Jökull - 01.01.2015, Page 37

Grain characteristics of tephra from Katla and Hekla eruptions
0
0,5
1
1,5
2
2,5
3
3,5
4
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
M
ea
n
g
ra
in
s
iz
e
(m
m
)
M
ea
n
g
ra
in
s
iz
e
(Ф
)
Dis tanc e from s ourc e (km)
Loðnugil bulk
Loðnugil top unit
Loðnugil upper middle unit
Loðnugil lower middle unit
Loðnugil bottom unit
Geldingasker middle unit
Geldingasker bottom unit
Leiðólfsfell top unit
Leiðólfsfell middle unit
Leiðólfsfell bottom unit
Blágil
Varmárfell bulk
Varmárfell top unit
Varmárfell middle unit
Varmárfell bottom unit
S IL K -L N
V armárfell top
unit
0.5
0.25
0.125
0.063
1
Mean grain s ize of S IL K -L N
plotted agains t dis tanc e from s ourc e
Figure 7. Mean grain size along
the axis of thickness of SILK-LN
tephra plotted against distance
from source (centre of Katla
caldera). Colours denote sam-
ples from locations listed to the
right. – Meðalkornastærð eftir
þykktarás SILK-LN gjóskulagsins
á móti fjarlægð frá upptökum
(í miðri Kötluöskjunni). Litaðir
þríhyrningar tákna einstakar
einingar og heildarsýni SILK-
LN gjóskulagsins á mismunandi
stöðum.
0
10
20
30
40
50
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
W
ei
g
h
t
%
Dis tance from s ource (km)
Varmárfell top unit
Varmárfell top unit
F ine proportions of S IL K -L N plotted agains t dis tance from s ource
≤4 Φ (≤0.063 mm)
≤5 Φ (≤0.031 mm)
≤6.5 Φ (≤0.011 mm)
Varmárfell top unit
Figure 8. Weight % of fine par-
ticles of the SILK-LN tephra
plotted against distance from
source. – Þyngdarprósenta fín-
efnis í SILK-LN gjósku á móti
fjarlægð frá upptökum.
(Þórarinsson, 1968). Therefore are all Hekla-1947
samples collected for this project bulk samples.
Where thickness of the tephra layers changed later-
ally more than one sample was collected and treated
individually. The sampling locations all lie in the axis
of thickness: Vestan Hafrafells, Fljótsdalsheiði and
Hamragarðaheiði, at a distance of 19 km, 29.5 km and
42 km from Hekla summit, respectively (Figure 4).
The Hekla-1947 tephra consists of grey brown and
brownish black pumice and ash with occasional red
grains (Figure 5). West of Hafrafell the thickness of
the layer was up to 5 cm and there the brownish black
tephra originally at the top had been washed down into
the coarser bottom unit.
Grain size characteristics
Because of lack of primary structures in the Hekla
1947 tephra it was decided to use results from grain
size analyses of 14 samples from 8 locations close
to the axis of thickness published by Thorarinsson
(1954), collected shortly after the tephra fall in 1947.
The smallest sieve used by Thorarinsson (1954) was
4 Φ or 0.063 mm, so grain intervals below 0.063 mm
cannot be plotted separately. Grain parameters were
recalculated as for the new samples.
The samples collected in 2013 were analysed
down to 10 Φ (0.001 mm), (Figure 10). The sample
closest to source, at 19 km distance, is distinctly bi-
modal, with the coarse mode between 4 and 8 mm and
JÖKULL No. 65, 2015 37