In the Arctic region, Iceland is an important source of dust due to ash production from volcanic eruptions. In addition, dust is resuspended from the surface into the atmosphere as several dust storms occur each year. During volcanic eruptions and dust storms, material is deposited on the glaciers where it influences their energy balance. The effects of deposited volcanic ash on ice and snow melt were examined using laboratory and outdoor experiments. These experiments were made during the snow melt period using two different ash grain sizes (1 ϕ and 3.5 ϕ) from the Eyjafjallajökull 2010 eruption, collected on the glacier. Different amounts of ash were deposited on snow or ice, after which the snow properties and melt were measured. The results show that a thin ash layer increases the snow and ice melt but an ash layer exceeding a certain critical thickness caused insulation. Ash with 1 ϕ in grain size insulated the ice below at a thickness of 9-15 mm. For the 3.5 ϕ grain size, the insulation thickness is 13 mm. The maximum melt occurred at a thickness of 1 mm for the 1 ϕ and only 1-2 mm for 3.5 ϕ ash. A map of dust concentrations on Vatnajökull that represents the dust deposition during the summer of 2013 is presented with concentrations ranging from 0.2 up to 16.6 g m −2 .

Due to poor preservation and lack of proximal tephra thickness data, no comprehensive isopach map has existed for the tephra layer from the major eruption of the Katla volcano in 1918. We present such a map obtained by combining existing data on the thickness of the 1918 tephra in soil profiles with newly acquired data from the 590 km2 Mýrdalsjökull ice cap which covers the Katla caldera and its outer slopes. A tephra thickness of 20–30 m on the ice surface proximal to the vents is inferred from photos taken in 1919. The greatest thicknesses presently observed, 30–35 cm, occur where the layer outcrops in the lowermost parts of the ablation areas of the Kötlujökull and Sólheimajökull outlet glaciers. A fallout location within the Katla caldera is inferred for the presently exposed tephra in both outlet glaciers, as estimates of balance velocities imply lateral transport since 1918 of ~15 km for Kötlujökull, ~11 km for Sólheimajökull and about 2 km for Sléttjökull. Calculations of thinning of the tephra layer during this lateral transport indicate that the presently exposed tephra layers in Kötlujökull and Sólheimajökull were respectively over 2 m and about 1.2 m thick where they fell while insignificant thinning is inferred for the broad northern lobe of Sléttjökull. The K1918 layer has an estimated volume of 0.95+/-0.25 km3 (corresponding to 1.15+/-0.30x10**12 kg) whereof about 50% fell on Mýrdalsjökull. About 90% of the tephra fell on land and 10% in the sea to the south and southeast of the volcano. The volume estimate obtained contains only a part of the total volume erupted as it excludes water-transported pyroclasts and any material that may have been left on the glacier bed at the vents. While three main dispersal axes can be defined (N, NE and SE), the distribution map is complex in shape reflecting tephra dispersal over a period of variable wind directions and eruption intensity. In terms of airborne tephra, Katla 1918 is the largest explosive eruption in Iceland since the silicic eruption of Askja in 1875.

<p>Explosive eruptions in ice-covered volcanoes may deposit large volumes of tephra on the glaciated slopes.  The tephra can influence surface ablation and alter mass balance.  Ice melting by an eruption can change glacier geometry and temporarily alter the flow of outlet glaciers.  Conversely, when assessing the size of past tephra-producing eruptions in an ice-covered volcano the glacier complicates such estimates.  The effects of ice flow, dilation and shear need to be considered.  A tephra layer may get buried in the accumulation area, be transported by glacier flow and progressively removed over years-to-centuries by ice flow, eolian transport of exposed tephras and sediment transport in glacial rivers.  Here we report on a case study from the Mýrdalsjökull ice cap that covers the upper parts of the large Katla central volcano in south Iceland.  Most eruptions start beneath the 300-700 m thick ice cover within the Katla caldera, melt large volumes of ice and cause major jökulhlaups.  They also produce tephra layers that are preserved in soils around the volcano.  The most recent eruption in Katla occurred in October-November 1918, when a large tephra layer was deposited in a 3-weeks long eruption. By using a combination of (1) data obtained at or near the vent area within the SE-part of the Katla caldera in the year following the eruption, (2) mapping of the tephra as exposed at the present time in the ablation areas in the lower parts of the outlet glaciers, and (3) simple models of ice flow based on balance velocities and knowledge of mass balance, we estimate the location of fallout and the original thickness of the presently exposed tephra.  Photos taken in the vent area in 1919 indicate a tephra thickness of 20-30 m on the ice surface proximal to the vents.  The greatest thicknesses presently observed, 30-35 cm, occur where the layer outcrops in the lowermost parts of the ablation areas of the Kötlujökull and Sólheimajökull outlet glaciers.  A fallout location within the Katla caldera is inferred for the presently exposed tephra, as estimates of balance velocities imply lateral transport since 1918 of ~15 km for Kötlujökull, ~11 km for Sólheimajökull and about 2 km for the broad northern lobe of Sléttjökull.  The calculations indicate that ice transport with associated dilation of the glacier through the accumulation areas has resulted in significant thinning.   Thus, the layer that is now 0.3-0.35 m thick in the fastest flowing outlets is estimated to have been four to seven times thicker when it fell on the accumulation area within the ice-filled caldera.  In contrast, changes have been minor in the slowly moving Sléttjökull.  These findings allow for the construction of an isopach map for the glacier.  The results indicate that just under half of the total airborne tephra produced in the eruption fell within the Mýrdalsjökull glacier, with the remaining half spread out over a large part of Iceland.  These methods potentially allow for reconstruction of several tephra layers from ice-covered volcanoes in Iceland and elsewhere. </p>