Jökull - 01.01.2021, Qupperneq 41

Reviewed research article
The bedrock and tephra layer topography within the glacier
filled Katla caldera, Iceland, deduced from dense RES-survey
Eyjólfur Magnússon1, Finnur Pálsson1, Alexander H. Jarosch2, Tayo van Boeckel1,
Hrafnhildur Hannesdóttir1,3 and Joaquín M. C. Belart1,4
1Institute of Earth Sciences, University of Iceland (IES-UI), Sturlugata 7, IS-102 Reykjavík, Iceland
2ThetaFrame Solutions, Kufstein, Hörfarterstrasse 14, 6330 Kufstein, Austria
3Icelandic Meteorological Office, Bústaðavegi 7–9, 105 Reykjavík, Iceland
4National Land Survey of Iceland, Stillholti 16–18, 300 Akranes, Iceland
Corresponding author eyjolfm@hi.is; https://doi.org/10.33799/jokull2021.71.039
Abstract — We present results from recent low frequency radio echo-sounding (RES) campaigns over the ice-
covered caldera of Katla central volcano, beneath the Mýrdalsjökull ice cap, southern Iceland. The current
RES-survey both partly repeats and enhances the RES-profile grid of a previous survey in 1991 with denser
sounding lines and improved instruments. The RES-data, obtained in 2012–2021, include ∼760 km of 2D
migrated RES-profiles covering an area of 116 km2. Around 14 km2 subsections of this area were surveyed with
RES-profiles 20 m apart allowing 3D migration of the RES-data. Our study confirms findings from previously
published bedrock mapping, including main topographic features, ice volume stored within the 100 km2 caldera
(45±2 km3, in autumn 2019) and maximum ice thickness (740±40 m). However, the significantly expanded
level of detail and features observed in the new bedrock map reveals further evidence of a complex and eventful
formation history of the caldera interior. This bedrock map is unprecedented in terms of detail for an ice-
covered volcano. The new RES-data allows for a unique comparison of bedrock maps obtained from RES-data
with 2D and 3D migration, demonstrating the limitations of 2D migrated RES-data in areas of high topographic
variability. Reflections from the 1918 Katla eruption tephra layer within the ice were detected in a much wider
area within the caldera than in the 1991 RES-data. We also observe a second internal layer at 420–580 m depth
within the northern part of the caldera, identified here as the tephra from the 1755 Katla eruption. The 1918
tephra layer is typically observed at 200–300 m below the glacier surface. However, the layer depth varies
from ∼100 m depth at the western rim of the caldera down to 460 m depth, where geothermal activity beneath
ice cauldrons melts ice from below. At the most prominent geothermal areas all ice beneath the 1918 tephra
has been melted leaving the tephra at the bed. Furthermore, the obtained tephra layer maps reveal footprints
of some previously unidentified geothermal areas.
INTRODUCTION
Many of the most active volcanoes in Iceland are ice-
covered. The high elevations, found at many cen-
tral volcanoes, produce an environment where more
snow accumulates in winter than melts during sum-
mer, favouring glacier formation. An eruption within
a glacier is usually accompanied by jökulhlaups and
tephra fall (e.g. Gudmundsson et al., 2008) and can be
a serious threat, causing death of people and livestock
and damage of agricultural land and infrastructure in-
cluding houses, roads, bridges, dams, and hydropower
stations. The interaction of ice and volcanism is
also of broader scientific interest, with meltwater-
magma interactions causing explosive fragmentation
that can result in widespread airborne tephra. A sub-
JÖKULL No. 71, 2021 39