Jökull - 01.01.2014, Side 24

B. A. Óladóttir et al.,
Katla is one of the best known volcanic systems
in Iceland when it comes to eruption history. Its activ-
ity is mostly explosive, eruption frequency has been
estimated for the last 8400 years and volume esti-
mates exist for many of the historical eruptions (e.g.
Larsen, 2000, 2010; Óladóttir et al., 2008). Field
observations around the Katla volcano indicate that
Katla eruptions occurring towards the end of prehis-
toric time in Iceland, i.e. before 870 AD, were larger
and formed thicker tephra layers than the historical
eruptions, i.e. after 870 AD, and that these large erup-
tions were more frequent before the settlement of Ice-
land (e.g. Figure 3.13 in Larsen, 2010). Selected pre-
historic Katla tephra layers have been mapped and
their volumes estimated with the aim of comparing
them to known volumes of historical tephra layers.
Geological context
Iceland is one of the most active and productive sub-
aerial volcanic regions on Earth, with volcanic events
occurring on average every 3–4 years (Guðmunds-
son et al., 2008), and a postglacial magma output
rate of ≥5 km3/100 years (Thordarson and Höskulds-
son, 2008). Iceland covers 103,000 km2, of which
the Neovolcanic zones cover about 30%. The volcan-
ism follows four main areas, the Snæfellsnes, North-
ern, Western and Eastern volcanic zones (Figure 1),
of which the Eastern one is responsible for >80%
of eruptions and 60% of erupted magma in post-
glacial times (Sæmundsson, 1979, Thordarson and
Höskuldsson, 2008). Due to the fact that the most ac-
tive volcanic systems are, at least partly, covered with
glaciers, explosive volcanism is the dominant type
(77%) although about 90% of eruptions are basaltic
(Thordarson and Höskuldsson, 2008).
The most active Icelandic volcanic systems are
Grímsvötn, Bárðarbunga–Veiðivötn, Hekla and Katla
(Thordarson and Larsen, 2007, Larsen and Eiríksson,
2008b). The eruption frequency of these systems is
known from written documentation and tephra stud-
ies (e.g. Sigurður Þórarinsson, 1967; Larsen et al.,
1998; Larsen, 2000; Óladóttir et al., 2008, 2011a) but
little is known of erupted volumes and much of that
knowledge is focused on the largest or the most re-
cent eruptions (e.g. Thorarinsson, 1954; 1967; Thor-
arinsson and Sigvaldason, 1972; Larsen and Thorar-
insson, 1977; Grönvold et al., 1983; Róbertsdóttir,
1992; Larsen et al., 1992; 1999; 2013; Larsen, 2010;
Guðmundsson et al., 2013). Some of Europe’s largest
explosive eruptions occur in Iceland but maps of less
than 10% of the Icelandic tephra layers have been
published (Thordarson and Höskuldsson, 2008).
Katla
The Katla volcanic system is located on the southern
part of the EVZ (Figure 1). It is the most produc-
tive volcanic system in Iceland in historical time but
has the fourth highest eruption frequency (Thordarson
and Larsen, 2007). The volcanic system consists of
a glacier covered central volcano in the southwestern
part and a fissure swarm that reaches northeast (Fig-
ure 1). The central volcano features a 100 km2 (14 by
9 km) caldera within which all observed eruption sites
are located (Larsen, 2010).
Activity on the Katla volcanic system is well
known during the historical period (from the settle-
ment of the Vikings in ∼870 AD) from written ac-
counts, soil and ice sections (e.g. Thorarinsson, 1959;
1975; 1981; Jakobsson, 1979; Einarsson et al., 1980;
Larsen, 2000; 2010). Volumes on land of nine his-
torical eruptions have been estimated to range from
0.04–0.8 km3 of uncompacted (UCP) or freshly fallen
tephra (Larsen, 2010), in addition to the large Eldgjá
eruption that produced ∼4.5 km3 of UCP tephra. The
total volume of K-1755, one of the most voluminous
Katla layer in historical time, has been estimated at
1.5 km3 UCP (including tephra that fell outside of Ice-
land; Thorarinsson, 1975).
The prehistoric time is less well known but based
on tephra studies it has been estimated that a mini-
mum of ∼280 basaltic explosive eruptions have taken
place in Katla in the ∼7300 prehistoric years stud-
ied (Óladóttir et al., 2005). Most of these erup-
tions originate from the central volcano itself, beneath
the glacier that, according to sulphur content in the
tephra, has existed during the last 8400 years (Óla-
dóttir et al., 2007). There have not been many ef-
fusive lava-forming eruptions on the Katla volcanic
system during prehistoric time but 5–10 relatively mi-
nor eruptions are known, most of them probably older
than 4000 years (Larsen, 2000; Jóhannesson et al.,
1990).
24 JÖKULL No. 64, 2014