Jökull - 01.01.2021, Side 19

The 1918 Katla eruption
The timing of events is given in this paper as re-
ported in the contemporary sources, except where the
old division into one-eight (Icelandic eykt) of the so-
lar day was used, then the time is given as PM or AM
as appropriate. The term “miðmundi” refers to 1:30
PM, which is the time when the humming sound of
the jökulhlaup was heard by farmers some 15 km east
of Kötlujökull. This implies that the jökulhlaup had
broken up through the glacier at about 1:30 PM and
was flowing on its surface.
The course of events in the 1918 jökulhlaup can
be summarized as follows (Jóhannsson 1919; Sveins-
son 1919; Karlsson 1994; Tómasson 1996): In the
first phase of the jökulhlaup, the meltwater broke
through the ice of the Kötlujökull glacier, probably
several kilometers above its snout and flowed sub-
aerially/supraglacially down the glacier in three forks
(Figure 9). The second phase began when the meltwa-
ter had created a subglacial channel below the west-
ern part of Kötlujökull and broke out from under
the southwest corner of the glacier, breaking up the
glacier snout and carrying huge icebergs onto Mýr-
dalssandur and into the sea. The previous channels
became inactive and all meltwater was confined to this
channel from then onwards (Figure 9).
The 1918 Katla jökulhlaup is the largest such
event observed in the 20th century in Iceland, with
a peak discharge estimated at about 300,000 m3/s
(Tómasson 1996). Comparison with descriptions of
earlier jökulhlaups and the size of the areas that they
affected (Larsen, 2018), classifies the 1918 event to-
gether with 1755 and 1625 CE as the largest three
jökulhlaups known to have accompanied Katla erup-
tions.
It is beyond the scope of the present paper to
make a full account of Katla 1918 eruption. Besides
the large tephra layer, juvenile pyroclasts formed sub-
glacially on the first day of the eruption may have
had a similar volume (Tómasson, 1996; Larsen, 2018;
Gudmundsson et al., 2018). The total volume of
fragmented material (tephra, water-transported pyro-
clasts) may therefore have been in the range of 2 km3,
equivalent to 0.5–0.9 km3 DRE (Dry Rock Equiva-
lent).
SUMMARY AND CONCLUSIONS
The explosive basaltic 1918 Katla eruption lasted 23
days, beginning on October 12 and ending on Novem-
ber 4. The eruption produced 0.9–1.0 km3 of airfall
tephra, corresponding to 1.1–1.2 km3 of freshly fallen
tephra. Two phases of intense activity, with a lull in
between, occurred on eruption days 1–3 and 11–13.
The highest reliably estimated eruption column height
was 14.3 km a.s.l. on day 1.
The most intense tephra fall in the vicinity of Mýr-
dalsjökull occurred on October 12–14 (days 1–3) and
October 22–24 (days 11–13). The longest consequent
tephra fall lasted for 13 hours, depositing up to 4 cm
of ash, on October 24–25 (days 13–14).
Two separate eruption columns were observed on
October 20. The location of the most active vent(s)
had apparently shifted, implying that a new fissure or
new source vents had opened up, which then fed the
second intense phase.
Figure 11. Aerial photograph of a 6 km wide part of the Mýrdalssandur plain from Hjörleifshöfði hill (middle
left) to the Blautakvísl river (right). Note the channels (dark grey) and bars (lighter grey) between Hjörleifs-
höfði and Blautakvísl. The surface of the bars still shows pockets where stranded ice bergs melted and furrows
left by the jökulhlaups of the first days. The channels that were cleared of ice by the flood water, as described
by Kjartan L. Markússon, were modified in the years to come by the Sandvatn river but the bars were at least
partly preserved. ©Landmælingar Íslands 1975. – Loftmynd sem sýnir um 6 km breiða spildu á neðanverðum
Mýrdalssandi, frá Hjörleifshöfða að Blautukvísl með farvegum (dekkri) og eyrum (ljósari). Yfirborð eyranna
er eins og jökulhlaupið skildi við þær í október 1918. Það er mishæðótt með bollum eftir jaka og vatnsrásum
með annarri stefnu en farvegirnir. Eftir fyrsta gosdaginn mynduðust straumálar þar sem jökum var skolað burt
eða þeir grafnir niður, skv. lýsingu Kjartans L. Markússonar. Næstu árin eftir 1918 rann Sandvatnið austan
Hjörleifshöfða og farvegirnir sem hér sjást hafa vafalaust breyst við það en eyrarnar hafa varðveist að hluta.
©Landmælingar Íslands 1975.
JÖKULL No. 71, 2021 17