Jökull - 01.01.2021, Page 25

The 1918 Katla eruption
surements from over 60 locations in the period 1940–
1981 (e.g. Bjarnason and Thorarinsson, 1940; Thor-
arinsson, 1944, 1958, 1968, 1981) and the present au-
thors have made additional measurements at 290 lo-
cations (Figure 1). The scarcity of data in the area
beyond 60 km distance north of Katla is due to the
very poor preservation in these mostly barren high-
lands. Contemporary records demonstrate that minor
fallout occurred in most parts of the country (Larsen
et al., this issue). For the more distal and more recent
surveys, samples have been taken and the origin of the
tephra confirmed by microprobe analyses.
Figure 1. Katla 1918, sampling locations and
overview of sampling 1939–2018. Only points where
the layer was detected are included on the map.
The outlet glaciers of Mýrdalsjökull are indicated:
Sól: Sólheimajökull, K: Kötlujökul, Sl: Sléttjökull. –
Sýnatökustaðir þar sem gjóskulagið frá Kötlugosinu
1918 hefur fundist. Elstu mælingarnar eru frá 1939
en þær síðustu frá 2018.
On Mýrdalsjökull we have sampled the 1918
tephra and measured the thicknesses in the outlet
glaciers of Sólheimajökull, Kötlujökull and Sléttjök-
ull (Figure 1). The tephra on these outlet glaciers was
deposited on the accumulation area of Mýrdalsjökull,
where it was buried and transported within the glacier
by flow until it emerges when surface melting exposes
the layer. In the ablation area the exposed tephra layer
forms a distinct band, aligned perpendicular to ice
flow, extending across the outlet glacier. During sum-
mer, meltwater on the ice surface may transport parts
of the tephra away, especially the fines (Jónsdóttir,
2015). In order to prevent possible loss of fines from
the tephra layer, samples were acquired, either by: (1)
cutting out the tephra from the ice with a chainsaw,
or (2) by sampling the layer right at the point where
it becomes exposed by the ice melting. At Sléttjök-
ull the tephra forms a near continuous layer mixed
with some ice. In contrast, at Kötlujökull and Sól-
heimajökull, the tephra in the ice emerges as a 1–2 m
thick layer of ice and tephra. The ice is melted by
ablation, leaving an exposed 30–35 cm thick layer of
compacted tephra on the glacier surface. This 1–2 m
thick layer formed as a mix of snow accumulation and
fallout of tephra; contemporary observations indicate
repeated snowfall on Mýrdalsjökull over the 23 days
of the eruption (Sveinsson, 1919; Jóhannsson, 1919).
Additional estimates of the thickness of tephra
within the caldera at the end of the eruption are based
on the analysis of photos taken in September 1919
(Gudmundsson and Högnadóttir, 2001) and calcula-
tions based on estimates of annual mass balance, bal-
ance velocities and layer thinning during ice flow us-
ing our measurements in Sólheimajökull and Kötlu-
jökull.
Surface velocity measurements and submeter
DGPS measurements on Mýrdalsjökull, over a period
of 109 days (12 May–29 August) in the summer of
2001, were used for comparison with calculated ice
flow velocities.
TEPHRA THICKNESS ON
MÝRDALSJÖKULL
Observations in the Katla caldera in 1919
Conditions in the Katla caldera after the eruption
were explored in three separate inspection trips to the
eruption site in the summer and autumn of 1919 (G.
Sveinsson, 1919; P. Sveinsson, 1930; Jónsson, 2008).
The photos taken by Kjartan Guðmundsson on 23
JÖKULL No. 71, 2021 23