Jökull - 01.01.2021, Side 45

Bedrock and tephra layer topography within the Katla caldera
plane and threatening the bridge across the river, and
Múlakvísl, 9 July 2011, washing away the bridge over
the river (Guðmundsson and Högnadóttir, 2011; Jóns-
son and Þórarinsdóttir, 2011; Jóhannesson, 2012).
These jökulhlaups are all thought to have been associ-
ated with sudden energy release at the bed, from shal-
low magmatic intrusions within the bedrock, due to
minor subglacial eruptions or via hydrothermal pro-
cesses. Gudmundsson et al. (2007) associated the
1999 event with a small eruption at the glacier base
given the short estimated time of melting at the bed
(hours or days). Chemical analysis, however, indi-
cated that magma had not been in direct contact with
the glacier ice or the subglacial water released in the
2011 event (Galeczka et al. 2014). Study on the re-
lated seismicity (Sgattoni et al., 2019) was, however,
not conclusive in regard to whether magmatic or hy-
drothermal processes produced the melt released dur-
ing these three events.
The jökulhlaups in 1955, 1999 and 2011 were
sudden and unforeseen. Following the 2011 jökul-
hlaup, attempts were made to monitor the cauldrons
by surveying RES-profiles with low frequency radar
(5 MHz) to look for signs of water accumulation. This
was done simultaneously with the ground DGNSS
surface elevation profiling. RES-profiling across the
Mýrdalsjökull cauldrons has continued, following the
same track as accurately as possible, once or twice ev-
ery year since May 2012 looking for signs of signifi-
cant water accumulation as temporally elevated reflec-
tions beneath the cauldrons (Magnússon et al., 2017).
Similar RES-monitoring has also been successfully
adopted for the Eastern Skaftá cauldron in Western
Vatnajökull since 2014 (Magnússon et al., 2021). In
2016, the monitoring on Mýrdalsjökull initiated with
a support from the Icelandic Road Administration Re-
search Fund, developed into the research project Katla
Kalda, supported by Rannís via the Icelandic Re-
search Fund. The basic objective of the Katla Kalda
project is to improve understanding of the collection
and drainage of water from subglacial geothermal ar-
eas. To achieve this, the repeated RES-survey, ini-
tiated in 2012, was continued and expanded. High
resolution (sub-meter) optical satellite images (Pléi-
ades) have been repeatedly acquired for the purpose
of producing surface DEMs and detecting surface ele-
vation changes within the cauldrons. Surface changes
of Mýrdalsjökull in the past decades have also been
studied (Belart et al., 2020). Continuously recording
GNSS instruments were deployed in various ice caul-
drons to obtain records of jökulhlaup timing, dura-
tion and surface subsidence. Conductivity variation in
the rivers draining these jökulhlaups was also studied
(Einarsson, 2019). An automatic weather station (to
estimate the surface energy balance) has been oper-
ated during summers and information on surface melt-
ing as well as motion has been obtained by deploying
mass balance stakes in and around ice cauldrons in
spring and revisiting them in the autumn. Finally, high
Figure 2. a) The location of RES-data used in the pre-existing bedrock DEM (b) of our study area (Björns-
son et al., 2000, with further unpublished improvements, by the IES-glaciology group, for limited area based
RES-profile and point survey in 2000–2003). c) The location of RES-data used to create the new bedrock DEM
(d) presented here. The white dotted line in d) indicates which part of the bedrock DEM is revised here. The
background shaded relief images in a and c show the glacier surface in 2010 (Jóhannesson et al., 2013) and the
red hachured polygon indicates the rim of the Katla caldera (Björnsson et al., 2000). The cyan lines in a–d show
glacier margin and nunataks in 2010. e) An example of a 2D migrated RES-profile, measured in 2016, from
A to B (locations shown in c). Red lines indicate traced bedrock, yellow dotted lines the traced 1918 tephra
layer. – a) Lega íssjármælilína (að mestu mældar 1991) sem eldra botnkort (b) byggir á. c) Lega mælilína til
grundvallar þess korts sem hér er birt (d). Svæðið utan hvítu punktalínunnar er óbreytt frá eldra korti. Myndir
a og c sýna yfirborð jökulssins (skuggamynd og hæðarlínur) og útmörk (blágrænar línur) sumarið 2010. Rauð
hökuð lína á a–d, sýnir brún Kötluöskjunnar. e) Dæmi um íssjársnið með tvívíðri staðsetningarleiðréttingu
endurkastsflata (e. migration), mælt vorið 2016 frá A til B (staðsetning sýnd á c). Rauðar línur sýna rakin
botnendurköst en gula punktalínan endurköst frá gjóskulaginu sem féll á jökulinn í gosinu 1918.
JÖKULL No. 71, 2021 43