The explosive, basaltic Katla eruption in 1918, south Iceland, II: Isopach map, ice cap deposition of tephra and layer volume

Abstract

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.

The Influence of Burial Rate on Variability in Tephra Thickness and Grain Size Distribution in Iceland

“…bimodal GSD distribution presented here (Gudmundsson et al, 2021). Vertical sub-sampling the K1918 tephra layer to measure the GSD of would have provided additional stratigraphic information about the bimodal grain distributions however, because of the shallow depths of fallout considered in this study the cause of any vertical variations in GSD could be the result of two quite different processes.…”

“…A third of them had a reduced thickness, the rest maintained the same thickness or, in one instance, became thicker. The K1918 tephra is likely to have compacted by around 20 % since deposition (Gudmundsson et al, 2021;Larsen et al, 2021). Scaling-up to account for compaction indicated that retention of tephra was better than site measurements suggest, with all but one site losing less material than predicted and retaining a great portion of the initial deposit thickness.…”

“…The ash plume reached an altitude of 14-15 km high and dispersed an estimated 1.1-1.2 km 3 of tephra (Larsen et al, 2021;Gudmundsson et al, 2021). There were two primary tephra production phases during the eruption, with less activity in between (Gudmundsson et al, 2021). Tephra was largely deposited to the north-east of the volcano.…”

“…It was a subglacial basaltic eruption, taking ~2 h to melt through the overlying ice, causing a large jökulhlaup (glacial outburst flood) and ash plume (Owen et al, 2019;Gísladóttir et al, 2021). The ash plume reached an altitude of 14-15 km high and dispersed an estimated 1.1-1.2 km 3 of tephra (Larsen et al, 2021;Gudmundsson et al, 2021). There were two primary tephra production phases during the eruption, with less activity in between (Gudmundsson et al, 2021).…”