Jökull - 01.01.2021, Page 4

Larsen et al.
Figure 1. The Mýrdalsjökull ice cap and neighbouring areas. The Katla caldera rim (Björnsson et al., 2000) is
shown as a grey hatched line. Katla marks the 1918 eruption site. Base map topography ArcticDEM (Porter et
al., 2018), glacier topography Lidar (Jóhannesson et al., 2013). – Mýrdalsjökull og nágrenni. Brún Kötluöskj-
unnar er táknuð með grárri slitinni línu. Katla er sýnd þar sem gosstöðvarnar 1918 voru. Landslag samkvæmt
ArcticDEM hæðarlíkani nema jökulyfirborð sem er samkvæmt Lidar mælingum.
The last eruption to break through the ice in the
Katla caldera (Figure 1) began around 3 PM on Octo-
ber 12 1918, accompanied by widespread tephra fall
and a catastrophic jökulhlaup.1 Detailed descriptions
exist of the course of events during the 23 days of
the eruption (Sveinsson 1919; Jóhannsson 1919) as
well as photographs of the eruption plume and jökul-
hlaup deposits (National Museum of Iceland/Institute
of Earth Sciences; Photographic archives of Westman
Islands).
Considerable work has been carried out to map
the 1918 tephra layer. Efforts were made in 2018, 100
years after the eruption of 1918, to constrain the mag-
nitude of the eruption by collecting new data and com-
bine with older data sets. New measurements of the
tephra layer, obtained within and close to the Mýrdals-
jökull ice cap were combined with the existing data
set on the thickness of the tephra layer.
This paper presents a description of the Katla 1918
eruption: Precursors, eruption plume and tephra fall,
based on contemporary sources, including a compre-
hensive isopach map of the Katla 1918 tephra, in the
proximal and medial areas, and an overview of dis-
tal deposition. Flood routes for the two phases of the
jökulhlaup of October 12 on Mýrdalssandur are de-
scribed, based on eyewitness descriptions and analy-
sis of aerial photographs from 1945 and 1946. Infor-
mation on the preservation of the tephra is provided
from contemporary thickness records in comparison
with measured thicknesses at selected locations.
1Jökulhlaups that were 1–2 orders of magnitude smaller than those associated with known eruptions occurred in 1955, 1999 and 2011.
The cause of these events has been debated, with both volcanic and geothermal origin suggested (e.g. Russell et al., 2010; Galeczka et al.,
2014). These events will not be considered further here.
2 JÖKULL No. 71, 2021