Jökull - 01.01.2015, Page 29

Reviewed research article
Grain characteristics of tephra from the sub-glacial SILK-LN
Katla eruption ∼3400 years ago and the sub-aerial Hekla
eruption in 1947
Edda Sóley Þorsteinsdóttir, Esther Ruth Guðmundsdóttir and Guðrún Larsen
Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland
eth64@hi.is
Abstract — Chemical composition of the tephra from the Katla and Hekla volcanoes is similar but the eruption
environment is quite different, the Katla volcano is ice-covered whereas the Hekla volcano is ice-free. The
∼3400 years old Katla SILK-LN and the Hekla 1947 tephra layers were studied with respect to spatial and
temporal changes in grain size and their grain characteristics with main focus on the finer fraction of the tephra
grains. The results show obvious difference both in mean grain size and the fraction of finest particles from
the two volcanoes. The Hekla 1947 tephra has significantly higher mean grain size and much lower content
of material finer than ≤4 Φ (0.063 mm). The grains from the Katla tephra are elongated and are even needle
shaped, but the grains from the Hekla tephra are more equant. The silicic Katla eruptions produce greater
quantities of ash ≤4Φ than Hekla eruptions with similar magma composition and are therefore more likely to
affect the human environment, including health of people and aviation.
INTRODUCTION
The recent eruptions in Eyjafjallajökull 2010 and
Grímsvötn 2011 (Gudmundsson et al., 2012; Hreins-
dóttir et al., 2014) have increased interest in the trans-
port of fine-grained tephra and the proportion of par-
ticles 0.063 mm and smaller generated by different
types of explosive eruptions. The fine-grained tephra
particles tend to stay in the air for a long time and
can be transported very far. Fine particles from an
explosive eruption may cause critical engine failure
if an airplane encounters an ash cloud. Serious dis-
turbances in flight transportations can therefore occur
during an explosive eruption (e.g. Þorkelsson, 2012).
Furthermore, wind-blown tephra can cause trouble
in the area around the volcano for a long time af-
ter the eruption has ended. Example of such wind-
blown material is the fine tephra from the Eyjafjalla-
jökull 2010 and Grímsvötn 2011 eruptions which has
reached Reykjavík on a number of occasions (Nichol-
son et al., 2014).
The project described in the following chapters is
part of a larger project aimed at the characterization
of tephra from different types of explosive Icelandic
eruptions through analysis of grain sizes and grain
shapes. This paper deals with tephra of similar chem-
ical composition from two eruptions occurring in dif-
ferent environmental settings, the ice-covered Katla
volcano and the ice-free Hekla volcano.
The main objective is to investigate whether the
effect of the eruption environment is discernible in
the grain characteristics of the tephra from these two
eruptions. Spatial and temporal changes in the propor-
tion of tephra 0.063 mm and smaller are of particular
interest. Also, will the tephra grains reflect the differ-
ent eruption environment and can difference in grain
shape affect the long-range transport?
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