Jökull - 01.01.2021, Síða 7

The 1918 Katla eruption
Figure 3. Drawing by Samúel Eggertsson (1919, redrawn here) showing the eruption plume with lightning
flashes as seen from Reykjavík on October 12, also explaining how the height of the plume was determined
from Reykjavík. – Teikning Samúels Eggertssonar (1919, endurteiknuð) sýnir gosmökkinn með eldingaleiftrum
eins og hann sást frá Reykjavík 12. október. Myndin sýnir einnig hvernig hæð gosmakkarins var ákvörðuð.
1919). From then onwards no large eruption column
was described but black smoke was noted on Octo-
ber 27 and November 2. On November 3 and 4 some
steam was seen coming from the “crater”.
In the following days unfavourable weather did
not allow observations of the eruption site until
November 10, when a “smoke veil” was observed.
The eruption was, however, considered to have ended
on November 4 (Sveinsson, 1919; Jóhannsson, 1919).
Tephra fall and course of events
The eruption and the tephra fall, as experienced in
the populated areas in the vicinity of Mýrdalsjökull,
can be divided into two intense phases with less in-
tense activity in between. The first intense phase
lasted through day 1 to 3 (October 12–14) and the sec-
ond phase occurred on days 11–13 (October 22–24).
From day 16 (October 26), the tephra fall was mostly
towards northerly directions over unpopulated areas
with scant information on intensity (Figure 5).
During the first hours, after breaking through the
ice on October 12, the eruption plume was carried
ESE. No contemporary records of tephra fall exist
from the unpopulated region of glaciers and sandur
plains east of the volcano. Tephra fall began 30–
40 km to the ESE of the volcano around 3:30 PM
(Loftur Guðmundsson, in Ásbjörnsson, 1994) on Oc-
tober 12. Around 8 PM in the evening the ground
in Álftaver (Figure 1) had become “black by sand”,
a description commonly used of black Katla ash on
the ground. The tephra fall gradually decreased, but
continued until 6 AM in the morning of October 13
(Sveinsson, 1919).
A slight change in wind direction late on Oc-
tober 12 carried the eruption plume towards E and
ENE over Skaftártunga, where tephra fall began in the
evening and continued until morning on October 13,
forming a ≤1/2 inch layer of ash (∼1 cm; 1 Danish
inch = 2.61 cm). At Síða and in Fljótshverfi farther
to the east the tephra fall began in the early hours of
October 13. By morning, the ground was covered by
black ash that prevented livestock from grazing. It
was noted by the farmers in Skaftártunga (Sveinsson,
1919) that this first ash was much finer than the tephra
that fell later during the eruption.
In the late morning of October 13 the wind shifted
and carried tephra towards W, reaching Reykjavík
(Figure 5). The tephra falling in Reykjavík that day
was collected from flat surfaces at three different loca-
tions and was measured to be 35, 38 and 44 g/m2, re-
spectively, or 39 g/m2 on average. Using a density of
1000 kg/m3 this translates to a thickness of 0.04 mm
(Eggertsson, 1919).
On October 14–15 the weather was calm and visi-
bility poor in the areas south and east of Katla. Tephra
fall was reported from Akureyri and several locations
in North Iceland on October 14 (Morgunblaðið, 15
Oct.). Minor tephra fall on the village of Vík and the
districts to the east occurred on October 14 and 15. On
JÖKULL No. 71, 2021 5