Jökull - 01.01.2015, Side 31

Grain characteristics of tephra from Katla and Hekla eruptions
3. Effusive basaltic eruptions are least common, oc-
curring outside the glacier along the margins of the
central volcano or on the fissure swarm, where the
two most voluminous eruptions took place. Where
fissures lie partly under the glacier the eruptions are
partly explosive.
The eruption history of Katla during the last ∼8400
years is fairly well known, especially the basaltic
eruptions. The majority of the eruptions start below
the ice cap, both basaltic and silicic eruptions. In this
paper the focus is on the explosive silicic eruptions.
Holocene silicic Katla (SILK) tephra in Iceland
Seventeen Holocene silicic Katla (SILK) tephra lay-
ers are known in Icelandic soils (Larsen et al., 2001;
Óladóttir et al., 2005, 2008; Þorsteinsdóttir, 2015).
The youngest one, SILK-YN, is about 1620 years old
(1675±12 14C years BP, Dugmore et al., 2000) and
the oldest has an age of ∼8100 years based on soil
accumulation rates (estimated after Óladóttir et al.,
2008). Tephra dispersal and shape of tephra lobes
in six eruptions suggest rather short-lived and small
eruptions with relatively low plumes. Volumes range
from 0.05 to 0.27 km3, implying that these eruptions
were generally smaller than the basaltic ones (Larsen
et al., 2001). SILK tephra has nevertheless been
found overseas, e.g. in the Faroe Islands and Ireland
(Wastegård, 2002; Hall and Pilcher, 2002).
The majority of the SILK layers have similar
major element composition, with SiO2 content be-
tween ca. 63–67 Wt% (Larsen et al., 2001, Þorsteins-
dóttir, 2015). This dacitic composition separates them
from the rhyolitic Vedde-type tephra with 69–72 Wt%
SiO2, such as the Suduroy tephra (Wastegård, 2002).
The source of the silicic eruptions is believed to be
inside or at the margins of the Katla caldera (Larsen et
al., 2001). There are indications that the caldera was
never ice-free during the last 8400 years, or at least
that there was water present in the caldera. Óladóttir
et al. (2007) have shown that the sulphur degassing of
the basaltic Katla tephra has been arrested, indicating
that the basaltic Katla tephra was quenched by contact
with water in hydromagmatic/phreatomagmatic erup-
tions. It is therefore assumed that when the silicic
eruptions occurred the volcano had an ice cover or at
least a caldera lake.
The SILK tephras have specific characteristics.
Most of them are of olive-green to grey-green colour
and have elongated glass grains in varying amounts.
Three tephra layers have more distinct characteris-
tics than the others both in the size and shape of the
grains with prominently needle shaped grains. The
term "needle layer" was first used about these layers
(Ólafsson et al., 1984), in English UN (upper needle),
MN (middle needle) and LN (lower needle) layers.
The prefix SILK was added later (Larsen, 2000).
Figure 2. Isopach map of the SILK-LN tephra layer
(adapted from Larsen et al., 2001). Sampling locations
are shown as pink circles: (Va)Varmárfell, (Bl) Blágil,
(Le) Leiðólfsfell, (Óf) Syðri Ófæra, (Ge) Geldingasker,
(St) Strútur, (Lo) Loðnugil, (Hv) Hvammur, (Gr) Gróf-
ará and (Fr) Framgil. – Þykktarkort af SILK-LN gjósku-
laginu (Larsen o.fl., 2001). Sýnatökustaðir eru sýndir
sem bleikir hringir.
The SILK-LN tephra
The SILK-LN tephra layer is the second largest of the
SILK tephra layers. The eruption site has not been ex-
actly located and is here assumed to be in the mid-part
of the caldera. The area on land which the SILK-LN
tephra layer covers is >15,000 km2 within the 0.1 cm
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