Jökull - 01.01.2021, Síða 65

Bedrock and tephra layer topography within the Katla caldera
ward dragging, caused by subglacial melting beneath
K6, occurs. Down-glacier from point C, the time in-
tegrated downward drag, due to the geothermal area
beneath K19, increases.
The saddle point down-glacier from K19 (marked
D) is however hard to explain in same way as for the
one at C. The higher velocity and shorter distances in-
dicate that tephra now at D fell up-glacier from K19
and therefore passed the current geothermal area be-
neath K19 without much downward dragging due to
subglacial melting. Saddle point D is therefore likely
a sign of sporadic geothermal activity beneath K19.
Dating of the deeper tephra layer
The horizontal surface velocity at the part of the ice
divide between Kötlujökull and Entujökull, shown in
Figure 8b and 8d (indicated with blue line from point
C to point D) , is 0–3 m a−1 in the model simulation
for the glacier surface in September 2016, explained
above (Jarosch et al., 2020). The motion is along the
ice divides towards a saddle point in the flow where
ice motion is only vertical (downwards). This is there-
fore an ideal location to estimate the age of the deeper
tephra layer by comparison of the downward vertical
motion from the model simulation with the depth of
the tephra layer. Doing so requires the assumption
that glacier geometry as well as the ice flow remains
fixed. For this to hold true the annual mass balance
(in metres of ice equivalent) should equal the annual
downward motion at the divides. The model results
in an average downward motion of 4.3 m a−1, for the
line between C and D on Figure 8b, while the annual
mass balance measured ∼900 m northeast of profile
end D, was on average 3.9 m of ice equivalent, for
the 11 successful years of measurement in the period
2001–2020 (Ágústsson et al., 2013 and more recent
unpublished data from JÖRFÍ field surveys). Assum-
ing that both the obtained mass balance required to
maintain equilibrium is correct and that the average
of obtained mass balance measurements is representa-
tive for the period and this part of the ice divide (Fig-
ure 8b), would mean that the annual mass balance in
ice equivalent has been 0.4 m a−1 too low during this
20 year period to maintain the elevation of the ice cap
at this location.
By calculating the cumulative downward ice mo-
tion of the stationary velocity field with time we can
estimate the age of the ice with depth and compare re-
sults with the depth of the two tephra layers. Figure
8d displays the estimated depth for tephra layers from
the 1918, 1823 and 1755 eruptions of Katla as well as
the 1845 eruption of Hekla at the location where RES-
profiles were surveyed across the ice divides. This
estimate results in the 1918 tephra layer at 15–30 m
greater depth than the observed depth of the 1918
tephra. This may be partly explained by the glacier
being thicker in 1918 than at present causing the start-
ing elevation of the tephra layer to be higher than the
one assumed in this model. In 1960 the glacier surface
in this area was ∼15 m higher than in 2016 (Belart et
al., 2020), hence a 20–30 m higher glacier surface in
1918 than in 2016 seems likely. It should be noted,
however, that thicker ice would likely also result in
faster downward motion, which would partly compen-
sate for the effect of a higher starting elevation.
When the possible candidates for the deeper
tephra layer are considered, the estimated depth of
a layer from 1755 fits almost perfectly the observed
depth of the deep tephra layer. The eruption in 1755
was probably the largest eruption in Katla since the
Eldgjá eruption in the 10th century (Larsen et al.,
2013) maybe apart from the eruption in 1918 (Larsen
et al., 2021; Gudmundsson et al., 2021). The eruption
in 1755 is therefore likely to have deposited a thick
tephra layer on the glacier surface. Therefore, it is un-
likely that tephra from smaller eruptions prior to 1755
and therefore at greater depth (e.g. the 1721 Katla
eruption) would be detected if the 1755 tephra were
not. Tephra falls on to the glacier surface from erup-
tions occurring between 1755 and 1918 probably all
formed relatively thin layers not detectable with our
radar; these were all either relatively small Katla erup-
tions (Larsen et al., 2013) or originating from other
volcanoes than Katla. The almost exact match of the
modelled depth for 1755, while the match is poorer
for 1918, could be due to net growth of the glacier in
the period 1755 to 1918 (e.g. Björnsson, 2017). The
starting elevation of the tephra falling in 1755 may
have been closer to the 2016 glacier surface than the
one in 1918. On average the mass balance in the pe-
JÖKULL No. 71, 2021 63