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Gudmundsson et al.
ison only for isopachs in the range 1–20 cm, but they
indicate that Katla 1918 may be slightly larger than
Katla 1625, and considerably larger than the layers
from 1262 and 1721. For comparison, the tephra layer
from Eyjafjallajökull in 2010 (bulk volume 0.27 km3)
is also shown (Gudmundsson, et al., 2012). This in-
dicates that the Katla 1918 layer was about four times
more voluminous than Eyjafjallajökull in 2010 and it
had almost five times the bulk volume of the Hekla
1947 tephra layer (0.21 km3, Thorarinsson, 1968).
This confirms that Katla 1918 is the largest explo-
sive eruption in Iceland since the rhyolitic Askja 1875
eruption (Carey et al., 2009).
The results indicate that about half of the airborne
tephra was deposited on Mýrdalsjökull. It is likely
that for the weaker eruptions of Katla, e.g. the erup-
tions in the 19th century (Thorarinsson, 1975), a rela-
tively minor proportion of the tephra erupted was de-
posited outside the ice cap.
The large thickness of tephra on Mýrdalsjökull is
not surprising, given the large proximal thicknesses
observed in recent eruptions in Grímsvötn and Eyja-
fjallajökull. In the eruption in Grímsvötn in 2004
(Jude-Eton et al., 2012; Oddsson et al., 2012) where
the bulk volume of tephra was only 0.02 km3, prox-
imal thickness was 7–8 m, and in the eruption in
Eyjafjallajökull in 2010 (Gudmundsson et al., 2012,
bulk tephra volume 0.27 km3), the maximum proxi-
mal thickness was about 30 m. The Katla 1918 erup-
tion lasted 23 days with varying activity at the vents,
and pervasive magma-water interaction. The disper-
sive power of the weaker phases of the eruption is ex-
pected to have been low, with most of the tephra de-
posited not far from the vents, contributing to thick
proximal, predominantly phreatomagmatic deposits.
It is likely that this also holds for several of the pre-
vious Katla eruptions considered by Óladóttir et al.
(2018) and Schmith et al. (2018).
The assessments made here of stretching and thin-
ning of tephra layers exposed in the lower half of the
ablation areas of glaciers may prove useful for es-
timating the original thickness of older, ice-covered
tephra layers. This may for instance apply in Vatna-
jökull, where several tens of tephra layers from the
last eight centuries are exposed in the outlet glaciers
(Larsen et al., 1998) and ice core drilling has con-
firmed how tephra layers are preserved in the accu-
mulation areas (Steinþórsson, 1977).
CONCLUSIONS
A first comprehensive isopach map of the tephra layer
formed in the eruption of Katla in 1918 is presented,
based on sampling spanning almost eight decades.
The key findings are:
1. The volume of airborne tephra erupted is estimated
as 0.95±0.25 km3, indicating that Katla 1918 is the
most voluminous explosive eruption in Iceland since
the rhyolitic eruption of Askja in 1875.
2. Tephra near the vents in the easternmost part of the
Katla caldera was 20–30 m thick, and the tephra ex-
ceeded 1 m thickness over large parts of the 100 km2,
ice-filled caldera.
3. About 50% of the tephra fell within Mýrdalsjökull
ice cap. Most of the tephra fell on land; only about
10% fell into the ocean.
4. The tephra fall is mostly fine-grained, consistent
with phreatomagmatic fragmentation being dominant
for long periods during the eruption. A substantial
part of the erupted material was water -transported.
This material is not considered here. Hence, the re-
sults do not provide and estimate of the total magma
erupted.
Acknowledgements
MHJ acknowledges an EU Marie Skłodowska-Curie
fellowship in 2016–2018. The work of JG was sup-
ported by an Icelandic Research Fund, Landsvirkjun
and Fræðslusjóður Suðurlands, and that of TJ by the
GOSVÁ program on volcanic hazard assessment in
Iceland. The field effort in 2018 was supported by
the Chief of Police in South Iceland, (Lögreglustjór-
inn á Suðurlandi) and the Icelandic Road Authority
(Vegagerðin). Joaquin Beloz at the Icelandic Geodetic
Survey gave us access to the oblique air photos from
1937. MTG and ÞH acknowledge support from the
University of Iceland Research Fund. Finally we also
would like to thank Olgeir Sigmarsson and our many
colleagues and friends that have throughout the years
given a hand in the fieldwork. Constructive reviews
32 JÖKULL No. 71, 2021