Finnur Pálsson

The thermal regime of sub-polar glaciers mapped by multi-frequency radio-echo sounding

Radio-echo soundings provide an effective tool for mapping the thermal regimes of polythermal glaciers on a regional scale. Radar signals of 320-370 MHz penetrate ice at sub-freezing temperatures but are reflected from the top of layers of ice which are at the melting point and contain water. Radar signals of 5-20 MHz, on the other hand, see through both the cold and the temperate ice down to the glacier bed. Radio-echo soundings at these frequencies have been used to investigate the thermal regimes of four polythermal glaciers in Svalbard: Kongsvegen, Uversbreen. Midre Lovenbreen and Austre Broggerbreen. In the ablation area of Kongsvegen, a cold surface layer (50-160m) thick was underlain by a warm basal layer which is advected from the temperate accumulation area. The surface ablation of this cold layer may be compensated by freezing at its lower cold-temperate interface. This requires that the free water content in the ice at the freezing interface is about 1% of the volume. The cold surface layer is thicker beneath medial moraines and where cold-based hanging glaciers enter the main ice stream. On Uversbreen the thermal regime was similar to that of Kongsvegen. A temperate hole was found in the otherwise cold surface layer of the ablation area in a surface depression between Kongsvegen and Uversbreen where meltwater accumulates during the summer (near the subglacial lake Setevatnet, 250 m a.s.l.). Lovenbreen was frozen to the bed at the snout and along all the mountain slopes but beneath the central part of the glacier a warm basal layer (up to 50m) thick was fed by temperate ice from two cirques. On Austre Broggerbreen. a temperate basal layer was not detected by radio-echo soundings but the basal ice was observed to be at the melting point in two boreholes.

Surges of glaciers in Iceland

Abstract Surges are common in all the major ice caps in Iceland, and historical reports of surge occurrence go back several centuries. Data collection and regular observation over the last several decades have permitted a detailed description of several surges, from which it is possible to generalize on the nature of surging in Icelandic glaciers. Combining the historical records of glacier-front variations and recent field research, we summarize the geographic distribution of surging glaciers, their subglacial topography and geology, the frequency and duration of surges, changes in glacier surface geometry during the surge cycle, and measured velocity changes compared to calculated balance velocities. We note the indicators of surge onset and describe changes in ice, water and sediment fluxes during a surge. Surges accomplish a significant fraction of the total mass transport through the main outlet glaciers of ice caps in Iceland and have important implications for their hydrology. Our analysis of the data suggests that surge-type glaciers in Iceland are characterized by gently sloping surfaces and that they move too slowly to remain in balance given their accumulation rate. Surge frequency is neither regular nor clearly related to glacier size or mass balance. Steeply sloping glaciers, whether hard- or soft-bedded, seem to move sufficiently rapidly to keep in balance with the annual accumulation.

Estimating the Spatial Distribution of Precipitation in Iceland Using a Linear Model of Orographic Precipitation

A linear model of orographic precipitation that includes airflow dynamics, condensed water advection, and downslope evaporation is adapted for Iceland. The model is driven using coarse-resolution 40-yr reanalysis data from the European Centre for Medium-Range Weather Forecasts (ERA-40) over the period 1958–2002. The simulated precipitation is in good agreement with precipitation observations accumulated over various time scales, both in terms of magnitude and distribution. The results suggest that the model captures the main physical processes governing orographic generation of precipitation in the mountains of Iceland. The approach presented in this paper offers a credible method to obtain a detailed estimate of the distribution of precipitation in mountainous terrain for various conditions involving orographic generation of precipitation. It appears to be of great practical value to the hydrologists, glaciologists, meteorologists, and climatologists.

Gradual caldera collapse at Bárdarbunga volcano, Iceland, regulated by lateral magma outflow

Driven to collapse Volcanic eruptions occur frequently, but only rarely are they large enough to cause the top of the mountain to collapse and form a caldera. Gudmundsson et al. used a variety of geophysical tools to monitor the caldera formation that accompanied the 2014 Bárdarbunga volcanic eruption in Iceland. The volcanic edifice became unstable as magma from beneath Bárdarbunga spilled out into the nearby Holuhraun lava field. The timing of the gradual collapse revealed that it is the eruption that drives caldera formation and not the other way around. Science, this issue p. 262 Magma flow from under the Bárdarbunga volcano drove caldera collapse during the 2014 eruption. INTRODUCTION The Bárdarbunga caldera volcano in central Iceland collapsed from August 2014 to February 2015 during the largest eruption in Europe since 1784. An ice-filled subsidence bowl, 110 square kilometers (km2) in area and up to 65 meters (m) deep developed, while magma drained laterally for 48 km along a subterranean path and erupted as a major lava flow northeast of the volcano. Our data provide unprecedented insight into the workings of a collapsing caldera. RATIONALE Collapses of caldera volcanoes are, fortunately, not very frequent, because they are often associated with very large volcanic eruptions. On the other hand, the rarity of caldera collapses limits insight into this major geological hazard. Since the formation of Katmai caldera in 1912, during the 20th century’s largest eruption, only five caldera collapses are known to have occurred before that at Bárdarbunga. We used aircraft-based altimetry, satellite photogrammetry, radar interferometry, ground-based GPS, evolution of seismicity, radio-echo soundings of ice thickness, ice flow modeling, and geobarometry to describe and analyze the evolving subsidence geometry, its underlying cause, the amount of magma erupted, the geometry of the subsurface caldera ring faults, and the moment tensor solutions of the collapse-related earthquakes. RESULTS After initial lateral withdrawal of magma for some days though a magma-filled fracture propagating through Earth’s upper crust, preexisting ring faults under the volcano were reactivated over the period 20 to 24 August, marking the onset of collapse. On 31 August, the eruption started, and it terminated when the collapse stopped, having produced 1.5 km of basaltic lava. The subsidence of the caldera declined with time in a near-exponential manner, in phase with the lava flow rate. The volume of the subsidence bowl was about 1.8 km3. Using radio-echo soundings, we find that the subglacial bedrock surface after the collapse is down-sagged, with no indications of steep fault escarpments. Using geobarometry, we determined the depth of magma reservoir to be ~12 km, and modeling of geodetic observations gives a similar result. High-precision earthquake locations and moment tensor analysis of the remarkable magnitude M5 earthquake series are consistent with steeply dipping ring faults. Statistical analysis of seismicity reveals communication over tens of kilometers between the caldera and the dike. CONCLUSION We conclude that 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 the collapse rate and the intensity of the 180-day-long eruption. By combining our various data sets, we show that the onset of collapse was caused by outflow of magma from underneath the caldera when 12 to 20% of the total magma intruded and erupted had flowed from the magma reservoir. However, the continued subsidence was driven by a feedback between the pressure of the piston-like block overlying the reservoir and the 48-km-long magma outflow path. Our data provide better constraints on caldera mechanisms than previously available, demonstrating what caused the onset and how both the roof overburden and the flow path properties regulate the collapse. The Bárdarbunga caldera and the lateral magma flow path to the Holuhraun eruption site. (A) Aerial view of the ice-filled Bárdarbunga caldera on 24 October 2014, view from the north. (B) The effusive eruption in Holuhraun, about 40 km to the northeast of the caldera

Ice-volume changes, bias-estimation of mass-balance measurements and changes in subglacial lakes derived by LiDAR-mapping of the surface of Icelandic glaciers

Abstract 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.

Response of Hofsjökull and southern Vatnajökull, Iceland, to climate change

Disclosed is a dimmable fluorescent lamp operating apparatus which comprises a fluorescent lamp, electrical characteristic detecting element for detecting an electrical characteristic of the fluorescent lamp, an inverter circuit for driving the fluorescent lamp, and a feedback controlling circuit for controlling the drive frequency of the inverter circuit such that the electrical characteristic of the fluorescent lamp becomes a predetermined value, wherein the feedback controlling circuit includes temperature detecting element for detecting the temperature of the fluorescent lamp and wherein the frequency characteristic bandwidth of the feedback controlling circuit is varied based on the detected fluorescent lamp temperature.

Changes in jökulhlaup sizes in Grímsvötn, Vatnajökull, Iceland, 1934-91, deduced from in-situ measurements of subglacial lake volume

A record of volumes of jokulhlaups from the subglacial Grimsvotn lake, Vatnajokull, Iceland, has been derived for the period 1934-91. The change in lake volume during jokulhlaups is estimated from the lake area, ice-cover thickness and the drop in lake level. The jokulhlaup volumes have decreased gradually during this period of low volcanic activity and declining geothermal power. The two jokulhlaups in the 1930s each discharged about 4.5 km (peak discharge 25-30×10 3 m 3 s −1 ). In the 1980s, jokulhlaup volumes were 0.6.-1.2 km 3 (peak discharge 2×10 3 m 3 s −1 ). The lake level required to trigger a jokulhlaup has risen as an ice dam east of the lake has thickened. Water flow in a jokulhlaup ceases when the base of a floating ice shelf covering Grimsvotn settles to about 1160 m a.s.l. Apparently, the jokulhlaups are cut off when the base of the ice shelf collapses on to a subglacial ridge bordering the lake on its eastern side. The decline in melting rates has resulted in a positive mass balance of the 160-170 km Grimsvotn ice-drainage basin. Comparison of maps shows that the average positive mass-balance rate was 0.12 km 3 a −1 (25% of the total accumulation) in the period 1946-87. A gradually increasing positive mass balance has prevailed since 1954, reaching 0.23 km 3 a −1 in 1976-86 (48% of total accumulation )

New insights into the subglacial and periglacial hydrology of Vatnajökull, Iceland, from a distributed physical model

We apply a time-dependent distributed glaciohydraulic model toVatna- jo« kull ice cap, Iceland, aiming to determine the large-scale subglacial drainage structure, theimportance of basallyderivedmeltwater, theinfluence ofapermeable glacierbedand Vatnajo« kulls discharge contribution to major rivers in Iceland.The model comprisestwo coupled layersthatrepresentthesubglacialhorizonperchedona subsurfaceaquifer inthe western sector and bedrock in the eastern sector.To initialize and drive the simulations, we use digital elevation models of the ice surface and bed, the1999/2000 measured mass balance and an estimate of subglacial geothermal heat fluxes. The modelled subglacial flow field differs substantially from that derived by hydraulic-potential calculations, and the corresponding distribution of basal effective pressure shows a strong correlation between low effective pressure and surge-prone areas in northeastern and southern sectors of Vatnajo« kull. Simulations suggest that geothermally derived basal melt may account for upto π5% of the annualglacial discharge, and buried aquifers may evacuate up to π30% ofsubglacialwater.Time-dependenttestsyieldestimatesoftheglacialdischargecomponent in various outlet rivers and suggest a possible seasonal migration of subglacial hydraulic divides.This study of present-dayVatnajo« kull hydrology formsthe starting point for inves- tigationsof its future evolution.

The impact of jokulhlaups on basal sliding observed by SAR interferometry on Vatnajokull, Iceland

We have analyzed InSAR data from the ERS-1/ERS-2 tandem mission, to study the ice dynamics of Vatnajokull, Iceland, during jokulhlaups from the Skaftacauldrons and the Grimsvot n geothermal area, which drained under the Tungnaarjokull and Skeiðararjokull outlets, respectively. During the initial phase of a Grimsvotn jokulhlaup in March 1996, the velocity of Skeiðararjokull 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 jokulhlaup from the Skaftacauldrons in October 1995 the velocity on Tungnaarjokull 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 Skafta ´. A reduced glacier speed as the flood peaked in Skaftaindicates evolution of the subglacial drainage system from sheet to tunnel flow. The glacier acceleration and local uplift, observed in the early phase of both jokulhlaups, 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.

The hyaloclastite ridge formed in the subglacial 1996 eruption in Gjálp, Vatnajökull, Iceland: present day shape and future preservation

Abstract In the Gjálp eruption in 1996, a subglacial hyaloclastite ridge was formed over a volcanic fissure beneath the Vatnajökull ice cap in Iceland. The initial ice thickness along the 6 km-long fissure varied from 550 m to 750 m greatest in the northern part but least in the central part where a subaerial crater was active during the eruption. The shape of the subglacial ridge has been mapped, using direct observations of the top of the edifice in 1997, radio echo soundings and gravity surveying. The subglacial edifice is remarkably varied in shape and height. The southern part is low and narrow whereas the central part is the highest, rising 450 m above the pre-eruption bedrock. In the northern part the ridge is only 150–200 m high but up to 2 km wide, suggesting that lateral spreading of the erupted material occurred during the latter stages of the eruption. The total volume of erupted material in Gjálp was about 0.8 km3, mainly volcanic glass. The edifice has a volume of about 0.7 km3 and a volume of 0.07 km3 was transported with the meltwater from Gjálp and accumulated in the Grímsvötn caldera, where the subglacial lake acted as a trap for the sediments. This meltwater-transported material was removed from the southern part of the edifice during the eruption. Variations in basal water pressure may explain differences in edifice form along the fissure. Partial floating of the overlying ice in the northern part is likely to have occurred due to high water pressures, reducing confinement by the ice and allowing lateral spreading of the edifice. The overall shape of the Gjálp ridge is similar to that of many Pleistocene hyaloclastite ridges in Iceland. Future preservation of the Gjálp ridge will depend on the rate of glacial erosion it will suffer. Besides being related to future ice flow velocities, the erosion rate will depend on the rate of consolidation due to palagonitization and shielding from glacial erosion while depressions in the ice are gradually filled by ice flow directed towards the Gjálp hyaloclastite ridge.