Jökull - 01.01.2015, Síða 44

Þorsteinsdóttir et al.
consistent with the idea that external water existed at
the eruption site during the eruption. In this particular
case the external water is glacier melt water.
When external water and high viscosity magma
come together in an eruption vast amount of fines are
produced (Self and Sparks, 1978) which is likely in
the case of the SILK-LN tephra layer. According to
Rose and Durant (2009) silicic eruptions produce high
proportions of fine material (30 to >50%) in compari-
son to basaltic eruptions (∼1 to 4 %).
The upward coarsening of the SILK-LN tephra
could reflect changes in the eruptive behaviour, ei-
ther increased vigour carrying larger grains higher and
thereby increasing their range of flight or less effec-
tive fragmentation due to isolation of the magma/vent
from the melt water or due to draining of water in a
jökulhlaup, allowing formation of larger grains in the
later stages of the eruption. Stronger wind during the
latter part of the eruption could also have increased the
range of flight resulting in deposition of more coarse
grained tephra at any given location.
No internal layering was observed within the
Hekla 1947 tephra layer in the field in 2013 and only
bulk samples were collected. According to Thorar-
insson (1954, 1968) changes with time were observed
at several locations where the Hekla 1947 tephra was
collected freshly fallen. In the samples measured by
Thorarinsson (recalculated for this study) the mean
grain size of the lower unit was -1.00 Φ (2 mm),
-0.07 Φ (1.04 mm) and 1.19 Φ (0.44 mm) at 32, 49
and 68 km, respectively and that of the upper unit
was -0.01 Φ (1.00 mm), 0.48 Φ (0.71 mm) and 2.62 Φ
(0.16 mm) at the same locations (Figures 11 and 14).
Hekla is not covered with a glacier although
perennial snow was present and there is not sufficient
amount of water in the area that could have caused
a hydromagmatic eruption. The coarse grained grey
brown tephra in the bottom unit was most likely pro-
duced during a magmatic eruption (dry eruption) such
as is common with Hekla volcano. The change to finer
brownish black tephra was related to decreasing SiO2
content (Þórarinsson, 1968) and most likely to lower
eruption column.
Grain morphology
The grain shape results show a difference between the
Hekla 1947 and SILK-LN tephra layers. In the case
of the ruggedness there is not a significant difference
between Hekla and Katla tephra. In fact the rugged-
ness value of Hekla 1947 samples lies between the
two SILK-LN samples (Tables 1 and 2).
There is a prominent difference between the elon-
gation values of the Katla tephra and the Hekla tephra
(Figure 16). The Katla grains have much more elon-
gated shape than the Hekla grains that have more
equant and stocky shape. The elongation values in the
Hekla tephra are exactly the same at a location close
to the source and at a location further away whereas
those of the Katla tephra change with distance. The
Katla grains that travel furthest from the source have
smoother surface and more elongated shapes while
the ones traveling shorter distances have more uneven
surfaces and are less elongated. Therefore it appears
that small, elongated, smooth surfaced grains settle
0,40
0,50
0,60
0,70
0,80
0 20 40 60
E
lo
n
g
at
io
n
Dis tanc e from s ourc e (km)
E longation vs dis tance Geldingasker middle unit
Geldingasker bottom unit
Varmárfell middle unit
Varmárfell bottom unit
Vestan Hafrafells
Hamragarðaheiði
Figure 16. Elongation parameters of the SILK-LN tephra (triangles) and Hekla 1947 tephra (circles) plotted
against distance. Lower values represent more elongated grains. – Ílengd SILK-LN gjósku (þríhyrningar) og
Heklu 1947 gjósku (hringir) á móti fjarlægð. Lægri gildi – ílengri korn.
44 JÖKULL No. 65, 2015