Jökull - 01.01.2016, Page 72

Þorsteinsdóttir et al.
basaltic Katla tephra was quenched by contact with
water in hydromagmatic/phreatomagmatic eruptions.
This indicates that the caldera was never ice-free dur-
ing the last 8400 years, or at least there was water
present. It is assumed that the silicic eruptions have
been taking place under similar conditions as the
basaltic Katla eruptions, under environmental condi-
tions dominated by ice and meltwater.
Dispersal maps exist for six silicic tephra layers
and the distribution and the shape of the tephra lobes
of these silicic eruptions suggest that they were rather
short-lived, with a relatively low eruption plume (Lar-
sen et al., 2001). Volume calculations of these six
silicic tephra layers (Table 2) show that the silicic
eruptions were generally smaller than known basaltic
eruptions (Larsen et al., 2001; Larsen, 2010, Óladóttir
et al., 2014). Silicic Katla tephra of Holocene age
has nevertheless been found overseas, e.g. in the Faroe
Islands and Ireland (Hall and Pilcher, 2002; Waste-
gård, 2002) and as ocean-transported pumice. e.g. on
the coasts of Scotland, Norway and Svalbard (Newton
1999; Larsen et al., 2001). These silicic Katla erup-
tions were classified as hydromagmatic by Larsen et
al. (2001).
This silicic tephra from Katla volcano has specific
characteristics in the field. Most of them are of olive-
green to grey-green color and have elongated glass
grains in varying amounts. Three of the 17 known
silicic tephra layers have more distinctive characteris-
tics. This difference lies both in the size and shape
of the grains, as the grains are prominently needle
shaped. The term „needle layer“ was first used about
these layers by Ólafsson et al. (1984), the prefix SILK
(silicic Katla layer) was added later (Larsen, 2000).
The needles form when vesicles are drawn out into
tubes, often flattened, with very thin walls that break
into needle-like grains or thin glass plates (Figure 3).
The needle grains are very delicate and brittle because
of these thin walls. The elongated grains have been
measured up to more than 8 cm long and 1–2 cm
wide. More equant grains, sometimes massive, occur
as well in most of the layers. The needle layers have a
very unusual appearance and are unique in Iceland, al-
though there are some characteristic features in these
layers, thin flat grains, that are similar to the tephra
that was formed in the Öræfajökull eruption in 1362
(Larsen, 2000; Thorsteinsdóttir, 2012). It is worth
mentioning that Dr. Grant Heiken, author of the Atlas
Table 2. Volumes of the youngest six SILK tephra lay-
ers on land. CPT stands for compacted tephra vol-
ume and UCP uncompacted or freshly fallen tephra
(Larsen et al., 2001). – Rúmmál yngstu sex SILK
gjóskulaganna (á landi). CPT er rúmmál á samþjapp-
aðri gjósku og UCP rúmmál á ósamþjappaðri eða ný-
fallinni gjósku.
SILK Tephra CPT km3 UCP km3
Layer YN 0.04 0.08
Layer UN 0.16 0.27
Layer MN 0.03 0.05
Layer LN 0.12 0.20
Layer N4 0.07 0.11
Layer N2 0.04 0.06 Figure 3. Grains of silicic Katla (SILK) tephra. The
largest grains are about 4 cm long. – Korn súrrar
Kötlu (SILK) gjósku (Ljósm./Photo. Guðrún Larsen).
72 JÖKULL No. 66, 2016