Jökull - 01.01.2015, Page 35

Grain characteristics of tephra from Katla and Hekla eruptions
grains. Plume height, eruption intensity, changes in
wind strength and fragmentation are all factors that
influence the way of transport and how far the grains
have travelled (e.g. Sparks et al., 1981; Carey and
Sparks, 1986; Dellino and Volpe, 1996; Rose and Du-
rant, 2009; Bonadonna and Haughton, 2005; Eiríks-
son, 1993; Guðmundsdóttir, 1998; Óladóttir, 2003;
Þorsteinsdóttir, 2012).
Grain morphology analysis
Grain morphology analysis was carried out on se-
lected samples from both the H-1947 and SILK-LN
tephra. The parameters ruggedness, elongation and
circularity were measured using an image analyzing
program (Eiríksson et al., 1994). The Hekla tephra
was cleaned using an ultrasonic bath to remove or-
ganic material adherent to the grains but no cleaning
was needed for the Katla tephra.
Tephra grain morphology has been used to study
whether the grains are a product of fragmentation
by hydromagmatic activity or due to exsolution of
magmatic gases and to distinguish between different
phases within an eruption (e.g. Eiríksson and Wigum,
1989; Dellino and Volpe, 1995; Gudmundsdóttir,
1998; Óladóttir, 2003; Dellino and Liotino, 2002;
Büttner et al., 2002; Þorsteinsdóttir, 2012; Cioni et al.,
2014). Grain morphology studies have also been used
to investigate the aerodynamic properties of tephra
particles, i.e. how far and for how long they stay up in
the air. Mele et al. (2011) concluded that settling ve-
locity of tephra grains is highly influenced by shape,
not only size and density.
Scanning electron microscopy
SEM (scanning electron microscope) images were
taken on a TM3000 electron microscope (Hitachi
High-Tech. Corp. 2010) on selected Hekla 1947 and
silicic Katla tephra samples. The purpose was to
demonstrate potential differences between the tephra
produced in these two volcanoes that might reflect the
different eruptive environments.
RESULTS – THE SILK-LN TEPHRA
Field characteristics
Field observations of the SILK-LN tephra show
changes in macroscopic characteristics i.e. bedding,
grain size, colour etc., within and between sample lo-
cations (Figure 2). Altogether 23 samples were col-
lected at ten locations. At four locations, Loðnugil,
Geldingasker, Leiðólfsfell and Varmárfell, the tephra
was bedded and sample from each unit was collected.
Where no bedding was apparent only a bulk sam-
ple was collected. All the samples were analysed for
grain size and the results plotted as histograms show-
ing mean grain size, sorting and Wt% of fines ≤0.063
and ≤0.011 mm. All histograms and tables appear in
Þorsteinsdóttir (2015).
The SILK-LN tephra is olive- to greyish green and
bedded where it is undisturbed. Figure 3 shows the
tephra layer at Loðnugil. Elongated, "needle"-shaped
grains with drawn-out (elongated) vesicles are promi-
nent but denser grains are also present. Both grain size
and colour change with time, the darkest layer being
the most coarse grained.
Grain size characteristics
Results from Loðnugil demonstrate the changes in
grain size distribution in the SILK- LN tephra with
time (Figure 6). In the two lowermost units the grains
are mostly smaller than 1 mm with a distinct peak be-
tween 0.25 and 0.125 mm, the mean grain size is 0.15
and 0.14 mm and the largest grains 1–1.5 cm. The two
upper units are bimodal with additional peaks at the
coarse part of the histograms, between 2–4 and 1–
2 mm respectively. The largest grains are over 2 cm
in diameter. At the four locations where the tephra
layer was bedded the amount of fine ash ≤0.063 mm
changed from ca. 30–43 Wt% in the bottom unit, to
25–40 Wt% in the middle unit and to ca. 11–29 Wt%
in the top unit.
Changes in mean grain size with distance (22–
65 km from center of caldera) along the axis of thick-
ness are shown on Figure 7. Individual units are
shown separately. Mean grain size changes from
0.14–0.43 mm at 22 km to 0.19–0.10 mm at 65 km.
Largest grains change from cm-sized clasts at 22 km
to mm-sized grains at 65 km. The mean grain size of
the bottom units is halved every 70–75 km whereas
that of the coarser units appears to be halved every
20–22 km.
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