caldera. A persistently seismically active area around a geothermal area within Askja caldera has a b-value around 1 but has a strong annual cycle of seismicity. We attribute the annual cycle to varying load from the snow cover modulating the seismicity. Seismicity driven by the intrusion of a large dyke has a b-value well above 1, driven by the high pore fluid pressures and thermal gradients around the dyke.

Dynamics of the Askja caldera July 2014 landslide, Iceland, from seismic signal analysis: precursor, motion and aftermath

Abstract. Landslide hazard motivates the need for a deeper understanding of the events that occur before, during, and after catastrophic slope failures. Due to the destructive nature of such events, in situ observation is often difficult or impossible. Here, we use data from a network of 58 seismic stations to characterise a large landslide at the Askja caldera, Iceland, on 21 July 2014. High data quality and extensive network coverage allow us to analyse both long- and short-period signals associated with the landslide, and thereby obtain information about its triggering, initiation, timing, and propagation. At long periods, a landslide force history inversion shows that the Askja landslide was a single, large event starting at the SE corner of the caldera lake at 23:24:05 UTC and propagating to the NW in the following 2 min. The bulk sliding mass was 7–16 × 1010 kg, equivalent to a collapsed volume of 35–80 × 106 m3. The sliding mass was displaced downslope by 1260 ± 250 m. At short periods, a seismic tremor was observed for 30 min before the landslide. The tremor is approximately harmonic with a fundamental frequency of 2.3 Hz and shows time-dependent changes of its frequency content. We attribute the seismic tremor to stick-slip motion along the landslide failure plane. Accelerating motion leading up to the catastrophic slope failure culminated in an aseismic quiescent period for 2 min before the landslide. We propose that precursory seismic signals may be useful in landslide early-warning systems. The 8 h after the main landslide failure are characterised by smaller slope failures originating from the destabilised caldera wall decaying in frequency and magnitude. We introduce the term “afterslides” for this subsequent, declining slope activity after a large landslide.

Interaction between central volcanoes and regional tectonics along divergent plate boundaries: Askja, Iceland

Activity within magmatic divergent plate boundaries (MDPB) focuses along both regional fissure swarms and central volcanoes. An ideal place to investigate their mutual relationship is the Askja central volcano in Iceland. Askja consists of three nested calderas (namely Kollur, Askja and Oskjuvatn) located within a hyaloclastite massif along the NNE-SSW trending Icelandic MDPB. We performed an extensive field-based structural analysis supported by a remote sensing study of tectonic and volcanic features of Askja’s calderas and of the eastern flank of the hyaloclastite massif. In the massif, volcano-tectonic structures trend N 10° E to N 40° E, but they vary around the Askja caldera being both parallel to the caldera rim and cross-cutting on the Western side. Structural trends around the Oskjuvatn caldera are typically rim parallel. Volcanic vents and dikes are preferentially distributed along the caldera ring faults; however, they follow the NNE-SSW regional structures when located outside the calderas. Our results highlight that the Askja volcano displays a balanced amount of regional (fissure-swarm related) and local (shallow-magma-chamber related) tectonic structures along with a mutual interaction among these. This is different from Krafla volcano (to the north of Askja) dominated by regional structures and Grimsvotn (to the South) dominated by local structures. Therefore, Askja represents an intermediate tectono-magmatic setting for volcanoes located in a slow divergent plate boundary. This is also likely in accordance with a northward increase in the spreading rate along the Icelandic MDPB.

The 2014 Lake Askja rockslide‐induced tsunami: Optimization of numerical tsunami model using observed data

AbstractA large rockslide was released from the inner Askja caldera into Lake Askja, Iceland, on 21 July 2014. Upon entering the lake, it caused a large tsunami that traveled about ∼3 km across the lake and inundated the shore with vertical runup measuring up to 60–80 m. Following the event, comprehensive field data were collected, including GPS measurements of the inundation and multibeam echo soundings of the lake bathymetry. Using this exhaustive data set, numerical modeling of the tsunami has been conducted using both a nonlinear shallow water model and a Boussinesq‐type model that includes frequency dispersion. To constrain unknown landslide parameters, a global optimization algorithm, Differential Evolution, was employed, resulting in a parameter set that minimized the deviation from measured inundation. The tsunami model of Lake Askja is the first example where we have been able to utilize field data to show that frequency dispersion is needed to explain the tsunami wave radiation pattern and that shallow water theory falls short. We were able to fit the trend in tsunami runup observations around the entire lake using the Boussinesq model. In contrast, the shallow water model gave a different runup pattern and produced pronounced offsets in certain areas. The well‐documented Lake Askja tsunami thus provided a unique opportunity to explore and capture the essential physics of landslide tsunami generation and propagation through numerical modeling. Moreover, the study of the event is important because this dispersive nature is likely to occur for other subaerial impact tsunamis.

Oxygen isotopes in melt inclusions and glasses from the Askja volcanic system, North Iceland

Primitive melt inclusions trapped during the earliest stages of fractional crystallisation are able to preserve oxygen isotope ratios inherited from mantle-derived melts. However, assimilation of low-δ18O hydrothermally altered crustal material and mixing with magmas held in shallow reservoirs may exert a strong control on the δ18O of melt inclusions trapped during later stages of crystallisation. Oxygen isotope ratios in olivine- and plagioclase-hosted melt inclusions and glasses from tephra samples collected from the Askja central volcano and Askja volcanic system indicate significant differences in the mechanisms of magma supply and storage between the northern and southern segments of the Askja volcanic system. Melt inclusions from the Holuhraun fissure eruption, ∼20km south of Askja, mostly preserve δ18O signatures of +4.1‰ to +5.4‰, suggesting that this magma underwent minimal modification by magma mixing or crustal assimilation prior to its eruption. By contrast, melt inclusions and glasses from the Nýjahraun fissure eruption, ∼60km north of Askja, have δ18O between +3.1‰ and +4.0‰. These relatively evolved melt inclusions (∼3.9–4.3wt.% MgO) were probably trapped during late-stage fractional crystallisation in a shallow magma storage zone. Melt inclusions from two phreatomagmatic tuff sequences within the Askja caldera have δ18O between +2.1‰ and +5.2‰, and this variability cannot be explained by mixing with low-δ18O rhyolitic or andesitic contaminants in the upper crust. Instead, mixing of the ascending magmas with hydrated, low-δ18O basaltic magmas is invoked, thus acquiring a low δ18O signature with minimal modification to the magma’s bulk composition. Such magma bodies are likely to be found throughout the upper 11km of the crust beneath Askja. Assimilation of low-δ18O meta-basalt in the upper crust is also likely to affect the δ18O of ascending magmas.

Subsidence of Askja caldera 2000–2009: Modelling of deformation processes at an extensional plate boundary, constrained by time series InSAR analysis

Many calderas in the world show long-term unrest in the form of elevated rates of deformation and seismicity, related to pressure changes and magma movements within their magmatic plumbing systems. We present new observations of the style of deformation at the Askja caldera, Iceland, since 2000, using interferometric analysis of synthetic aperture radar images (InSAR) acquired by the Canadian RADARSAT-2 satellite. When combined with previously acquired detailed geodetic observations, by various techniques, we obtain an overview of Askja's behaviour through more than four decades. The combined dataset reveals that, during this non-eruptive period, Askja continuously subsides at a rate of 2.5–3cm/yr in 2000–2009, somewhat lower than the ~5cm/yr rate inferred for the 1983–1998 period. This behaviour of Askja is evaluated and compared to those of other restless calderas. The wrapped interferograms show three main features: (1) concentric fringes depicting subsidence in the centre of the main Askja caldera, (2) oval fringes elongated along the rift portraying subsidence and (3) subsidence in an area north of the Vatnajökull glacier. The average line-of-sight (LOS) velocity from ground to satellite was used as input for inverse modelling, of a deflating pressure source beneath the caldera, embedded in a homogeneous, elastic half-space. Two different source geometries were tested: a point pressure source and a horizontal penny shaped crack. The concentric fringes centred in the Askja caldera are best fit by a point source located at 65.05°N 16.78°W, at a depth of 3.2–3.8km with a volume decrease of 0.0012–0.0017km3/yr from 2000 to 2009, consistent with previous studies. Provisional 2D FEM models including structural complexities in the crustal layers indicate that the tectonic setting of Askja plays an important role in the continuous, long-term high subsidence rates observed there. In order to fully understand the cause and effects of the complicated tectonic setting we encourage the use of a more realistic rheological model of the area, which could lead to reinterpretation of previous model results.

A model for syn-eruptive groundwater flow during the phreatoplinian phase of the 28–29 March 1875 Askja volcano eruption, Iceland

We present a groundwater flow model that integrates geological observations, field data, effective permeabilities for fractured lava flows, and historical eyewitness records to explain the change from wet to dry explosive activity that occurred in March 1875 at Askja volcano, Iceland. This rhyodacite eruption is the third largest silicic explosive event in Iceland since settlement in this region (approximately 1140 B.P.). It was part of a larger volcano-tectonic episode that took place in the North Volcanic Zone from 1874 to 1876. This eruption is unique in that it features both abrupt and gradual shifts between wet and dry explosive activities. It consisted of an initial and sudden shift from dry subplinian to wet phreatoplinian activity followed by a gradual shift from wet phreatoplinian to increasingly dry pyroclastic density currents to a final dry Plinian phase. The Askja caldera overlaps the Öskjuvatn caldera and it is bounded by steep-sided hyaloclastic mountains on all sides. Its floor is filled by Holocene and historical lava flows which are intensely fractured. The source of water that fuelled the phreatoplinian activity is not obvious, as eyewitness records rule out snow, surface water, and heavy rainfall. Hence we use 3D numerical simulations of the syn-eruptive groundwater flow in the Askja caldera, including an accurate estimation of the effective permeabilities of fractured lava flows, to explain the possible changes in eruptive style. We propose that the water necessary to maintain the phreatoplinian phase was stored within the lava pile and that the rapid groundwater flow through the fractured lavas provided enough water to drive the hour-long phreatoplinian eruption. Our model describes how the decline of the groundwater table caused the progressive dry-out of the wet phase and led to the dry character of the following Plinian activity. This study demonstrates how the availability of groundwater and the geology of the volcanic edifice can strongly affect the eruptive style of a volcanic eruption.

1983–2003 decaying rate of deflation at Askja caldera: Pressure decrease in an extensive magma plumbing system at a spreading plate boundary

1983-2003 decaying rate of deflation at Askja caldera: Pressure decrease in an extensive magma plumbing system at a spreading plate boundary

Deflation of the Askja volcanic system: Constraints on the deformation source from combined inversion of satellite radar interferograms and GPS measurements

The Askja central volcano in northern Iceland has been continuously subsiding at least since 1983. GPS and optical leveling tilt measurements show subsidence of at least 75 cm from 1983 to 1998 in the center of the Askja caldera, without any eruptive activity. We have performed an interferometric analysis of Synthetic Aperture Radar images (InSAR) of the area, utilizing data from the ERS satellites. We observe subsidence of the Askja caldera and its fissure swarm, up to a distance of 25 km from the volcano. We evaluate the geometry of the magma chamber at Askja, from a combined inversion of satellite radar interferograms and GPS measurements. Several models were tested, including a Mogi point source as well as an ellipsoidal source. The use of an ellipsoidal source instead of a Mogi source gives an estimate of the dimensions of the magma chamber and its deflating pressure, whereas these parameters are not independently resolved if a Mogi source is used. Two-source models were also considered in order to explain the additional subsidence observed along the Askja fissure swarm. We tested a model using two Mogi sources at different depths, a shallow ellipsoidal cavity with a deeper Mogi source, and then a shallow Mogi source with a deeper elongated ellipsoid, oriented along the fissure swarm. Results indicate that an ellipsoidal source at about 3 km depth can accommodate most of the deflation occurring in the caldera. Residual subsidence occurs along the Askja fissure swarm suggesting the existence of a deeper source of contraction. We interpret this signal in terms of subsidence of the plate boundary.

Net gravity decrease at Askja volcano, Iceland: constraints on processes responsible for continuous caldera deflation, 1988–2003

Askja caldera in northeast Iceland has been in a state of unrest for decades. Ground-deformation surveys show that the rate of deformation, i.e., deflation, is much higher then observed at any other dormant volcano in Iceland. This work presents the results from microgravity and deformation studies at Askja from 1988 to 2003. The deflation reaches a maximum of −0.46 m in the centre of the caldera, relative to a station outside the caldera, during the study period. The source of deformation is inferred to be at ∼3 km depth and a recent study infers a second deeper source at ∼16 km depth. The deflation is consistent with a subsurface volume change of −0.018 km 3. We find a net microgravity decrease of 115 μGal in the centre of the caldera relative to the same station. This corresponds to a subsurface mass decrease of 1.6×10 11 kg between 1988 and 2003 based on the use of a point source model. A combination of magma drainage and cooling and contraction of the shallow magma reservoir at 3 km depth is our favoured model, consistent with the integrated observations. We suggest that extensional tectonic forces generate space in the ductile part of the crust to accommodate ongoing magma drainage from the shallow magma chamber.

Volcanic and tectonic processes coinciding with glaciation and crustal rebound: an early Holocene rhyolitic eruption in the Dyngjufjöll volcanic centre and the formation of the Askja caldera, north Iceland

A pronounced volcanic production maximum on the rift zones through Iceland coincided with rapid crustal rebound during and after glacier melting at the Pleistocene/Holocene boundary. At peak glaciation, ice thickness over central Iceland may have reached 1,500–2,000 m, causing 400–500-m depression of the crust. Rapid climatic improvement caused glacier melting and removal of the ice load within about 1,000 years. Low mantle viscosity resulted in rapid crustal rebound which was completed in about 1,000 years, with an average rate of uplift on the order of nearly half a metre per year over central Iceland. High volcanic production rate is documented by tephrachronological dating and volume estimates of several large-volume monogenetic lava shields and polygenetic volcanic centres along the plate boundary. A Plinian rhyolitic eruption, dated at about 10 ka within the Askja caldera in the Dyngjufjoll volcanic centre, left a pumice deposit which serves as a marker horizon during this remarkable, high-intensity period in the history of the volcano. At the time of the eruption, glaciers had retreated from the coastal areas of the country but the central, elevated parts were covered with a thinning glacier. The Plinian eruption (1–2 km3 dense rock equivalent) was triggered by pressure release caused by glacier melting and volatile supersaturation. Distal deposits of rhyolitic pumice are found in soil sections in coastal areas of eastern and northern Iceland where the pumice occurs between tephra layers from other sources which have been dated by independent methods. A few proximal deposits are preserved within the Dyngjufjoll centre. These provide age constraints on major tectonic and volcanic events during the period of crustal rebound, before and after complete glacier removal. The rhyolitic pumice is sandwiched between thick layers of phreatomagmatic basaltic tephra formed in an open melt-water lake. Sharp contacts between the deposits suggest quick succession of events, but lack of mixing between magmas indicates separate vent locations. The vent location for the 10-ka Plinian eruption has been obliterated by later events but available evidence, supported by a gravity survey, suggests its location in the central part of the Dyngjufjoll volcano. The eruption formed a subsidence structure, presently seen as an embayment in the hyaloclastite mountain block in the southern part of the Askja caldera where a second Plinian eruption occurred in 1875 A.D. The Plinian 10-ka eruption occurred while thinning glaciers were still present over central Iceland, but the Askja caldera formed after lavas started to flow over ice-free surfaces. The morphology of the caldera faults suggests non-simultaneous movement, and the tectonics are not easily compatible with conventional caldera models. A model is proposed involving uplift of tectonically well-defined crustal blocks to the north and west of the Askja caldera, combined with downsagging caused by voluminous outpouring of basaltic lava. The southern and eastern borders of the caldera are remnants of a subsidence following the 10-ka Plinian eruption, partly reactivated by the 1875 A.D. Plinian eruption. The model provides a satisfactory explanation for the enigmatic Oskjuop pass, and it is in agreement with a gravity survey of the Dyngjufjoll centre. The uplift coincided with rapid crustal rebound which was amplified by crustal deformation (doming) of the volcanic centre caused by high magmatic pressure in the plumbing system of the volcano. This is supported by emission of very large lava flows produced in the first millennia of the Holocene.

Continuous deflation of the Askja caldera, Iceland, during the 1983–1998 noneruptive period

The Askja volcano at the spreading plate boundary in north Iceland consists of nested calderas, the latest formed in an eruption in 1875. Several eruptions have occurred since in Askja, the most recent in 1961. Precise leveling has been conducted yearly at Askja since 1983. In 1993, a dense GPS network was measured in and around the Askja caldera consisting of more than 20 points, and we remeasured this complete network for the first time in 1998. Askja subsided during the period from 1983 to 1998. Observed deformation fits broadly with a “Mogi” point source model with best fitting location near the center of the main Askja caldera (65.0448°N, 16.7805°W) at a depth of 2.8 km. From 1983 to 1991 the yearly subsidence rate between the end points of a leveling profile decayed gradually from ∼10 mm/yr to an average of ∼7 mm/yr in the 1991–1998 period. Total subsidence at the Askja center in the 1983–1998 period is at least 75 cm, and the integrated volume of surface subsidence is ∼0.037 km3. This period has been a “quiet” period at Askja with no eruptions, large earthquakes or known dike injections. Such a high rate of “background” deformation has not been observed at other volcanoes in Iceland. The eruption of a neighboring volcano in 1996 has not resulted in a modified deformation pattern at Askja, indicating no pressure connection at depth between the two systems. Solidification may account for part of the observed contraction and subsidence in Askja.

Post-eruptive gravity changes from 1990 to 1996 at Krafla volcano, Iceland

The 1975–1984 Krafla rifting episode was a major lava- and dyke-producing event associated with the release of extensional strain accumulated over more than 200 years at the divergent plate boundary in North Iceland. The present work provides a unique example of gravity decreases and increases sustained over a long period following a major eruptive episode at a rift volcano. After height correction, persistent net gravity decreases over the source of observed Mogi-type deflation occur with gravity increases occurring further away from this centre of deformation. Gravity decreases are interpreted in terms of drainage from a shallow magma chamber. The net gravity decreases require that at least 4×10 10 kg of magma must have been drained during the 6-year observation period. Assuming a density of 2700 kg m −3, this magma would occupy 1.5×10 7 m 3 and by analogy with results obtained for Kilauea, this implies a magma chamber volume change of 4.1×10 6 m 3. This is consistent with the chamber volume change deduced from ground deformation data assuming a Poisson's ratio of 0.25 and a Mogi source. Net gravity increases are more spatially extensive and are most likely caused by migration of the steam–water interface and/or closure of micro-fractures in lavas above the magma chamber during post-eruptive cooling and contraction. We present a model for the Krafla magma chamber in which a cooling, contracting and draining magma body causes subsidence at the surface. These results contrast with observations from the Askja caldera, Iceland, where post-eruptive deflation has been shown to be accompanied by negligible net gravity changes above the Mogi-type source in the caldera. Long-term post-eruptive deflation and magma drainage have not been observed at subduction-related volcanoes; this may be a function of a difference in magma viscosity.