Jökull - 01.01.2015, Side 46

Þorsteinsdóttir et al.
• The portion of fine material 0.063 mm (4 Φ) and
smaller increases with distance but the changes
are not very prominent. The fine material
0.011 mm (6.5 Φ) and smaller shows almost no
sign of changes with distance.
• Grains found closer to the source have more
rugged surfaces and less elongated shapes than
the ones further away.
The Hekla-1947 tephra
• Changes with time were prominent in samples
of freshly fallen tephra (Thorarinsson, 1954)
with grey brown coarser grained base and finer
grained brownish black top.
• Changes in grain size with distance are promi-
nent, the mean grain size (bulk samples)
changes from 0.79 mm (0.34 Φ) at 40 km to
0.23 mm (2.10 Φ) at 68 km.
• There was a clear evidence of deficiency in fine
material 0.063 mm and smaller in the samples
collected especially for this study which can
possibly be explained by reworking.
• Changes in grain shape with distance are not
significant. Grains further away from the source
have slightly smoother surfaces and more circu-
lar shapes.
Comparison of grain characteristics of silicic Katla
and Hekla tephra
• The mean grain size of Hekla 1947 tephra layer
changes more prominently with distance than in
the SILK-LN tephra layer.
• In both tephra layers where bedding is visi-
ble considerable changes are noticed during the
course of the eruption.
• The proportion of fines ≤0.063 mm is consid-
erably higher in the SILK-LN tephra layer than
in Hekla 1947 tephra layer. At a distance be-
tween 22 and 65 km this portion is 24–43 Wt%
along axis of thickness of SILK-LN but 1–33
Wt% for Hekla 1947.
• Obvious differences were noticed in the grain
shape results. The biggest difference observed
was in the elongation and circularity values, the
grains of the SILK-LN layer are more elongated
than the Hekla 1947 grains. The Hekla 1947
grains have smoother surfaces than the SILK-
LN grains at 33 km from the source but more
uneven surfaces than the SILK-LN grains at
65 km.
ACKNOWLEDGEMENTS
This work was supported by the International Civil
Aviation Organization (ICAO) as a part of the Cata-
logue of Icelandic Volcanoes Project of the Icelandic
Meteorological Office and the Institute of Earth Sci-
ences, University of Iceland. We are grateful to
Bergrún A. Óladóttir, Tinna Jónsdóttir and Agnes
Ö. Magnúsdóttir for assistance and good company
in the field, and to Johanne Schmith and Jón Eiríks-
son for assistance with the grain size and grain shape
analyses. We thank Hafliði Hafliðason and Stefan
Wastegård for their constructive reviews of this paper.
ÁGRIP
Kornastærðareinkenni súrra til ísúrra gjóskulaga frá
tveim eldfjöllum, Kötlu og Heklu, voru rannsökuð.
Efnasamsetning gjóskunnar er svipuð en umhverfi
gosstöðvanna er ólíkt, Katla er þakin jökli en Hekla
er jökullaus. Tvö gjóskulög, Kötlulagið SILK-LN um
3400 ára gamalt og Heklulagið úr gosinu 1947 voru
skoðuð með tilliti til kornastærðarbreytinga gjóskunn-
ar með fjarlægð frá upptökum og með tíma í gosun-
um. Athygli var sérstaklega beint að finustu gjósk-
unni. Einnig voru kornalögunareinkenni gjóskunnar
athuguð og borin saman. Greinilegur munur er bæði
á meðalkornastærð og á hlutfalli fínefnis í gjóskulög-
unum frá þessum tveim eldfjöllum. Meðalkornastærð
gjóskunnar úr Heklugosinu 1947 er talsvert hærri og
hlutfall fínefnis, ≤4Φ (0.063 mm), er miklu minna í
öllum fjarlægðum samanborið við SILK-LN gjósk-
una úr Kötlu. Kötlugjóskan varð grófari með tíma en
Heklugjóskan fínni. Munur á kornalögun Heklu- og
Kötlugjóskunnar er mjög greinilegur, kornin í Kötlu-
gjóskunni eru ílöng og jafnvel nálarlaga en kornin í
Heklugjóskunni fremur jafnása.
46 JÖKULL No. 65, 2015