Vatnajokull Ice Cap Research Articles

Icecap and Subglacial Crustal Deformation Inferred From SAR Pixel Tracking: The 2014 Dike Intrusion Episode in the Bárðarbunga Volcanic System, Iceland

AbstractBárðarbunga is an active volcano beneath the Vatnajökull icecap in Iceland, where a subglacial dike intrusion occurred in 2014. This area has been studied with interferometric synthetic aperture radar, an important geodetic method that measures crustal deformation; however, ice/snow cover on volcanoes impedes the mapping of crustal deformation because of decorrelation problems. Previous geodetic observations have reported deformation signals at ice‐free regions of a major dike formed in 2014 in the Bárðarbunga volcanic system, but direct observations of the subglacial crustal deformation associated with the dike intrusion have been limited. We applied a pixel tracking approach to various satellite synthetic aperture radar (SAR) data over the northern part of the Vatnajokull icecap and the Holuhraun plain. The pixel tracking data revealed not only crustal deformation fields in the ice‐free region of the magma intrusion, which covers only about 20% of the entire length of dike, but also icecap surface movements over the subglacial part of the dike in the ablation area of the Vatnajökull icecap. Signals above the icecap, suggesting subsidence due to subglacial graben formation, are consistent with the dike propagation path inferred from seismicity during the episode. By subtracting the scaled pre‐diking signals from the co‐diking signals, we corrected for the steady‐state icecap flow signals to derive the subglacial crustal deformations. We showed that the inferred subglacial crustal deformation signals can significantly contribute to the improvement of dike opening/faulting distributions. Applying the pixel tracking to satellite images will enable mapping subglacial crustal deformation in the case of subglacial volcanic activity.

Integration of SAR Data Into Monitoring of the 2014–2015 Holuhraun Eruption, Iceland: Contribution of the Icelandic Volcanoes Supersite and the FutureVolc Projects

We report how data from satellite and aerial synthetic aperture radar (SAR) observations were integrated into monitoring of the 2014-2015 Holuhraun eruption in the Barðarbunga volcanic system, the largest effusive eruption in Iceland since the 1783-84 Laki eruption. A lava field formed in one of the most remote areas in Iceland, after the propagation of a ~50 km-long dyke beneath the Vatnajokull ice cap, where the Barðarbunga caldera is located. Due to the 6 month duration of the eruption, mainly in wintertime, daily monitoring was particularly challenging. During the eruption, the European volcanological project FutureVolc was ongoing, allowing collaboration of many European experts on volcano monitoring activities. Icelandic volcanoes are also a permanent Supersite within the Geohazard Supersites and Natural Laboratories (GSNL) initiative, with support from the Committee on Earth Observation Satellite (CEOS) in the form of a large collection of SAR images. SAR data were acquired by Cosmo-SkyMed (CSK) and TerraSAR-X (TSX) satellites and complemented by aerial SAR images. The large set of SAR satellite data significantly contributed to the daily monitoring during the unrest at Barðarbunga caldera, the Holuhraun eruption and the year following the eruption. Detection of surface changes using both SAR amplitude and phase information was conducted throughout the whole duration of the volcano-tectonic event, and in the following months, to quantify and track the evolution of volcanic processes at Holuhraun and geothermal activity at Barðarbunga volcano. Combination of SAR data with other data sets, e.g. satellite optical images and geodetic Global Positioning System (GPS) measurements, was essential for the evaluation of the volcanic hazard in the whole area. International collaboration within the Futurevolc project formed the basis for successful analyses and interpretation of the large SAR data set. Information was provided at Scientific Advisory Board meetings of the Icelandic Civil Protection and used in decision-making, as well as for supporting field-deployment and air-based surveys.

The collapse of Bárðarbunga caldera, Iceland

Lying below Vatnajokull ice cap in Iceland, Barðarbunga stratovolcano began experiencing wholesale caldera collapse in 2014 August 16, one of the largest such events recorded in the modern instrumental era. Simultaneous with this collapse is the initiation of a plate boundary rifting episode north of the caldera. Observations using the international constellation of radar satellites indicate rapid 50 cm d^(−1) subsidence of the glacier surface overlying the collapsing caldera and metre-scale crustal deformation in the active rift zone. Anomalous earthquakes around the rim of the caldera with highly nondouble-couple focal mechanisms provide a mechanical link to the dynamics of the collapsing magma chamber. A model of the collapse consistent with available geodetic and seismic observations suggests that the majority of the observed subsidence occurs aseismically via a deflating sill-like magma chamber.

InSAR observations and models of crustal deformation due to a glacial surge in Iceland

Surges are common at all the major ice caps in Iceland. Ice masses of gigatons may shift from the upper part of the outlet glacier towards the terminus in a few months, advancing the glacier front by up to several kilometres. The advancing ice front may be up to 100 m thick, increasing the load on crustal rocks correspondingly. We use the observed change in crustal loading during a surge of the western part of the Vatnajokull ice cap, Iceland, during 1993–1995 and the corresponding elastic crustal deformation, surveyed with interferometric synthetic aperture radar, to investigate the material properties of the solid Earth in this region. Crustal subsidence due to the surge reaches ∼75mm at the edge of the Siðujokull outlet glacier. This signal is mixed with a broad uplift signal of ∼12 mm/yr, relative to our reference area, caused by the ongoing retreat of Vatnajokull in response to climate change. We disentangle the two signals by linear inversion. Finite element modelling is used to investigate the elastic Earth response of the surge, as well as to confirm that no significant viscoelastic deformation occurred as a consequence of the surge. The modelling leads to estimates of the Young’s modulus and Poisson’s ratio of the underlying Earth. Comparison between the observed and modelled deformation fields is made using a Bayesian approach that yields the estimate of a probability distribution for each of the free parameters. Residuals indicate a good agreement between models and observations. One-layer elastic models result in a Young’s modulus of 43.2–49.7 GPa (95 per cent confidence) and Poisson’s ratio of 0–0.27, after removal of outliers. Our preferred model, with two elastic layers, provides a better fit to the whole surge signal. This model consists of a 1-km-thick upper layer with an average Young’s modulus of 12.9–15.3 GPa and Poisson’s ratio of 0.17, overlying a layer with an average Young’s modulus of 67.3–81.9 GPa and Poisson’s ratio of 0.25.

Holocene volcanic activity at Grímsvötn, Bárdarbunga and Kverkfjöll subglacial centres beneath Vatnajökull, Iceland

Assessment of potential future eruptive behaviour of volcanoes relies strongly on detailed knowledge of their activity in the past, such as eruption frequency, magnitude and repose time. The eruption history of three partly subglacial volcanic systems, Grimsvotn, Bardarbunga and Kverkfjoll, was studied by analysing tephra from soil profiles around the Vatnajokull ice-cap, which extend back to ~7.6 ka. Well known regional Holocene marker tephra (e.g. H3, H4, H5) were utilized to correlate profiles. Stratigraphic positions and geochemical compositions were used for fine-scale correlation of basaltic tephra. Around Vatnajokull ice-cap 345 tephra layers were identified, of which 70% originated from Grimsvotn, Bardarbunga or Kverkfjoll. The eruption frequency of each volcanic system was estimated; Grimsvotn has been the most active with an average of ~7 eruptions/100 years (range 4–14) during prehistoric time (before ~870 AD); Bardarbunga has been the second most active with ~5 eruptions/100 years (range 1–8); and Kverkfjoll has remained essentially calm with 0–3 eruptions/100 years but showing periodic activity with repose times of >1000 years. All three volcanic systems experienced lulls in activity from 5 ka to 2 ka, referred to as the “Mid-Holocene low”. This reduced eruption frequency appears to have resulted from a decrease in magma generation and delivery from the mantle plume rather than from changes in ice-load/glacier thickness. In prehistoric time, there was a time lag of 1000–3000 years between a peak of activity at volcanoes directly above the mantle plume versus at volcanoes located in the non-rifting part of the Eastern Volcanic Zone, closer to the periphery of the island. This time-space relationship suggests that a significant future increase in volcanism can be expected there, following increased levels of volcanism above the plume.

Climate variability and glacial processes in eastern Iceland during the past 700 years based on varved lake sediments

Properties of varved sediments from Lake Logurinn in eastern Iceland and their link to climate and glacial processes of Eyjabakkajokull, an outlet glacier of the Vatnajokull icecap, were examined. A varve chronology, which covers the period AD 1262-2005, was constructed from visual observations, high-resolution images, X-ray density and geochemical properties determined from X-radiography and X-ray fluorescence scanning. Independent dating provided by 137Cs analysis and eight historical tephras verify the varve chronology. The thickness of dark-coloured seasonal laminae, formed mainly of coarser suspended matter from the non-glacial river Grimsa, is positively correlated (r=0.70) with winter precipitation, and our 743-year-long varve series indicates that precipitation was higher and more varied during the later part of the Little Ice Age. Light-coloured laminae thickness, controlled mainly by the amount of finer suspended matter from the glacial river Jokulsa i Fljotsdal, increased significantly during the AD 1972 surge of Eyjabakkajokull. As a consequence of the surge, the ice-dammed Lake Haoldulon formed and recurrently drained and delivered significant amounts of rock flour to Lake Logurinn. Based on these observations, and the recurring cyclic pattern of periods of thicker light-coloured laminae in the sediment record, we suggest that Eyjabakkajokull has surged repeatedly during the past 743 years, but with an increased frequency during the later part of the Little Ice Age.

HIRLAM experiments on surface energy balance across Vatnajökull, Iceland

The aim of this study is to investigate the skill of the High Resolution Limited Area Model (HIRLAM) to reproduce the near-surface atmospheric conditions across the Vatnajokull Ice Cap in Iceland. This model-observation comparison study is based on a mesoscale glaciometeorological observation campaign, which has been performed during summer 1996 and provided a wealth of meteorological and glaciological data. Fine-scale hydrostatic HIRLAM experiments are based on downscaling ERA-40 analyses and the application of upper-air and surface data assimilation. The simulation results are compared to a subset of observations following a height transect across Breidamerkurjokull, a southern outlet glacier of Vatnajokull. After introduction of improvements, suggested by comparison of a reference run with observations, HIRLAM successfully simulates the surface energy balance and the driving meteorological parameters. For a correct simulation, a proper description of the constant and temporary varying physical properties of the underlying surface turned out to be crucial. The results are valuable for further improvement of operational mesoscale NWP in mountainous and high latitude environments.

Circulation and thermodynamics in a subglacial geothermal lake under the Western Skaftá cauldron of the Vatnajökull ice cap, Iceland

The subglacial, geothermal lake beneath the Western Skaftá cauldron (depression) in the Vatnajökull ice cap, Iceland, was accessed by hot water drilling through the overlying 300 m‐thick ice shelf. Most of the ca. 100‐m water column was near 4.7°C, but was underlain by a distinct ∼10 m‐deep water mass at 3.5°C. The sensible heat content of the lake water is approximately twice the potential energy dissipated in outburst floods, and the temperature of the lake may be an important factor in the development of subglacial water courses of jökulhlaups from the lake. The lake temperature is higher than the temperature of maximum density, implying that convective heat transfer can take place in the lake. The vertical temperature structure suggests a large‐scale recirculating flow in the lake, the rate of which was estimated from the lake temperatures and the chemical composition of a water sample.

Glacial-isostatic Adjustment and the Viscosity Structure Underlying the Vatnajökull Ice Cap, Iceland

We examine the dependence of glacial-isostatic adjustment (GIA) due to changes in the Vatnajokull Ice Cap, Iceland, on the underlying viscosity structure. Iceland offers a unique case study for GIA research, with a thinner elastic lithosphere underlain by a low-viscosity zone or asthenosphere, as opposed to regions such as Fennoscandia or North America described by a thicker lithosphere, while not necessarily featuring an asthenosphere.

Estimating the inundation area of a massive, hypothetical jökulhlaup from northwest Vatnajökull, Iceland

Jokulhlaups are the consequence of a sudden and significant release of meltwater from the edge of a glacier. Such floods are sourced commonly from ice-dammed lakes, but occasional volcanic eruptions beneath ice can produce intense jokulhlaups due to prodigious rates of meltwater release. Globally, volcanogenic jokulhlaups have caused fatalities and damage to infrastructure within effected catchments. Here, we present the results of one-dimensional hydraulic modelling of the inundation area of a massive, hypothetical jokulhlaup on the Jokulsa a Fjollum River in northeast Iceland; the floodwater source for this simulation is an eruption within the ice-filled caldera of Barðarbunga: an active volcano beneath the Vatnajokull ice cap. Remotely sensed data were used to derive a digital elevation model and to assign surface-roughness parameters. We used a HEC-RAS/HEC-GeoRAS system to host the hydraulic model; to calculate the steady water-surface elevation; to visualise the flooded area; and to assess flood hazards. Maximum discharge was set notionally at 180,000 m3 s−1 and the duration and volume of the jokulhlaup were placed at 39 h and 14 km3, respectively. During the simulated rise to maximum discharge, the mean velocity of the jokulhlaup was 2.8 m s−1 over a distance of 120 km. At the height of the jokulhlaup an area of 460 km2 was inundated. Modelling results showed that, along short reaches, stream-power values exceeded 11,000 W m−2; such energy conditions would have allowed boulders up to 10-m in diameter to be mobilised by the jokulhlaup. Unsteady flow was simulated along a 22-km reach of the flood tract and it revealed strong spatial and temporal variations in flood power. Besides providing insight into the erosional and depositional effects of a volcanogenic jokulhlaup, the modelling results enable estimates of the relative timing and location of likely flooding hazards.

Recent advances in understanding ice sheet dynamics

Abstract Glaciers and ice sheets play a dynamic role in Earth's climate system, influencing regional- and global-scale climate and responding to climate change on time scales from years to millennia. They are also an integral part of Earth's landscape in alpine and polar regions, where they are an active agent in isostatic, tectonic, and Earth surface processes. This review paper summarizes recent progress in understanding and modelling ice sheet dynamics, from the microphysical processes of ice deformation in glaciers to continental-scale processes that influence ice dynamics. Based on recent insights and research directions, it can be expected that a new generation of ice sheet models will soon replace the current standard. Improvements that can be foreseen in the near future include: (i) the addition of internally-consistent evolutionary equations for ice crystal fabric (anisotropic flow laws), (ii) more generalized flow laws that include different deformation mechanisms under different stress regimes, (iii) explicit incorporation of the effects of chemical impurities and grain size (dynamic recrystallization) on ice deformation, (iv) higher-order stress solutions to the momentum balance (Stokes' equation) that governs ice sheet flow, and (v) the continued merger of ice sheet models with increasingly complex Earth systems models, which include fully-coupled subglacial hydrological and geological processes. Examples from the Greenland Ice Sheet and Vatnajokull Ice Cap, Iceland are used to illustrate several of these new directions and their importance to glacier dynamics.

Volume sensitivity of Vatnajökull Ice Cap, Iceland, to perturbations in equilibrium line altitude

The sensitivity of Vatnajökull ice cap, Iceland, to perturbations in equilibrium line altitude (ELA) is analyzed by performing a series of model experiments using a shallow ice approximation (SIA) flow model. For this purpose a simple but realistic parameterization for the mass balance is used that accurately simulates the observed variability in surface mass balance over a period of nine years. We find that because of feedback between mass balance and altitude the ice cap either grows without bounds or settles to steady states depending on whether ELA is larger or smaller than a critical value ELAcrit. The largest modeled steady state is 60% of the current volume of the ice cap. The ice cap, as modeled, is therefore currently not close to a possible stable steady state. Past climate history and spatial and temporal variations in basal condition, such as surges, can be expected to have an influence on the ice cap and have to be taken into account when modeling the response of the ice cap to future climate change scenarios. For the neighboring ice cap, Hofsjökull, the relationship between ELA and volume is, in contrast, found to be simple, and Hofsjökull is close to a stable steady state with respect to the current climatic conditions. Introducing surges, which is not done here, will likely change the details of the ELA‐volume relationship of Vatnajökull, presumably by making the relationship between volume and ELA more complex, and possibly less sensitive, as a further nonlinear feature is added to the model.

The 1996 eruption at Gj�lp, Vatnaj�kull ice cap, Iceland: efficiency of heat transfer, ice deformation and subglacial water pressure

The 13-day-long Gjalp eruption within the Vatnajokull ice cap in October 1996 provided important data on ice–volcano interaction in a thick temperate glacier. The eruption produced 0.8 km3 of mainly volcanic glass with a basaltic icelandite composition (equivalent to 0.45 km3 of magma). Ice thickness above the 6-km-long volcanic fissure was initially 550–750 m. The eruption was mainly subglacial forming a 150–500 m high ridge; only 2–4% of the volcanic material was erupted subaerially. Monitoring of the formation of ice cauldrons above the vents provided data on ice melting, heat flux and indirectly on eruption rate. The heat flux was 5–6×105 W m-2 in the first 4 days. This high heat flux can only be explained by fragmentation of magma into volcanic glass. The pattern of ice melting during and after the eruption indicates that the efficiency of instantaneous heat exchange between magma and ice at the eruption site was 50–60%. If this is characteristic for magma fragmentation in subglacial eruptions, volcanic material and meltwater will in most cases take up more space than the ice melted in the eruption. Water accumulation would therefore cause buildup of basal water pressure and lead to rapid release of the meltwater. Continuous drainage of meltwater is therefore the most likely scenario in subglacial eruptions under temperate glaciers. Deformation and fracturing of ice played a significant role in the eruption and modified the subglacial water pressure. It is found that water pressure at a vent under a subsiding cauldron is substantially less than it would be during static loading by the overlying ice, since the load is partly compensated for by shear forces in the rapidly deforming ice. In addition to intensive crevassing due to subsidence at Gjalp, a long and straight crevasse formed over the southernmost part of the volcanic fissure on the first day of the eruption. It is suggested that the feeder dyke may have overshot the bedrock–ice interface, caused high deformation rates and fractured the ice up to the surface. The crevasse later modified the flow of meltwater, explaining surface flow of water past the highest part of the edifice. The dominance of magma fragmentation in the Gjalp eruption suggests that initial ice thickness greater than 600–700 m is required if effusive eruption of pillow lava is to be the main style of activity, at least in similar eruptions of high initial magma discharge.

A comparison of surface renewal theory with the observed roughness length for temperature on a melting glacier surface

The roughness lengths for momentum and temperature are calculated using the profile method on amelting glacier surface. Data from a 5-level 9-m meteorological mastpositioned near the edge of Breidamerkurjokull, an outlet glacier of the Vatnajokull ice cap Iceland, are used for the calculations. The data are selected to avoid the presence of the katabatic wind speedmaximum which would otherwise alter the scaling laws of the surface layer. The surface roughness length for momentum is determined to be 1.0 mm, similar to other estimates made on flat melting ice surfaces. The surface roughness length for temperature is found to be in good agreement with previously proposed surface renewal theories for the observed roughness Reynolds number range of 30 * 70.

Comment on “Satellite radar images capture a subglacial volcanic eruption in Iceland”] Comment: Subglacial eruptions and synthetic aperture radar images

In a news piece published in the May 4, 1999, issue of Eos,Garvin et al. present RADARSAT satellite synthetic aperture radar (SAR) images of the subglacial volcanic eruption in the Gnmsvotn caldera within the Vatnajokull ice cap, Iceland, that occurred December 18–28, 1998. As pointed out in this piece, the images show considerable changes in surface backscatter of the radar signal within the caldera during and after the eruption. The authors suggest that the dark patches with an area of approximately 4 km2, observed within Grímsvötn, are open water. On the basis of the size of the dark patches, Garvin et al. estimate the volume of ice melted to have been about 1 km3 and the energy provided for melting, E = 3 × 1017 J, to have been about 30% of that of the 1996 Gjalp eruption, which occurred 12 km north of the 1998 eruption site [Gudmundsson et al., 1997]. Our groundbased and aerial observations at Gnmsvotn, however,show that this estimate of melting and energy release is far too high.

The Surface Albedo Of The Vatnajökull Ice Cap, Iceland: A Comparison Between Satellite-Derived And Ground-Based Measurements

The temporal and spatial variations in the surface albedo of the Vatnajokull ice cap, Iceland, are investigated. A time series of the surface albedo is composed for the summer of 1996 using satellite radiance measurements from the Advanced Very High Resolution Radiometer (AVHRR). This time series is compared with ground measurements carried out during a glacio-meteorological experiment during the same summer on the ice cap. The AVHRR is able to reproduce the development in time of the surface albedo fairly well. The large systematic differences found for some of the stations on the ice are attributed to sub-pixel-scale variations in the albedo. An attempt is made to confirm this hypothesis using satellite radiance measurements carried out by the Thematic Mapper (TM) and measurements made with a portable albedometer. The TM has a pixel size of 30 × 30 m whereas the pixel size of the AVHRR is 1 × 1 km. Although the TM measurements show greater variability in the albedo than do the AVHRR measurements, the large systematic difference remains. Measurements with the portable albedometer show a large spread in the albedo at sites with large systematic differences. This implies that the scale of the albedo variations is smaller than the scale of the AVHRR and TM pixels.

Satellite radar images capture a subglacial volcanic eruption in Iceland

What are believed to be the first satellite synthetic aperture radar (SAR) images of a sub‐glacial eruption have been captured by the Canadian Space Agency's RADARSAT at the Grimsvotn volcano in Iceland. The high‐resolution images were made in late November 1998 before the eruption, in late December 1998 during the eruption, and in mid‐January 1999 after the eruption (Figure 1), all in spite of persistent cloud cover and darkness.The subglacial eruption, centered within Grimsvotn volcano, occurred between December 18 and 28, 1998. It began at the southern rim of the caldera, in the west‐central region of the Vatnajokull ice cap glacier, the largest in Europe with a total area of ∼8300 km2. During the eruption's early stages, five craters were active along an eruptive fissure, extending east‐west along the southern caldera rim.

Interferometric SAR observations of ice topography and velocity changes related to the 1996, Gjalp subglacial eruption, Iceland

A major volcanic eruption beneath the Vatnajokull ice cap from 30 September to 13 October 1996 melted up to 500m of overlying ice and produced 3.5km3 of water that was later released catastrophically onto the Skeidararsandur outwash plain. Here, we present pre- and post-event topography and velocity field maps of the ice cap surface derived from ERS-1/2 synthetic aperture radar (SAR) interferometry. Within the errors of this method our results reveal local topographic and ice flow variations near the eruption site and incision of a 140m meltwater trench. A 24-hour, 50cm subsidence of the frozen surface of the Grimsvotn caldera lake was also detected. However, despite the large increases in geothermal heat flux and basal meltwater availability associated with this event, there appears to be no regional-scale ice subsidence and little to no alteration in flow dynamics of the Vatnajokull ice cap.

Effects of subglacial geothermal activity observed by satellite radar interferometry

We use one day Synthetic Aperture Radar (SAR) interferograms from data of the Earth Remote Sensing Satellites ERS‐1 and ERS‐2 to study ice flow and uplift of two surface depressions within the Vatnajökull ice cap, Iceland. The ice cauldrons are created by melting at sub‐glacial geothermal areas. Meltwater accumulates in a reservoir under the cauldrons over 2 to 3 years until it drains in a jökulhlaup under the ice dam surrounding the reservoir. The ice surface in the depressions drops down by several tens of meters during these draining events but rises again, as ice flows into the depressions, until a jökulhlaup occurs again. Using SAR interferograms we quantify an uplift rate of about 2 to 18 cm/day within the jökulhlaup cycle varying with the surface slope of the depressions. The uplift rate is high during the first months after a jökulhlaup when the cauldron is relatively deep with steep slopes, but the uplift rate decreases as the cauldron is gradually filled. A simple axisymmetric model simulating the ice‐flow into one of the depressions describes quantitatively the filling rate of the cauldron and qualitatively the shape of the ice flow field. The best‐fit model has an ice flow law parameter A0 that is about one order of magnitude lower than typically estimated for temperate glaciers.

Eight centuries of periodic volcanism at the center of the Iceland hotspot revealed by glacier tephrostratigraphy

A record of volcanic activity within the Vatnajokull ice cap has been obtained by combining data from three sources: tephrostratigraphic studies of two outlet glaciers, a 415-m-long ice core from northwestern Vatnajokull, and written records. The record extends back to a.d. 1200 and shows that the volcanic activity has a 130–140 yr period. Intervals of frequent eruptions with recurrence times of three to seven years alternate with intervals of similar duration having much lower eruption frequency. In comparison with other parts of the plate boundary in Iceland, eruption frequency is greater, episodes of unrest are longer, and intervals of low activity are shorter. The high eruption frequency may be the result of a more sustained supply of magma, owing to the area's location above the center of the Iceland mantle plume. When combined with historical data on eruptions and earthquakes, our data indicate that rifting-related activity in Iceland as a whole is periodic and broadly in phase with the volcanic activity within Vatnajokull.