Jökull - 01.01.2015, Page 39

Grain characteristics of tephra from Katla and Hekla eruptions
the fine one between 0.063 and 0.125 mm. Largest
grains were >1.6 cm and mean grain size is 0.8 mm.
Samples at 29.5 and 42 km distance are unimodal,
the coarse mode had shifted to 2–4 mm and 1–2 mm
respectively but the fines have become insignificant.
Mean grain size is 2.4 and 1.3 mm, respectively.
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Ves tan Hafrafells
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P hi Φ
Mean grain size: 0.335 Φ (0,793 mm)
Sorting: 3,423 Φ
Wt % ≤4 Φ (≤0,063 mm): 17,83
Wt % ≤6,5 Φ (≤0,011 mm): 1,43
W
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%
Hekla-1947
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F ljóts dals heiði bulk
Mean grain size: -1,264 Ф (2,402 mm)
Sorting: 1,792 Ф
Wt % ≤4 Ф (≤0,063 mm): 2,74
Wt % ≤6,5 Ф (≤0,011 mm): 0,19
Hekla-1947
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Hamragarðaheiði bulk
Mean grain size: -0.361 Ф (1.284 mm)
Sorting: 1.123 Ф
Wt % ≤4 Ф (≤0,063 mm): 1.51
Wt % ≤6,5 Ф (≤0,011 mm): 0.15
Hekla-1947
C
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P hi Φ
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Figure 10. Examples of grain size distribution graphs
of Hekla-1947 tephra near the axis of maximum
thickness, west of Hafrafell, at Fljótsdalsheiði and
Hamragarðaheiði. The violet bars represent the grain
size distribution weight % and the blue line the cumu-
lative weight % plotted against Φ. – Kornastærðargröf
við þykktarás Heklu-1947, vestan Hafrafells, á Fljóts-
dalsheiði og Hamragarðaheiði. Fjólubláu stöplarnir
sýna þyngdarprósentu og bláa línan uppsafnaða
þyngdarprósentu á móti kornastærð (Φ).
The mean grain size of all samples is plotted
against distance on Figure 11. The mean grain size
of the bulk samples changes from 9.4 mm at 3.5 km
distance to 0.23 mm at 68 km and is halved every 13–
14 km, somewhat less than Thorarinsson (1954) found
by using median grain size. A systematic variability
appears in samples collected from the two units where
the bottom grey brown unit is always more coarse-
grained than the top brownish black unit in the same
location. The maximum grain size closest to Hekla
had longest diameters in tens of cm and up to 4 cm at
30 km distance (Thorarinsson, 1954).
The proportion of ash ≤4 Φ (≤0.063 mm) in the
Hekla-1947 layer is not very prominent. We assume
that the grain size data from Thorarinsson (1954)
presents a realistic proportion of ash ≤0.063 mm. The
fines have to be looked at separately (Figure 12) in the
bulk samples and in the grey brown and black tephra
pairs of Thorarinsson (1954). In the bulk samples the
proportion of ash ≤0.063 mm is less than 10 Wt%, ex-
cept at 68 km where it is about 24 Wt% (excluding the
possibly reworked tephra collected at 19 km in 2013).
In samples of the grey brown tephra the proportion of
ash ≤0.063 mm does not exceed 8 Wt% and changes
irregularly with distance, e.g. it is only 5.6 Wt% at
74 km. In samples of the black tephra it varies signif-
icantly, from about 8 to 33 Wt%, but changes irregu-
larly with distance.
The samples collected at 42 km in summer of 2013
and at 49 km presumably in 1947 are similarly low in
fines ≤0.063 mm. This may indicate that conditions
during deposition, and not reworking, caused this ap-
parent lack of fines at this distance from source.
Fine material ≤5 Φ (≤0.031 mm) and ≤6.5 Φ
(≤0.011mm) was not analysed in samples from Thor-
arinsson (1954). The preservation of the newer sam-
ples is questionable but the similarly low amount of
fines at 42 km and 49 km pointed out above suggest
that the proportions at 42 km, <7 Wt% and <1 Wt%
respectively, are at least indicative that proportions of
very fine ash are relatively low in the H-1947 tephra.
Grain shape characteristics
Grain shape analyses on the Hekla-1947 tephra were
carried out on samples from two locations; at W
of Hafrafell at 19 km and at Hamragarðaheiði 42 km
from source.
Changes within the 23 km distance are very small
and not significant. In general the Hekla-1947 grains
are closer to being equant/circular in shape than elon-
gated and have rather uneven surfaces. Grains closer
JÖKULL No. 65, 2015 39