Jökull - 01.01.2021, Page 23

Reviewed research article
The explosive, basaltic Katla eruption in 1918, south Iceland II.
Isopach map, ice cap deposition of tephra and layer volume
Magnús Tumi Gudmundsson1,2, Maria H. Janebo1, Guðrún Larsen1, Thórdís Högnadóttir1,
Thorvaldur Thordarson1,2, Jónas Gudnason3 and Tinna Jónsdóttir4
1Nordvulk, Institute of Earth Sciences, University of Iceland, Sturlugata 7, IS-102 Reykjavík, Iceland
2Faculty of Earth Sciences, University of Iceland, Sturlugata 7, IS-102 Reykjavík, Iceland
3Landsvirkjun, Háaleitisbraut 68, 108 Reykjavík, Iceland
4Orkustofnun, Grensásvegi 9, 108 Reykjavík, Iceland
Corresponding author mtg@hi.is; https://doi.org/10.33799/jokull2021.71.021
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±0.30×1012 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.
INTRODUCTION
The eruption of Katla in 1918 (October 12–November
4) with its extensive tephra fallout and a massive
jökulhlaup, caused by intense ice melting of the over-
lying glacier during the first phase of the eruption,
belongs to one of the most prominent natural events
that occurred in Iceland in the 20th century. Due to
Katla’s proximity to inhabited areas, with its 100 km2
ice-filled caldera beneath the Mýrdalsjökull ice cap,
past eruptions with the associated jökulhlaups have
had major impact on the neighboring farming dis-
tricts (e.g. Thorarinsson, 1975; Larsen, 2000; 2018).
For the same reasons, Katla remains one of the most
dangerous volcanoes in Iceland (Gudmundsson et al.,
2008) calling for large monitoring networks and de-
tailed response plans (e.g. Þorkelsson et al., 2005).
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