Jökull - 01.01.2014, Síða 23

Reviewed research article
Volume estimates of nine Katla tephra layers
(∼1860 BC – 870 AD)
Bergrún Arna Óladóttir1, Guðrún Larsen2 and Olgeir Sigmarsson2,3
1The Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland
2Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, IS-101 Reykjavík, Iceland
3LMV, CNRS - Université Blaise Pascal, Clermont-Ferrand, France
Corresponding author: bergrun@hi.is
Abstract – Activity of the Katla volcano, under the Mýrdalsjökull ice cap, is characterized by explosive basaltic
eruptions and occasional silicic eruptions. Tephrochronology indicates that it has erupted over 300 times
during the last ∼8400 years. About 40 soil sections, distributed around the volcano although the majority are
located to the east and northeast, have been measured in order to map the thickness of nine selected prehistoric
Katla tephra layers, younger than ∼1860 BC (Hekla-S tephra) but older than ∼870 AD (Settlement tephra,
Bárðarbunga–Veiðivötn volcanic system). The volume of these layers was calculated from isopach maps, using
the Surfer© Golden Software, and compared to published volume estimates of eight historical tephra layers
(younger than ∼870 AD), all of which are ≤1 km3 of freshly fallen tephra. Maximum thickness values similar
to those applied in the volume estimates of the historical tephra were used. The volume range of the prehistoric
tephra layers on land is from 0.2–2.7 km3, with the more voluminous layers being significantly larger and more
frequent than those produced by historical eruptions at Katla. The observed volume difference between the two
time periods could be related to changes to the magma plumbing system under the Katla volcano.
INTRODUCTION
Eruption frequencies of many Icelandic volcanic sys-
tems in historical time are well known (e.g. Larsen
and Eiríksson, 2008a; Óladóttir et al., 2008; see also
Sólnes et al., 2013 and refs. therein) and explosive
eruption frequency has been estimated for the most
active volcanic systems in prehistoric time (Óladóttir
et al., 2008; 2011a). However the eruption history of
these volcanic systems is only partly known as there is
little information available on the magnitude of erup-
tions, which is an important factor in understanding
volcanism.
Tephra deposits are an archive of past volcanic
activity containing information on explosive eruption
history and can be utilized to model eruption column
height, volume and mass of magma erupted, as well
as the eruption duration and its intensity (e.g. Carey
and Sparks, 1986; Carey and Sigurdsson, 1989; Pyle,
1989; 1995; Fierstein and Nathenson, 1992; Burden
et al., 2011; Bonadonna and Costa, 2012; Bonadonna
and Houghton, 2005; Burden et al., 2013; Engwell et
al., 2013). Erupted volume has mainly been estimated
based on isopach maps drawn from point measure-
ments of tephra thickness (e.g. Thorarinsson, 1967;
Larsen and Thorarinsson, 1977; Pyle, 1989; Fierstein
and Nathenson, 1992; Klawonn et al., 2014) using
different calculation methods to estimate the proxi-
mal thickness (the so-called exponential and Weibull
methods; Pyle, 1989; Bonadonna and Costa, 2012).
Recently inversion of mass per unit area measure-
ments (Connor and Connor, 2006) and statistical mod-
els have been applied directly to thickness-versus-
distance data (Burden et al., 2013).
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