Jökull - 01.01.2015, Side 41

Grain characteristics of tephra from Katla and Hekla eruptions
Table 2. Results of grain morphology measurements of the Hekla-1947 tephra. The parameters measured are
Elongation (lower values - more elongated grains), Ruggedness (lower values - more rugged grains) and Cir-
cularity (higher values - more circular grains). – Kornalögunarniðurstöður á Heklu-1947 gjósku. Mældar voru
þrjár breytur, ílengd (lægri gildi - ílengri korn), hrjúfleiki (lægra gildi - hrjúfari korn) og hringlögun (hærri
gildi - betri hringlögun).
Unit Elongation Ruggedness Circularity Distance (km)
Vestan Hafrafells 0.73 0.40 0.73 19
Hamragarðaheiði 0.73 0.41 0.75 42
(Figure 13). As in case with the Katla tephra the SEM
imaging was carried out to visually demonstrate the
grain shapes. More images can be seen in Þorsteins-
dóttir (2015).
DISCUSSION
Changes with distance along axes of thickness
There is prominent difference in the mean grain size
and the proportion of the fine material in the tephra
layers from these two volcanoes. At similar distances
from their source the Hekla 1947 tephra is much
coarser-grained than the Katla SILK-LN tephra (Fig-
ure 14).
The mean grain size of the SILK-LN tephra
changes fairly regularly with distance but varies
within bedded layers. The mean grain size of the finer
bottom units is halved every 70–75 km and that of the
coarser units every 20–22 km. The mean grain size
of Hekla 1947 does however change much more with
distance and is halved every 13–14 km, as expressed
in the bulk samples.
There is much more fine material in the Katla
tephra than in the Hekla tephra (Figure 15). The range
in proportion of fines ≤4 Φ (≤0.063 mm) is 24 to 43
Wt% at distances between 22 and 65 km in the SILK-
LN tephra (samples from axis of thickness) but in the
Hekla tephra this proportion is 1 to 33 Wt% at the
same distance (excluding samples within 19 km dis-
tance because no Katla tephra could be obtained at
that distance and also potentially remobilized sam-
ples). It is interesting that the changes in proportion
of fine material ≤6.5 Φ (≤0.011 mm) with distance in
the Katla tephra are little to none (Figure 8). Possible
reasons for little to no change with distance in grains
≤6.5 Φ (≤0.011 mm) can be aggregation or scaveng-
ing by or adhesion to larger grains.
The main differences in grain size between the
SILK-LN and the Hekla 1947 tephra can be summed
up in the following way: The Hekla 1947 tephra has
significantly higher mean grain size and much lower
content of material ≤4 Φ (≤0.063 mm) at all distances
than in the SILK-LN tephra. The mean grain size of
Hekla 1947 decreases more rapidly with distance than
that of the SILK-LN tephra. The higher mean grain
size and lower content of fine material in the Hekla-
1947 tephra compared to SILK-LN tephra is thought
to be connected to environmental conditions. Katla
is covered in ice and Hekla is situated in a dry area.
The interaction between water and magma in Katla
eruptions can produce extremely fine grained tephra
which explains the high amount of fines and lower
mean grain size in the Katla tephra (e.g. Morrisey et
al., 2000; White and Houghton, 2000; Zimanowski
et al., 1997; Wohletz, 1983). Mean grain size will
also decrease more slowly with distance in the case of
SILK-LN due to high proportion of fines with lower
terminal velocity and longer travel range, as compared
to the more coarse grained Hekla 1947.
Changes with time
The grain size measurements of the SILK-LN tephra
show an evolution from finer grained material in the
beginning toward a coarser grained material in the
latter part of the eruption (e.g. Figure 6). Where
the tephra was bedded the proportion of fine ash de-
creased from ca. 30–43 Wt% in the bottom unit to
ca. 11–29 Wt% in the top unit.
A possible reason for finer grained tephra in the
beginning phase of the eruption could be due to in-
creased fragmentation caused by optimal availability
of external water and the fact that SILK-LN tephra
layer is high in SiO2. The large amount of fine ash is
JÖKULL No. 65, 2015 41