Jökull - 01.01.2015, Side 45

Grain characteristics of tephra from Katla and Hekla eruptions
more slowly and travel further through the air than
larger grains with more rounded shape and uneven
surface. Walker et al. (1971) found that down to a
diameter of 1 mm cylinders have lower terminal ve-
locity than spheres. Here this seems also to be the
case for grains smaller than 1 mm.
When looking at the differences in grain shape as
seen in the SEM (Figures 9 and 13) the needle- or rod-
like grains that characterize the SILK-LN tephra are
almost completely missing in the Hekla 1947 tephra
where equant grains are most common. Another dif-
ference is that grains with smooth, fluidal surface are
much more common in the Hekla tephra than in the
SILK-LN tephra.
The main difference between the silicic Katla
tephra SILK-LN and the Hekla 1947 tephra regard-
ing grain morphology can be summed up in the fol-
lowing way. The grains of the SILK-LN tephra are
significantly more elongated than those of the Hekla
1947 tephra but the ruggedness appears to be simi-
lar. According to Eiríksson and Wigum (1989) grains
from silicic magma are readily more elongated than
the basaltic ones and magmatic grains tend to be more
elongated than the hydromagmatic grains. Accord-
ing to the elongation values (see Tables 1 and 2) the
SILK grains are very elongated which fits the descrip-
tion above regarding viscous silicic magma producing
more elongated grains. The elongated SILK-LN grain
shape is here believed to connect to the phreatomag-
matic explosive activity. It is suggested that the reason
for the very elongate shape is the break-up of viscous
silicic magma with abundant elongate vesicles as a re-
sult of a shock caused either by cooling contraction, a
sudden expansion of steam, or both. Both the Hekla
and Katla tephra grains become less rugged with dis-
tance from source. The Katla tephra becomes more
elongated farther from source, contrary to the Hekla
tephra that becomes more circular farther from source.
One reason for this difference could be the deficiency
of elongated grains in the Hekla tephra.
Potential effects of fine tephra on health and avia-
tion
The grain size data represents material deposited ter-
restrially, on the ground, and therefore gives min-
imum values for the amount of material that was
erupted, in particular for material ≤4Φ (≤0.063 mm)
that can travel over long distances, as exemplified by
the Hekla 1947 tephra deposited in Finland with grain
size of 3 to 15µm (0.003–0.015 mm, Salmi, 1948)
and the Eyjafjallajökull 2010 tephra (Gudmundsson
et al., 2012). Most samples from the SILK-LN tephra
are considered representative of what was deposited
(with one or two exceptions). The Hekla 1947 tephra
samples of Thorarinsson (1954) are considered repre-
sentative, the Hekla samples collected in 2013 less so.
The data shows that silicic Katla eruptions pro-
duce greater quantities of material ≤4 Φ (≤0.063 mm)
than Hekla eruptions of similar composition and are
therefore more likely to affect air traffic and the health
of people. These small grain sizes can reach up to the
stratosphere where air traffic takes place. The air must
not contain more than 4 mg of tephra per m3 for air
traffic to be safe (EASA Safety Information Bulletin,
2013). At distances of 65–70 km from source the pro-
portion of deposited material ≤4 Φ (≤0.063 mm) was
up to 42 Wt% and 33 Wt% for Katla and Hekla tephra,
respectively.
In case of the SILK-LN tephra the data shows that
grains equal and smaller than 6.5 Φ (0.011 mm or 11
µm) were present in amounts greater than 1-8 Wt% at
distances of 65 km. These grains are breathable, i.e.
can be inhaled. The elongate shape of the grains could
be of concern because fibrous material, with length-
diameter ratio of 3, can be hazardous when the fibers
are longer than 5 µm (Horwell and Baxter, 2006).
Checking for the presence of such very small grains
will have to be subject of another study.
SUMMARY
The SILK-LN tephra
• During the course of the eruption the SILK-LN
tephra tends to get more coarse-grained and the
Wt% of the fine-grained tephra decreases sig-
nificantly with time.
• The SILK-LN tephra becomes gradually more
fine-grained away from the source. The
mean grain size (bulk samples) changes from
0.23 mm (2.12 Φ) at 22 km to 0.13 mm (2.94 Φ)
at 65 km.
JÖKULL No. 65, 2015 45