Jökull - 01.01.2021, Qupperneq 3

Reviewed research article
The explosive basaltic Katla eruption in 1918, south Iceland I:
Course of events, tephra fall and flood routes
Guðrún Larsen1, Maria H. Janebo1 and Magnús Tumi Gudmundsson1,2
1Nordvulk, Institute of Earth Sciences, University of Iceland (IES-UI), Sturlugata 7, IS-102 Reykjavík, Iceland
2Faculty of Earth Sciences, University of Iceland (IES-UI), Sturlugata 7, IS-102 Reykjavík, Iceland
Corresponding author glare@hi.is; https://doi.org/10.33799/jokull2021.71.001
Abstract — The 23-day long eruption of the ice-covered Katla volcano in 1918 began on October 12 and
was over by November 4. Seismicity preceding and accompanying the onset had already started by 11:30 on
October 12, while the eruption broke through the glacier around 3 PM. The plume rose to 14–15 km on the
first day. The eruption caused widespread tephra fall, accompanied by lightning and thunder. Tephra fall from
the intense first phase (October 12–14) was reported from Höfn, 200 km east of Katla, Reykjavík, 150 km to
the west and Akureyri, 240 km to the north. The initial phase was followed by more sporadic activity for a
week, and a second intense phase (October 22–24), with heavy tephra fall in populated areas east and south
of the volcano. Skaftártunga (25–35 km east of Katla), was the worst hit farming district, with reported tephra
thickness of 6.5–10 cm in total, collecting into drifts tens of cm thick. The Vík village suffered almost continuous
tephra fall for 13 hours on October 24 and 25, leaving a 2 to 4 cm thick tephra layer on the ground. Tephra
reached Reykjavík four times but minor tephra fallout («1 mm) occurred. Tephra also reached northern, western
and eastern Iceland. In addition to producing the 0.9–1.0 km3 tephra layer, which may as freshly fallen have
been 1.1–1.2 km3, the eruption was accompanied by a jökulhlaup that flooded the Mýrdalssandur plain and
neighbouring areas. The jökulhlaup on October 12 had two separate phases. The first phase is considered
to have flowed supraglacially down the lower parts of the Kötlujökull outlet glacier into the Leirá, Hólmsá
and Skálm rivers (northern fork), and the Sandvatn and Múlakvísl rivers (southern fork). It was much more
widespread than the second phase which emerged from below the glacier snout and was confined to the western
part of Mýrdalssandur. That phase carried huge icebergs and massive sediment load onto the sandur plain.
INTRODUCTION
Volcanism in Iceland is dominated by basaltic erup-
tions. Environmental factors such as large ice caps,
lakes and high groundwater levels within the volcanic
zones influence the style of the eruptions (Thordar-
son and Larsen, 2007). In Iceland explosive basaltic
eruptions outnumber the effusive basaltic ones almost
by 4 to 1 (Thordarson and Höskuldsson, 2008), and
by far the largest number of the explosive basaltic
eruptions occur in ice-capped or ice-covered volca-
noes (Larsen, 2002). One consequence of eruptions
below ice are the often hazardous jökulhlaups (glacial
outburst floods) (Gudmundsson et al., 2008).
The central part of the Katla volcano is covered by
the 590 km2 Mýrdalsjökull ice cap (Figure 1) with an
ice thickness of 300–750 m within a 100 km2 caldera
(Björnsson et al., 2000). It has erupted at least 300
times during the last 8400 years with an eruption fre-
quency of 2–4 eruptions/century, on average (Óla-
dóttir et al., 2005, 2008). Over the last millennium
Katla eruptions have averaged two per century (Thor-
arinsson, 1975; Larsen, 2000). These can be large
events that severely affect the environment through
extensive and often heavy tephra fall and catastrophic
jökulhlaups (Thorarinsson, 1975; Tómasson, 1996;
Larsen, 2000; Gudmundsson et al., 2008).
JÖKULL No. 71, 2021 1