Jökull - 01.01.2021, Page 24

Gudmundsson et al.
Extensive work has been carried out over the past
decades to study Katla and its eruptions, notably the
eruption history through tephrochronology and other
tephra studies (e.g. Thorarinsson, 1975; Larsen, 2000;
Larsen et al., 2001; Óladóttir et al., 2008; 2014;
2018; Schmith et al., 2018), studies of the jökul-
hlaup of 1918 (Tómasson, 1996; Duller et al., 2008;
Larsen, 2018), glacier and ice bedrock mapping on
Mýrdalsjökull (Björnsson et al., 2000; Mackintosh
et al., 2000) and through various geophysical moni-
toring and research (e.g. Einarsson and Brandsdóttir,
2000; Jónsson and Kristjánsson, 2000; Gudmundsson
et al., 1994; Sturkell et al., 2007). However, until
now, no comprehensive isopach map has existed for
the tephra layer formed in 1918.
Conditions for tephra preservation for the Katla
1918 tephra layer were not favourable over large parts
of the fallout area, which includes large regions of
vegetation-poor highlands and sandur plains in the
lowlands. The eruption occurred in late autumn, af-
ter the growth period of vegetation. Snow cover is
patchy and sporadic in south Iceland in the autumn
months, a period of frequent strong winds. These ef-
fects contribute to low preservation potential of the
tephra. Fallout occurred over an extended period. The
most intense fallout in inhabited areas occurred dur-
ing 12–14 and 22–24 October (Larsen et al., this is-
sue). All of the above resulted in a complicated dis-
tribution pattern and considerable variation in tephra
thickness in soils. In addition, limited information has
been available on tephra thicknesses on the ice cap.
The 1918 tephra has been preserved in Mýrdalsjökull
and can be accessed on most of the outlet glaciers of
the 590 km2 ice cap. In 2018, on the 100th anniver-
sary of the eruption, an effort was made to sample the
tephra in Mýrdalsjökull and its immediate surround-
ings as well as analyse the existing contemporary in-
formation on conditions at and around the vents in the
south-eastern part of the Katla caldera. In this paper,
we combine the results of this work with the exten-
sive data on the Katla 1918 tephra layer to compile
an isopach map and estimate the volume of tephra
erupted. An overview of contemporary records of
tephra fallout and of the jökulhlaup is provided in the
complementary paper by Larsen et al. (this issue).
THE KATLA 1918 ERUPTION
The eruption of Katla in 1918 began on October
12. It was preceded by earthquakes that were felt
in Mýrdalur. Around 3 PM, an eruption plume
was seen rising from Mýrdalsjökull and shortly af-
ter a catastrophic jökulhlaup was observed propagat-
ing down the outwash plain of Mýrdalssandur to the
east of the volcano, carrying huge amounts of water-
transported pyroclasts and sediment (e.g. Jóhannsson,
1919; Tómasson, 1996). Contemporary measure-
ments from Reykjavík indicate a 14 km high plume
in the late afternoon (Eggertsson, 1919). The ini-
tial phase was subglacial with extremely high melt-
ing rates. The visible eruption was preceded by sound
of running water by 1.5–2 hours and seismic tremor
by ∼3.5 hours (Larsen et al., this issue). This in-
dicates that the subglacial phase lasted 2–3.5 hours.
The main jökulhlaup was over by the morning of Oc-
tober 13, while the eruption continued until the first
days of November, ending on November 4. Fallout
persisted throughout with varying intensity (Larsen et
al., this issue) and tephra was dispersed in all direc-
tions, with the heaviest fallout occurring to the north
and northeast of the volcano. Tephra fallout was de-
tected in most parts of Iceland (Larsen et al., this is-
sue). In terms of tephra volume and jökulhlaup size,
Katla 1918 has been classified as belonging to the
larger eruptions of Katla in the last thousand years
(Larsen, 2000; Gudmundsson et al., 2005). The vents
that melted through the glacier were located in the
southeastern part of the Katla caldera, near the east-
ern rim, along a 1–2 km long volcanic fissure (Larsen
and Högnadóttir, this issue; Larsen et al., this issue).
The magma erupted in 1918 was basaltic, typical
for eruptions from Katla (e.g. Óladóttir et al., 2008).
The tephra is relatively fine-grained, showing clear
evidence of brittle fragmentation by magma-water in-
teraction for long periods during the eruption (Jóns-
dóttir, 2015).
DATA AND METHODS
Thickness measurements of the Katla 1918 layer have
been made over several decades in soil sections. Fig-
ure 1 summarises the data used for compilation of the
isopach map. Sigurður Þórarinsson published mea-
22 JÖKULL No. 71, 2021