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Non-surface mass balance is non-negligible for glaciers in Iceland. Several Icelandic glaciers are in the neo-volcanic zone where a combination of geothermal activity, volcanic eruptions and geothermal heat flux much higher than the global average lead to basal melting close to 150 mm w.e. a−1for the Mýrdalsjökull ice cap and 75 mm w.e. a−1for the largest ice cap, Vatnajökull. Energy dissipation in the flow of water and ice is also rather large for the high-precipitation, temperate glaciers of Iceland resulting in internal and basal melting of 20–150 mm w.e. a−1. The total non-surface melting of glaciers in Iceland in 1995–2019 was 45–375 mm w.e. a−1on average for the main ice caps, and was largest for Mýrdalsjökull, the south side of Vatnajökull and Eyjafjallajökull. Geothermal melting, volcanic eruptions and the energy dissipation in the flow of water and ice, as well as calving, all contribute, and thus these components should be considered in mass-balance studies. For comparison, the average mass balance of glaciers in Iceland since 1995 is −500 to −1500 mm w.e. a−1. The non-surface mass balance corresponds to a total runoff contribution of 2.1 km3a−1of water from Iceland.

The volume of glaciers in Iceland (∼3,400 km3 in 2019) corresponds to about 9 mm of potential global sea level rise. In this study, observations from 98.7% of glacier covered areas in Iceland (in 2019) are used to construct a record of mass change of Icelandic glaciers since the end of the 19th century i.e. the end of the Little Ice Age (LIA) in Iceland. Glaciological (in situ) mass-balance measurements have been conducted on Vatnajökull, Langjökull, and Hofsjökull since the glaciological years 1991/92, 1996/97, and 1987/88, respectively. Geodetic mass balance for multiple glaciers and many periods has been estimated from reconstructed surface maps, published maps, aerial photographs, declassified spy satellite images, modern satellite stereo imagery, and airborne lidar. To estimate the maximum glacier volume at the end of the LIA, a volume–area scaling method is used based on the observed area and volume from the three largest ice caps (over 90% of total ice mass) at 5–7 different times each, in total 19 points. The combined record shows a total mass change of −540 ± 130 Gt (−4.2 ± 1.0 Gt a−1 on average) during the study period (1890/91 to 2018/19). This mass loss corresponds to 1.50 ± 0.36 mm sea level equivalent or 16 ± 4% of mass stored in Icelandic glaciers around 1890. Almost half of the total mass change occurred in 1994/95 to 2018/19, or −240 ± 20 Gt (−9.6 ± 0.8 Gt a−1 on average), with most rapid loss in 1994/95 to 2009/10 (mass change rate −11.6 ± 0.8 Gt a−1). During the relatively warm period 1930/31–1949/50, mass loss rates were probably close to those observed since 1994, and in the colder period 1980/81–1993/94, the glaciers gained mass at a rate of 1.5 ± 1.0 Gt a−1. For other periods of this study, the glaciers were either close to equilibrium or experienced mild loss rates. For the periods of AR6 IPCC, the mass change rates are −3.1 ± 1.1 Gt a−1 for 1900/01–1989/90, −4.3 ± 1.0 Gt a−1 for 1970/71–2017/18, −8.3 ± 0.8 Gt a−1 for 1992/93–2017/18, and −7.6 ± 0.8 Gt a−1 for 2005/06–2017/18.

To date, most mass balance studies in Iceland have concentrated on the three largest ice caps. This study turns the focus towards smaller Icelandic glaciers, presenting geodetic mass-balance estimates for 14 of them (total area 1010 km 2 in 2017) from 1945 to 2017, in decadal time spans. These glaciers, distributed over the country, are subject to different climatic forcing. The mass balance, derived from airborne and spaceborne stereo imagery and airborne lidar, is correlated with precipitation and air temperature by a first-order equation including a reference-surface correction term. This permits statistical modelling of annual mass balance, used to temporally homogenize the mass balance for a region-wide mass balance assessment for the periods 1945–1960, 1960–1980, 1980–1994, 1994–2004, 2004–2010 and 2010–2017. The 14 glaciers were close to equilibrium during 1960–1994, with an area-weighted mass balance of 0.07±0.07 m w.e. a −1. The most negative mass balance occurred in 1994–2010, accounting for –1.20±0.09 m w.e. a −1 , or 21.4±1.6 Gt (1.3±0.1 Gt a −1 ) of mass loss. Glaciers located along the south and west coasts show higher decadal mass-balance variability and static mass-balance sensitivities to summer temperature and winter precipitation, –2.21±0.25 m w.e. a −1 K −1 and 0.22±0.11 m w.e. a −1 (10%) −1 respectively, while glaciers located inland, north and northwest, have corresponding mass-balance sensitivities of –0.72±0.10 m w.e. a −1 K −1 and 0.13±0.07 m w.e. a −1 (10%) −1 . These patterns are likely due to the proximity to warm (south and west) versus cold (northwest) oceanic currents.

Drangajokull is a maritime ice cap located in northwest (Vestfirðir) Iceland. Drangajokull's evolution is therefore closely linked to atmospheric and ocean variability. In order to better constrain the Holocene climate and glacier history of Vestfirðir we model the past evolution of Drangajokull ice cap. Simulations from 10 ka to present are forced by general circulation model output, ice-core-based temperature reconstructions, and sea-surface temperature reconstructions. Based on these 10-thousand year simulations, Drangajokull did not persist through the Holocene. We estimate that air temperatures were 2.5–3.0 °C higher during the Holocene Thermal Maximum than the local 1960–1990 average. Simulations support Drangajokull's late Holocene inception between 2 and 1 ka, though intermittent ice likely occupied cirques as early as 2.6 ka. Drangajokull is primarily a Little Ice Age ice cap: it expanded between 1300 and 1750 CE, with the most rapid growth occurring between 1600 and 1750 CE. The maximum Holocene extent of Drangajokull occurred between 1700 and 1925 CE, despite the lowest late Holocene temperatures, occurring between 1650 and 1720 CE. Between 1700 and 1925 CE temperatures were likely 0.6–0.8 °C lower than the 1950–2015 reference temperature. The modern equilibrium line altitude (ELA) is bracketed by topographic thresholds: a 1 °C temperature increase from the modern ELA would eliminate the ice cap's accumulation area, while a reduction of 0.5 °C would lead to the rapid expansion of the ice cap across Vestfirðir. The proximity of Drangajokull to topographic thresholds may explain its late inception and rapid expansion during the Little Ice Age.

Publisher's version (útgefin grein), Lava flow thicknesses, volumes, and effusion rates provide essential information for understanding the behavior of eruptions and their associated deformation signals. Preeruption and posteruption elevation models were generated from historical stereo photographs to produce the lava flow thickness maps for the last five eruptions at Hekla volcano, Iceland. These results provide precise estimation of lava bulk volumes: V1947–1948 = 0.742 ± 0.138 km3, V1970 = 0.205 ± 0.012 km3, V1980–1981 = 0.169 ± 0.016 km3, V1991 = 0.241 ± 0.019 km3, and V2000 = 0.095 ± 0.005 km3 and reveal variable production rate through the 20th century. These new volumes improve the linear correlation between erupted volume and coeruption tilt change, indicating that tilt may be used to determine eruption volume. During eruptions the active vents migrate 325–480 m downhill, suggesting rough excess pressures of 8–12 MPa and that the gradient of this excess pressure increases from 0.4 to 11 Pa s−1 during the 20th century. We suggest that this is related to increased resistance along the eruptive conduit., Icelandic Research Fund. Grant Number: 152266‐052

The 6-month long eruption at Holuhraun (August 2014–February 2015) in the Bárðarbunga-Veiðivötn volcanic system was the largest effusive eruption in Iceland since the 1783–1784 CE Laki eruption. The lava flow field covered ~84 km2 and has an estimated bulk (i.e., including vesicles) volume of ~1.44 km3. The eruption had an average discharge rate of ~90 m3/s making it the longest effusive eruption in modern times to sustain such high average flux. The first phase of the eruption (August 31, 2014 to mid-October 2014) had a discharge rate of ~350 to 100 m3/s and was typified by lava transport via open channels and the formation of four lava flows, no. 1–4,which were emplaced side by side. The eruption began on a 1.8 km long fissure, feeding partly incandescent sheets of slabby pāhoehoe up to 500 m wide. By the following day the lava transport got confined to open channels and the dominant lava morphology changed to rubbly pāhoehoe and ‘a’ā. The latter became the dominating morphology of lava flows no. 1–8. The second phase of the eruption (Mid-October to end November) had a discharge of ~100–50 m3/s. During this time the lava transport system changed, via the formation of a b1 km2 lava pond ~1 km east of the vent. The pond most likely formed in a topographical low created by a the pre-existing Holuhraun and the newHoluhraun lava flow fields. This pond became themain point of lava distribution, controlling the emplacement of subsequent flows (i.e. no. 5–8). Towards the end of this phase inflation plateaus developed in lava flowno. 1. These inflation plateaus were the surface manifestation of a growing lava tube system, which formed as lava ponded in the open lava channels creating sufficient lavastatic pressure in the fluid lava to lift the roof of the lava channels. This allowed new lava into the previously active lava channel lifting the channel roof via inflation. The final (third) phase, lasting from December to end-February 2015 had a mean discharge rate of ~50 m3/s. In this phase the lava transport was mainly confined to lava tubes within lava flows no. 1–2, which fed breakouts that resurfaced N19 km2 of the flow field. The primary lava morphology from this phase was spiny pāhoehoe, which superimposed on the ‘a’ā lava flows no. 1–3 and extended the entire length of the flow field (i.e. 17 km). Thismade the 2014–2015 Holuhraun a paired flow field,where both lava morphologies had similar length. We suggest that the similar length is a consequence of the pāhoehoe is fed from the tube systemutilizing the existing ‘a’ā lava channels, and thereby are controlled by the initial length of the ‘a’ā flows., The work was financed with crisis response funding from the Icelandic Government along with European Community's Seventh Framework Programme Grant No. 308377 (Project FUTUREVOLC) and along with the Icelandic Research fund, Rannis, Grant of Excellence No. 152266-052 (Project EMMIRS). Furthermore, Vinur Vatnajökuls are thanked for support.

Publisher's version (útgefin grein), Mass-balance measurements of Icelandic glaciers are sparse through the 20th century. However, the large archive of stereo images available allows estimates of glacier-wide mass balance in decadal time steps since 1945. Combined with climate records, they provide further insight into glacier-climate relationship. This study presents a workflow to process aerial photographs (1945-1995), spy satellite imagery (1977-1980) and modern satellite stereo images (since 2000) using photogrammetric techniques and robust statistics in a highly automated, open-source pipeline to retrieve seasonally corrected, decadal glacier-wide geodetic mass balances. In our test area, Eyjafjallajökull (S-Iceland, ~70 km2), we obtain a mass balance of, This study was funded by the University of Iceland (UI) Research Fund. Collaboration and travels between IES and LEGOS were funded by the Jules Verne research fund. We thank David Shean and two anonymous reviewers for their valuable comments, which greatly improved the manuscript. We thank Carsten Kristinsson at LMÍ for scanning the aerial photographs, Oleg Alexandrov for his helpful tips and advice on ASP, Luc Girod for his help in the MicMac forum and Deirdre Clark and Ken Moxham for the Englishlanguage editing of the manuscript. Pléiades images were acquired at research price thanks to the CNES ISIS program (http://www.isis-cnes.fr). This study uses the lidar mapping of the glaciers in Iceland, funded by the Icelandic Research Fund, the Landsvirkjun research fund, the Icelandic Road Administration, the Reykjavík Energy Environmental and Energy Research Fund, the Klima-og Luftgruppen research fund of the Nordic Council of Ministers, the Vatnajökull National Park, the organization Friends of Vatnajökull, LMÍ, IMO and the UI research fund. This study uses the GLIMS database of the outlines of Icelandic glaciers. E.B. acknowledges support from the French Space Agency (CNES) through the TOSCA program.

GPS campaigns on glaciers during jökulhlaups show how subglacial floods affect glacier motion and shed light on the dynamics of such floods. Three such campaigns have been carried out on southern and western Vatnajökull, southeast Iceland, over known jökulhlaup paths. Two slowly rising jökulhlaups from Grímsvötn and two rapidly rising jökulhlaups from the western and eastern Skaftá cauldrons were captured in these campaigns, with maximum discharge ranging from 240 to 3300 m3 s−1. Glacier surface movements measured in these campaigns are presented along with the corresponding discharge curves. The measurements are interpreted as indicating: (1) initiation of rapidly rising jökulhlaups with a propagating subglacial pressure wave, (2) decreased glacier basal friction during jökulhlaups, (3) subglacial accumulation of water in slowly rising jökulhlaups and (4) lifting of the glacier caused by subglacial water pressure exceeding overburden in both rapidly and slowly rising jökulhlaups. The latter two observations are inconsistent with assumptions that are typically made in theoretical and numerical modelling of jökulhlaups. Both viscous and elastic deformation of the glacier as well as turbulent hydraulic fracture at the ice/bedrock interface are important in the dynamics of the subglacial pressure wave at the front of rapidly rising jökulhlaups.

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 Bárðarbunga 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 Vatnajökull ice cap, where the Bárðarbunga 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 Bárðarbunga 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 Bárðarbunga 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.

In this paper we describe how recent high-resolution digital elevation models (DEMs) can be used to extract glacier surface DEMs from old aerial photographs and to evaluate the uncertainty of the mass balance record derived from the DEMs. We present a case study for Drangajokull ice cap, NW Iceland. This ice cap covered an area of 144 km(2) when it was surveyed with airborne lidar in 2011. Aerial photographs spanning all or most of the ice cap are available from survey flights in 1946, 1960, 1975, 1985, 1994 and 2005. All ground control points used to constrain the orientation of the aerial photographs were obtained from the high-resolution lidar DEM. The lidar DEM was also used to estimate errors of the extracted photogrammetric DEMs in ice-and snow-free areas, at nunataks and outside the glacier margin. The derived errors of each DEM were used to constrain a spherical semivariogram model, which along with the derived errors in ice-and snow-free areas were used as inputs into 1000 sequential Gaussian simulations (SGSims). The simulations were used to estimate the possible bias in the entire glaciated part of the DEM and the 95% confidence level of this bias. This results in bias correction varying in magnitude between 0.03m (in 1975) and 1.66m (in 1946) and uncertainty values between +/- 0.21m (in 2005) and +/- 1.58m (in 1946). Error estimation methods based on more simple proxies would typically yield 2-4 times larger error estimates. The aerial photographs used were acquired between late June and early October. An additional seasonal bias correction was therefore estimated using a degree-day model to obtain the volume change between the start of 2 glaciological years (1 October). This correction was largest for the 1960 DEM, corresponding to an average elevation change of -3.5m or approx. three-quarters of the volume change between the 1960 and the 1975 DEMs. The total uncertainty of the derived mass balance record is dominated by uncertainty in the volume changes caused by uncertainties of the SGSim bias correction, the seasonal bias correction and the interpolation of glacier surface where data are lacking. The record shows a glacier-wide mass balance rate of (B) over dot = -0.26 +/- 0.04m w.e.a(-1) for the entire study period (1946-2011). We observe significant decadal variability including periods of mass gain, peaking in 1985-1994 with (B) over dot = -0.27 +/- 0.11m w.e.a(-1). There is a striking difference when (B) over dot is calculated separately for the western and eastern halves of Drangajokull, with a reduction of eastern part on average similar to 3 times faster than the western part. Our study emphasizes the need for applying rigorous geostatistical methods for obtaining uncertainty estimates of geodetic mass balance, the importance of seasonal corrections of DEMs from glaciers with high mass turnover and the risk of extrapolating mass balance record from one glacier to another even over short distances., This work was carried out within SVALI funded by the Nordic Top-level Research Initiative (TRI) and is SVALI publication number 70. It was also financially supported by alpS GmbH. This work is a contribution to the Rannis grant of excellence project, ANATILS. We thank the National Land Survey of Iceland and Loftmyndir ehf. for acquisition and scanning of the aerial photographs. This study used the recent lidar mapping of the glaciers in Iceland that was funded by the Icelandic Research Fund, the Landsvirkjun Research Fund, the Icelandic Road Administration, the Reykjavik Energy Environmental and Energy Research Fund, the Klima- og Luftgruppen (KoL) research fund of the Nordic Council of Ministers, the Vatnajokull National Park, the organization Friends of Vatnajokull, the National Land Survey of Iceland and the Icelandic Meteorological Office.

Simulations of the post-Little Ice Age evolution of three outlet glaciers of southeast Vatnajökull, Iceland – Skálafellsjökull, Heinabergsjökull and Fláajökull – are presented. A coupled shallow-ice-approximation ice-flow and degree-day mass-balance model is applied that is calibrated with a 14 year record of in situ mass-balance measurements. The measured mass balance cannot be realistically represented by constant horizontal and vertical precipitation gradients. High-resolution (1 km) precipitation fields, derived from downscaled orographic atmospheric circulation models of precipitation, are required to capture the spatial variation of the winter mass balance. The observed ice volume around 1890 (15–30% larger than in 2000) can be simulated with 1°C lower temperatures and a 20% reduction in the annual precipitation, relative to the reference climate period, 1980–2000. The sensitivity of each glacier’s annual balance to a change in temperature is −1.51 to −0.97 m w.e. a−1 °C−1 and +0.16 to +0.65 m w.e. a−1 for a 10% increase in precipitation. A steady-state experiment applying a step increase in temperature of 2°C (3°C), and precipitation increase of 10%, results in a >50% (80–90%) decrease in ice volume.

The rifting episode associated with the Bardarbunga-Holuhraun eruption in 2014–2015 included the first observations of major dyke propagation under ice. Three shallow ice depressions (ice cauldrons) with volumes ranging from 1 to 18 million m3 formed in Dyngjujokull glacier above the 48-km-long lateral path of the magma, at 4, 7 and 12 km from the northern glacier edge. Aircraft-based radar altimetry profiling was used to map the evolution of the cauldrons and construct a time series of the heat transfer rates. Out of the three scenarios explored: (1) onset or increase of hydrothermal activity, (2) convection within vertical fissures filled with water overlying intruded magma and (3) subglacial eruptions, the last option emerges as the only plausible mechanism to explain the rapid heat transfer observed in a location far from known geothermal areas. The thermal signals at two of the cauldrons are consistent with effusive subglacial eruptions. The formation of the northernmost cauldron was more rapid, indicating faster heat transfer rates. Radio-echo sounding data indicate that in contrast to the other two cauldrons, an intrusion of eruptive products occurred into the glacier, reaching 50–60 m above bedrock with the increased magma-ice contact explaining the more rapid heat transfer. We propose that the ~2-m widening associated with graben formation increased the groundwater storage capacity of the bedrock, creating space for the meltwater to be stored, explaining the absence of meltwater pulses draining from Dyngjujokull.

[1] In total, Icelandic ice caps contain ∼3600 km3 of ice, which if melted would raise sea level by ∼1 cm. Here, we present an overview of mass changes of Icelandic ice masses since the end of the 19th century. They have both gained and lost mass during this period. Changes in ice volume have been estimated both through surface mass balance measurements (performed annually since ∼1990) and differencing of digital elevation models derived from various satellite and airborne observations. While the glaciers showed little mass loss as the 20th century began, losses increased rapidly after 1925, peaked in the 1930s and 1940s, and remained significant until the 1960s. After being near-zero or even positive during the 1980s and early 1990s, glacier mass budgets declined considerably, and have since the mid-1990s shown an average annual loss of 9.5±1.5 Gt a−1, contributing ∼0.03 mm a−1 to sea level rise. Since 1995 interannual variability in mass loss is high, ranging from 2.7 to 25.3±1.5 Gt a−1, corresponding to surface mass balances of −0.2 to −2.2 ± 0.15 m we a−1. This variability is driven by climate fluctuations and also by transient reduction of albedo due to volcanic eruptions.

During the 2010 Eyjafjallajokull eruption in South Iceland, a 3.2-km-long benmoreite lava flow was emplaced subglacially during a 17-day effusive-explosive phase from April 18 to May 4. The lava flowed to the north out of the ice-filled summit caldera down the outlet glacier Gigjokull. The flow has a vertical drop of about 700 m, an area of ca. 0.55 km2, the total lava volume is ca. 2.5·107 m3 and it is estimated to have melted 10–13·107 m3 of ice. During the first 8 days, the lava advanced slowly (

We report on satellite and ground-based observations that link glacier motion with subglacial hydrology beneath Skeiðarárjökull, an outlet glacier of Vatnajökull, Iceland. We have developed a technique that uses interferometric synthetic aperture radar (InSAR) data, from the European Remote-sensing Satellite (ERS-1/-2) tandem mission (1995–2000), to detect localized anomalies in vertical ice motion. Applying this technique we identify an area of the glacier where these anomalies are frequent: above the subglacial course of the river Skeiðará, where we observed uplift of 0.15–0.20 m d−1 during a rainstorm and a jökulhlaup, and subsidence at a slower rate subsequent to rainstorms. A similar pattern of motion is apparent from continuous GPS measurements obtained at this location in 2006/07. We argue that transient uplift of the ice surface is caused by water accumulating at the glacier base upstream of an adverse bed slope where the overburden pressure decreases significantly over a short distance. Most of the frictional energy of the flowing water is therefore needed to maintain water temperature at the pressure-melting point. Hence, little energy is available to enlarge water channels sufficiently by melting to accommodate sudden influxes of water to the base. This causes water pressure to exceed the overburden pressure, enabling uplift to occur.

Large volcanic eruptions on Earth commonly occur with a collapse of the roof of a crustal magma reservoir, forming a caldera. Only a few such collapses occur per century, and the lack of detailed observations has obscured insight into the mechanical interplay between collapse and eruption.We usemultiparameter geophysical and geochemical data to show that the 110-square kilometer and 65-meter-deep collapse of Bárdarbunga caldera in 2014–2015 was initiated through withdrawal of magma, and lateral migration through a 48-kilometers-long dike, from a 12-kilometers deep reservoir. Interaction between the pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow path explain the gradual, near exponential decline of both collapse rate and the intensity of the 180-day- long eruption., Civil Protection Department of the National Commissioner of the Icelandic Police. European Community’s Seventh Framework Programme grant no. 308377 (Project FUTUREVOLC). EU Seventh Framework Marie Curie project NEMOH no. 289976 CO2Volc ERC grant no. 279802, the Research Fund of the University of Iceland, the Irish Research Council, the Helmholtz Alliance on Remote Sensing and Earth System Dynamics (EDA), Bayerisches Geoinstitut through their DFG core facility for high pressure research, and UNAM/CIC Intercambio Académico.

This paper reports a study aimed to detect changes at the northern part of the Vatnajokull glacier, Iceland, by exploiting a stack of Digital Elevation Models (DEMs) generated with Synthetic Aperture Radar (SAR) imagery. The stack is covering the Bardarbunga volcanic system, subject to large topographical variations during the 2014–2015 eruption. In particular, the collapse of the caldera and the restructuring of the glacier, with for instance the formation of new cauldrons, is well represented by the DEM stack, and a quantitative and a geophysical analysis of the results becomes feasible.

We have analyzed InSAR data from the ERS-1/ERS-2 tandem mission, to study the ice dynamics of Vatnajökull, Iceland, during jökulhlaups from the Skaftá cauldrons and the Grímsvötn geothermal area, which drained under the Tungnaárjökull and Skeiðarárjökull outlets, respectively. During the initial phase of a Grímsvötn jökulhlaup in March 1996, the velocity of Skeiðarárjökull increased up to three-fold (relative to observed velocities in December 1995) over an area up to 8 km wide around the subglacial flood path. Accumulation of water was observed at one location in the flood path. During a small jökulhlaup from the Skaftá cauldrons in October 1995 the velocity on Tungnaárjökull increased up to four-fold over a 9 km wide area. The velocity increase was observed 1.5 days before the floodwater was detected in the river Skaftá. A reduced glacier speed as the flood peaked in Skaftá indicates evolution of the subglacial drainage system from sheet to tunnel flow. The glacier acceleration and local uplift, observed in the early phase of both jökulhlaups, supports the concept that increased water inflow in a narrow tunnel system causes water pressure to rise and forces water into areas outside the channels, thus reducing the coupling of ice with the glacier bed.

Many of the large outlet glaciers of Vatnajökull ice cap, Iceland, have a history of regular surges. The mass transport during surges can be up to 25% of the total ice flux. This is a considerable amount that affects the whole ice cap, the location of the ice divides, the flow field and the size and shape of the ice cap. Data from the surging outlet Dyngjujökull, on the northern side of Vatnajökull, which surged during the period 1998-2000, are presented: surface elevation changes, displacement and total mass tr ansport. The total gain in ice volume in the receiving area, due to the surge, is considerably smaller than the loss in the reservoir area. The difference is mainly due to enhanced melting rates on the larger surface area of the crevassed glacier surface, and increased turbulent fluxes above the surface, but also due to increased frictional melting at the bed during the surge. A two-dimensional vertically integrated numerical flow model, of standard shallow-ice approximation type, is used to show that a modeled glacier that is similar in size to Dyngjujökull and subject to the same mass balance has three times higher velocities than the measured velocity during the quiescent phase. Adding surges in the numerical model, by periodically increasing the sliding velocity, causes the glacier to retreat and oscillate around a smaller state when subject to the same mass-balance regime. Lowering the equilibrium line by 50 m lets the modeled surging glacier oscillate around a size similar to that of the present glacier, indicating that surging is an efficient long-term ablation mechanism.

Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like dykes, some tens of kilometres long1. Previous models of rifting events indicate either lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens2, 3, 4, or magma flowing vertically into dykes from an underlying source5, 6, with the role of topography on the evolution of lateral dykes not clear. Here we show how a recent segmented dyke intrusion in the Bárðarbunga volcanic system grew laterally for more than 45 kilometres at a variable rate, with topography influencing the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred primarily over 14 days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar images (InSAR), and graben formation. The strike of the dyke segments varies from an initially radial direction away from the Bárðarbunga caldera, towards alignment with that expected from regional stress at the distal end. A model minimizing the combined strain and gravitational potential energy explains the propagation path. Dyke opening and seismicity focused at the most distal segment at any given time, and were simultaneous with magma source deflation and slow collapse at the Bárðarbunga caldera, accompanied by a series of magnitude M > 5 earthquakes. Dyke growth was slowed down by an effusive fissure eruption near the end of the dyke. Lateral dyke growth with segment barrier breaking by pressure build-up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries.

Icelandic glaciers cover ∼11 000 km2 in area and store ∼3600 km3 of ice. Starting in 2008 during the International Polar Year, accurate digital elevation models (DEMs) of the glaciers are being produced with airborne lidar. More than 90% of the glaciers have been surveyed in this effort, including Vatnajökull, Hofsjökull, Myrdalsjökull, Drangajökull, Eyjafjallajökull and several smaller glaciers. The publicly available DEMs are useful for glaciological and geological research, including studies of ice-volume changes, estimation of bias in mass-balance measurements, studies of jökulhlaups and subglacial lakes formed by subglacial geothermal areas, and for mapping of crevasses. The lidar mapping includes a 500-1000 m wide ice-free buffer zone around the ice margins which contains many glacio-geomorphological features, and therefore the new DEMs have proved useful in geological investigations of proglacial areas. Comparison of the lidar DEMs with older maps confirms the rapid ongoing volume changes of the Icelandic ice caps which have been shown by mass-balance measurements since 1995/96. In some cases, ice-volume changes derived by comparing the lidar measurements with older DEMs are in good agreement with accumulated ice-volume changes derived from traditional mass-balance measurements, but in other cases such a comparison indicates substantial biases in the traditional mass-balance records.

We assess the volume change and mass balance of three ice caps in southern Iceland for two periods, 1979/1984 to 1998 and 1998 to 2004, by comparing digital elevation models (DEMs). The ice caps are Eyjafjallajokull (ca. 81 km²), Tindfjallajokull (ca. 15 km²) and Torfajokull (ca. 14 km²). The DEMs were compiled using aerial photographs from 1979 to 1984, airborne Synthetic Aperture Radar (SAR) images obtained in 1998 and two image pairs from the SPOT 5 satellite’s high-resolution stereoscopic (HRS) instrument acquired in 2004. The ice-free part of the accurate DEM from 1998 was used as a reference map for co-registration and correction of the vertical offset of the other DEMs. The average specific mass balance was estimated from the mean elevation difference between glaciated areas of the DEMs. The glacier mass balance declined significantly between the two periods: from -0.2 to 0.2 m yr -1 w. eq. during the earlier period (1980s through 1998) to -1.8 to -1.5 m yr -1 w. eq. for the more recent period (1998-2004). The declining mass balance is consistent with increased temperature over the two periods. The low mass balance and the small accumulation area ratio of Tindfjallajokull and Torfajokull indicate that they will disappear if the present-day climate continues. The future lowering rate of Eyjafjallajokull will, however, be influenced by the 2010 subglacial eruption in the Eyjafjallajokull volcano. Keywords: Remote sensing, glacier mass balance, regional warming, Eyjafjallajokull, Torfajokull, Tindfjallajokull (Published: 20 July 2011) Citation: Polar Research 2011, 30 , 7282, DOI: 10.3402/polar.v30i0.7282