Jökull - 01.01.2021, Page 11

The 1918 Katla eruption
Figure 6. a) The 1918 Katla tephra layer in soil near Gæsavatn, about 10 km north of Vík and 10 km south of
the eruption site. It is now 6–8 cm thick, at most. Just below the grass lies the 2010 Eyjafjallajökull tephra layer.
Photo:/Ljósm. Maria H. Janebo. b) The 1918 Katla tephra recently melted out of the ice at Sléttjökull, northern
outlet of Mýrdalsjökull, ice below. The tephra thickness is about 30 cm. Photo:/Ljósm. Guðrún Larsen, August
2018. – Kötlugjóskulagið frá 1918. a) Svört, 6–8 cm þykk Kötlugjóska í jarðvegi við Gæsavatn, um 10 km
sunnan gosstöðvanna og 10 km norðan Víkur. Uppundir grasrótinni er grá gjóska frá Eyjafjallajökli 2010. b)
Um 30 cm þykk, lagskipt Kötlugjóska á Sléttjökli í norðanverðum Mýrdalsjökli, nýkomin/nýbráðin út úr ís (sést
undir gjóskulaginu) í ágúst 2018, tæplega 100 árum eftir að hún féll.
The thickness of tephra at the present time is 1.5–
4 cm in Álftaver where the total or accumulated thick-
ness as freshly fallen was over 2.5 inches or 6–8 cm
(Jóhannsson 1919). In Skaftártunga the thickness is
now 0.5–6 cm where the total thickness was over 2.5
inches and 3–4 inches (6.5–10 cm) in lows (Sveins-
son, 1919). In Vík and vicinity the thickness now
measures 0–2 cm but was 2–4 cm after 13 hours of
tephra fall. There the thickness was measured during
and after the tephra fall, before rain compacted the
tephra (Jóhannsson, 1919).
On grassland the thickness of tephra may have
been overestimated because accurate measurements
on a rough surface can be problematic. However, re-
ports from farmers show that they distinguished be-
tween dry, new ash and wet, compacted ash. Although
a substantial part of the tephra outside the icecap may
have been redeposited and/or lost, the thicknesses
measured in the field now do not seem far off in ar-
eas where conditions for preservation (vegetated areas
except hayfields) were appropriate in 1918, given the
compaction by overlying soil.
The Katla 1918 tephra is best preserved within the
Mýrdalsjökull ice cap (Gudmundsson et al., this is-
sue). The original thickness ranged from about 10 cm
to some 25 m. The tephra was often covered by snow
after deposition, at least from October 20 onwards.
The tephra is exposed in the lower parts of the abla-
tion areas of all outlet glaciers of Mýrdalsjökull (Fig-
ure 6b) and can be accessed as it melts out of the ice
(Gudmundsson et al., this issue).
The new isopach map (Gudmundsson et al., this
issue), here presented in Figures 7 and 8, was com-
piled from thickness measurements in over 300 loca-
tions from various sources, including new data points
on the ice cap. As expected from the contemporary
descriptions of changing wind directions and repeated
tephra fall, the map shows several thickness axes. The
three most distinct axes trend N, NE and SE with mi-
nor axes towards the SSE, W and WNW. Very little
tephra was carried towards the SSW; the Vestmanna-
eyjar islands only reported tephra fall on two days
throughout the eruption (Figure 5).
Tephra deposited to the NE of Katla caused the
most severe damage to the farmlands and summer
pastures. The four farms within the present 5 cm
isopach line were temporarily or permanently aban-
doned (Figure 8). The north-axis is the largest, but
JÖKULL No. 71, 2021 9