Digital elevation models (DEMs) of several j & ouml;kulhlaup source areas were measured during the mapping of Icelandic glaciers with aerial lidar in 2008-2013. This included i) the j & ouml;kulhlaup in M & uacute;lakvisl from M & yacute;rdalsj & ouml;kull on 9 July 2011, which destroyed the bridge on the ring road, and ii) the j & ouml;kulhlaup in Vestari-J & ouml;kuls & aacute; from Hofsj & ouml;kull in late August 2013. Comparison of these DEMs with lidar measurements from before the j & ouml;kulhlaups reveals valuable information about the volume of the floods and the nature of subglacial water flow from j & ouml;kulhlaup source areas created by subglacial geothermal activity. DEM differencing shows lowering of the glacier surface of M & yacute;rdalsj & ouml;kull along several kilometres of the subglacial flood path from the main cauldrons that released the initial flash flood, which is most likely caused by melting of the glacier sole by warm flood water. It is not possible to derive an explicit estimate of the temperature of the flood water near the main cauldrons, but the DEM differencing indicates temperature on the order of 10 degrees C to tens of degrees Celsius. In recent years, three j & ouml;kulhlaups have been observed from Hofsj & ouml;kull and cauldrons have formed in the ice cap surface at three different locations. These measurements and observations of several other j & ouml;kulhlaups in Iceland in recent years show that the initial temperature of j & ouml;kulhlaup flood water may be quite variable from near the freezing point of water to tens of degrees Celsius. This variable temperature is likely to have important consequences for j & ouml;kulhlaups dynamics and may be part of the explanation for the highly variable development of j & ouml;kulhlaups. The j & ouml;kulhlaup in M & uacute;lakvisl in 2011 and from Hofsj & ouml;kull in 2013, as well as several other j & ouml;kulhlaups from M & yacute;rdalsj & ouml;kull and some other glaciers and ice caps in Iceland, highlight the importance of improving the understanding of j & ouml;kulhlaups and related natural hazards.
The caldera Lake & Ouml;skjuvatn, is at 1050 m elevation in Iceland's interior. It is a deep (217 m) dimictic lake formed after an eruption in 1875. Geothermal activity with gas and liquid inflows, down to 84 m depth, maintains a similar to 0.13 km2 permanent ice opening in winter. Remote sensing data revealed a progressively disappearing ice cover in the winter 2012. Physical and chemical conditions were explored in April 2012 (ice-free) and April 2013 (ice-cover). Measurements included CTD profiles and continuous temperature records. Meteorological observations from an automatic station show frequent southwesterly winds in the first quarter of 2012. Near-linear temperature increase with depth in April 2012 indicated effective whole lake vertical mixing. In contrast, the lake was weakly stratified in April 2013 with heat stored below 60 m depth. Moored temperature records in the winters of 2013 and 2014, revealed sustained under ice temperature, and hence a density rise in the upper 60 m, which is half the lake volume. The April 2012 concentrations of geochemical temperature indicators gave no indications of enhanced thermal activity. However, the concentrations of dissolved mineral constituents had decreased since 1975. Chloride and lithium decreased by 30 degrees lo but the geothermal indicators, silicate and sulphate, had decreased less, at similar to 15 degrees lo. The inflowing water, from local precipitation, had not changed. The estimated annual lake-air flux of carbon dioxide, 19000 tons in 2012, had substantially decreased. The unexpected winter ice loss in Lake & Ouml;skjuvatn in February-March 2012 was driven by a complex interplay of wind stress, the lake's seasonal deep water heat storage and geothermal activity. Inflow of geothermal gas at 84 m depth off the western shore enhances the vertical transport of heat to the lake's upper layer which, together with frequent southwesterly wind stress 2012, eventually induced vertical instability, whole lake turnover and a complete ice melt.
- The Skaft & aacute; cauldrons, a pair of surface depressions NW of Grimsv & ouml;tn on the Vatnaj & ouml;kull glacier, signify subglacial lakes created by geothermal heat sources within the underlying bedrock. These subglacial lakes continuously grow in volume and each produce j & ouml;kulhlaups every 1-5 years at the glacier outlet 35-40 km away. Seismic activity associated with the eight 2015-2021 j & ouml;kulhlaups from the Skaft & aacute; cauldrons was analyzed, reflecting striking similarities. Seismic activity associated with subglacial flood propagation and cauldron deepening was dominated by small transient signals, interpreted as icequakes from ice deformation and hydrofracturing as the water starts to migrate subglacially. Low-amplitude, low-frequency highly repetitive events were discovered during this period for the larger magnitude j & ouml;kulhlaups, possibly relating to stick-slip motion of the glacier at the bedrock. Seismic tremor events coincide with these large floods, but the origin of the tremor has been disputed. Sustained tremor (1-3 Hz) persists for 1.5-3 days, while high-amplitude tremor bursts (0.5-4 Hz) with durations up to tens of minutes and a strong relationship with increased electrical conductivity in the flood water are observed once most of the water has drained from the cauldrons, exhibiting higher amplitudes during larger j & ouml;kulhlaups. A probabilistic location method reveals that this 0.5-4 Hz tremor is co-located with the cauldrons and temporally coincides with the end of cauldron subsidence, indicating that the tremor is related to the rapid depressurization of the bedrock and is likely generated by either confined shallow level magmatic activity or enhanced geothermal activity such as vigorous boiling or hydrothermal explosions.
Numerous jokulhlaups have followed the river Jokulsa a Fjollum in northeast Iceland during the Holocene. Some of these fall under the category of catastrophic floods that carved out the present-day Jokulsargljufur canyon, over 100 km north of the present-day Vatnajokull. Sedimentary beds from 16 jokulhlaups between 6.3 and 4.1 ka ago are preserved in Vesturdalur, near the north end of the canyon. Chemical composition of volcanic glass in the jokulhlaup deposits correlates to three volcanic systems beneath Vatnajokull. The characteristics of the Bardarbunga volcanic system dominate in 12 beds, and those of Grimsvotn and Kverkfjoll in one bed each, two remain unsolved. The characteristics of the Bardarbunga glass in the jokulhlaup sediments are mostly low TiO2 and high MgO (TiO2 <1.6, MgO >7.3 w%). Seventeen basaltic "Low-Ti" tephra layers from Bardarbunga have been identified in soil sections in north Iceland from this period. Grain characteristics of the tephra indicate phreatomagmatic origin. Dispersal maps confirm source area below northwest Vatnajokull and tephra volume (bulk) of the order of 1 km3 for the largest layers. As the preserved soil sections are distal (>50 km) from source, it is likely that only the largest tephra layers have been preserved. The mid-Holocene floods confirm the existence of glaciers on Bardarbunga, Kverkfjoll, and Grimsvotn at that time. The magnitude of these jokulhlaups is not well constrained, but apparent cross sections out of Vesturdalur fit a peak discharge of order 50,000 m(3)/s and likely total volume of a few km3. These repeated jokulhlaups 6.3 to 4.1 ka ago did not cause large erosion at Vesturdalur. Their source areas were most likely the calderas of the central volcanoes, which may have changed in size and form since the mid-Holocene. Eruptions within the Bardarbunga caldera are a possible source for 12 of the floods. Bardarbunga may have hosted a geothermal area and a subglacial caldera lake similar to present day Grimsvotn, which may explain the repeated, apparently similar-magnitude jokulhlaups over this long period.
