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Gudmundsson et al.
constrain the location of the vents in the easternmost
part of the caldera, 1.5–2 km to the north of Háabunga
(Figure 2). Ice thickness is least at the eastern end,
270–300 m (Björnsson et al., 2000; Pálsson et al.,
2005).
Thickness based on the preserved tephra layer
The Katla 1918 tephra is exposed in the lower parts
of the ablation areas of all outlet glaciers of Mýrdals-
jökull, in most places 1–2 km from the glacier margin
(Figures 3 and 4). In order to use the exposed tephra
in the outlet glaciers to estimate thickness at the lo-
cations where the tephra fell, the relation between the
thickness observed in the outlet glaciers today and the
thickness at the time of deposition is needed. The
distance that the exposed tephra has been transported
since 1918 is, furthermore, needed to determine the
locations where the tephra fell.
To a first approximation, we assume that the con-
cept of balance velocity (e.g. Cuffey and Paterson,
2010) can be used. The balance velocity at a cer-
tain point on a glacier is defined as the depth-averaged
horizontal velocity of glacier flow needed to transport
the combined net mass balance (amount of snow ac-
cumulated minus the amount of surface melting) in
the upslope part of the glacier. Our estimate of the
balance velocity used here is constrained by mass bal-
ance measurements in the Katla caldera (Ágústsson et
al., 2013; Eyþórsson, 1945), estimates of the approx-
imate location of the equilibrium lines for each of the
three outlet glaciers Kötlujökull (1000 m a.s.l.), Sól-
heimajökull (1100 m a.s.l.) and Sléttjökull (1000 m
a.s.l.) (Björnsson, 1979; Jaenicke et al., 2006), and
information on bedrock geometry and ice thickness
on Mýrdalsjökull (Björnsson et al., Mackintosh et al.,
2000; Pálsson et al., 2005). A slightly negative net
mass balance for the Sólheimajökull and Kötlujökull
and their accumulation areas of -0.2 to -0.3 m is used,
broadly in agreement with the results of Belart et al.
(2020) on the geodetic mass balance of Mýrdalsjökull
over the last 60 years.
Only a rough first order estimate is used for Slétt-
jökull. It is a broad ice lobe that has retreated signifi-
cantly over the last 100 years. The lobe-like geometry
results in regular ice flow with velocities estimated in
the range 15–20 m/year around the equilibrium line,
and displacements of ∼2 km over the 100 years be-
tween 1918 and 2018 (Figure 4). This is in line with
the results of Mayer et al. (2016) who measured hor-
izontal velocity of 13 m/year for 2013–14 where the
tephra layer outcrops in the upper part of the ablation
area near the central part of Sléttjökull.
Figure 3. The Katla 1918 tephra layer in Mýrdalsjök-
ull. a) In Kötlujökull on 2 September 2012, the layer
is 35 cm thick. b) The 20 cm thick layer on 17 August
2018 at sampling location 2 in Sléttjökull (see Fig-
ures 1 and 4a for locations). – Gjóskulagið frá 1918
á Kötlujökli 2. september 2012, 35 cm þykkt (a) og í
Sléttjökli 17. ágúst 2018, 20 cm þykkt (b).
For both Kötlujökull and Sólheimajökull (Fig-
ure 4) the ice that accumulates within the caldera
flows through relatively narrow passages (Björnsson
et al., 2000; Pálsson et al., 2005) and down the out-
let glaciers, resulting in rising flow velocities in the
accumulation areas that peak near the equilibrium
line. Surface ablation measurements near the ter-
minus of Sólheimajökull since 2013, carried out by
students in glaciology at the University of Iceland
26 JÖKULL No. 71, 2021