Jökull - 01.01.2015, Blaðsíða 36

Þorsteinsdóttir et al.
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P hi Ф
L oðnugil top unit
Mean grain size: 1,720 Ф (0,304 mm)
Sorting: 2,958 Ф
Wt % ≤4 Ф (≤0,063 mm): 27,76
Wt % ≤6,5 Ф (≤0,011 mm): 4,55
S IL K -L N ~3400 years old
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L oðnugil upper middle unitS IL K -L N ~3400 years old
Mean grain size: 1,203 Ф (0,434 mm)
Sorting: 3,188 Ф
Wt % ≤4 Ф (≤0,063 mm): 24,95
Wt % ≤6,5 Ф (≤0,011 mm): 4,24
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L oðnugil lower middle unit
Mean grain size: 2,833 Ф (0,140 mm)
Sorting: 2,357 Ф
Wt % ≤4 Ф (≤0,063 mm): 36,74
Wt % ≤6,5 Ф (≤0,011 mm): 7,13
S IL K -L N ~3400 years old
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P hi Ф
L oðnugil bottom unit
Mean grain size: 2,726 Ф (0,151 mm)
Sorting: 2,400 Ф
Wt % ≤4 Ф (≤0,063 mm): 34,45
Wt % ≤6,5 Ф (≤0,011 mm): 6,56
S IL K -L N ~3400 years old
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Figure 6. Grain size distribution from four units of
SILK-LN at Loðnugil. The violet bars represent
the grain size distribution weight % (Wt%) and
the blue colored line the cumulative Wt% plotted
against Φ. – Kornastærðardreifing (Φ) fjögurra
sýna úr SILK-LN í Loðnugili. Fjólubláu stöplarnir
gefa þyngdarprósentu hvers sýnis og bláa línan
uppsafnaða þyngdarprósentu.
The Wt% of fines ≤0.063, ≤0.031 and
≤0.011 mm and changes with distance are shown on
Figure 8. All samples are included. As expected the
Wt% of fines (≤0.063 mm) increases with distance
from 25–37% at 22 km to 37–43% at 65 km, omitting
the sample from the top unit at 65 km that may be re-
worked. The sample at 35 km with the 45 Wt% lies at
the edge of the SILK-LN tephra layer. Similar trend is
seen in fines ≤0.031 mm, they are several Wt% lower
and increasing slightly with distance. However, fines
≤0.011 mm appear not to change with distance and
are below 10 Wt% in all samples. It should be kept
in mind that the Wt% of fines measured here is what
is deposited on the ground and represents a minimum
of the fines generated in the eruption and injected into
the air.
Grain shape characteristics
Grain shape analyses on SILK-LN tephra were car-
ried out on samples from two locations, at Geldinga-
sker and Varmárfell, 33 and 65 km from the source,
respectively. Only the two lowermost units were used
to ensure that the analyses were performed on primary
tephra fall units. About 50 grains were picked ran-
domly from a mixture of both 3 and 3.5 Φ fractions.
Grains closer to source, in Geldingasker, are more
rugged (lower values represent more rugged grains)
and less elongated (lower values represent more elon-
gated grains) than further away from source in Varm-
árfell (Table 1). In simple terms this indicates
that long smooth grains tend to travel farther than
short/circular, rugged grains. At both locations the
grains tend to become more rugged (with more un-
even outline) with time but such systematic change
is not seen in the other two parameters (Table 1).
More detailed results can be viewed in Þorsteinsdóttir
(2015).
The Scanning Electron Microscope TM3000 was
used to obtain SEM-images of the SILK-LN tephra
grains. The samples used came from Geldingasker
and Varmárfell. Five to six different types of grains
along with few sub-categories were noticed (Fig-
ure 9).
RESULTS - HEKLA 1947 TEPHRA
Field characteristics
The Hekla-1947 tephra layer did not show any bed-
ding in the field when sampled in 2013 although
descriptions of the tephra fall explicitly describe
changes both in colour and grain size of the tephra
36 JÖKULL No. 65, 2015