Jökull - 01.01.2005, Blaðsíða 54

Bergrún Arna Óladóttir et al.
1981), Veiðivötn 1477 AD and Vatnaöldur ∼870
AD (Larsen, 1984) and the prehistoric 10,300 year
old Saksunarvatn eruption (Mangerud et al., 1986;
Jóhannesdóttir et al., 2005). Another major contrib-
utor to formation of Holocene tephra layers in Iceland
is the Hekla volcano in S-Iceland, which is also lo-
cated in the Eastern Volcanic Zone and is character-
ized by intermediate to silicic volcanism (e.g. Thorar-
insson, 1967, 1981; Larsen and Thorarinsson, 1977;
Dugmore et al., 1995a). All of the abovementioned
volcanoes are located in the Eastern Volcanic Zone
(EVZ), (Figure 1). In the following, “Katla volcano”
(or Katla) refers to the ice-covered central volcano,
and “Katla volcanic system” (or Katla system) refers
to both the central volcano and the associated fissure
swarm.
Icelandic soils contain a record of explosive vol-
canism in the form of tephra layers extending back 8–
9000 years (7–8000 14C years BP, e.g. Larsen 2000;
Hafliðason et al., 2000). In lake sediments the record
can reach as far back as 10–12,000 years (9–10,500
14C years BP, e.g. Björck et al., 1992; Jóhannesdóttir
et al., 2005). It is a custom to divide the tephra record
into two groups; the historical part, covering the pe-
riod since the Viking settlement (870 AD to present),
and the prehistoric part, which refers to all Holocene
tephra layers pre-dating the Viking settlement. The
Holocene tephra record is a valuable source of in-
formation on the postglacial eruption history in Ice-
land and is a very reliable record for constructing the
longer-term eruption history at a given volcano. It also
provides the means for assessing the nature of erup-
tive activity and magma evolution over specific time
periods.
The aim of this study is to evaluate the history
and eruption frequency of the Katla volcanic system
during the Holocene. This was accomplished by de-
tailed logging and sampling of closely spaced soil pro-
files that contain a record of explosive volcanism at
Katla volcano over the last 8.4 ka. The Holocene
tephra stratigraphy of the region is presented and we
use pre-existing 14C-dates of tephra layers (Table 1)
in conjunction with the calculated soil accumulation
rate (SAR) to establish a comprehensive chronology
for the tephra layers preserved in the soils to the east
of the volcano. Major-element composition is the key
criterion used to identify the source volcano for in-
dividual tephra layers. It is shown that eruption fre-
quency and compositional variations are greater in the
prehistoric era compared to that in historical time.
KATLA VOLCANIC SYSTEM:
GEOLOGIC SETTING AND ERUPTION
HISTORY
The Katla volcanic system is situated in the south-
ern sector of the Eastern Volcanic Zone (EVZ) in
South-Iceland and takes it name from the notorious
subglacial Katla fissure. The 80-km-long system is
made up of an ice-capped central volcano and an as-
sociated SW-NE trending fissure swarm (Jakobsson
1979; Figure 1). The central volcano (1450 m), is
the second largest of its kind in Iceland and features
a 110 km2 ice-filled summit caldera that is up to
700 m deep (Björnsson et al., 2000). The Katla fis-
sure, which is the inferred source vent for all historical
eruptions at the volcano since the 12th century, is sit-
uated in the southern part of the caldera. The fissure
swarm is demarcated by SW-NE and E-W trending
volcanic fissures of Holocene and historical age and
represents the ice-free part of the system (Figure 1).
Furthermore, the available data indicates that the sys-
tem has been characterized by Fe-Ti basalt volcanism
throughout its history and felsic rocks of intermediate
and silicic compositions represent a minor component
in the erupted products (e.g. Jakobsson, 1979; Larsen,
1979; Jóhannesson and Sæmundsson, 1998; Lacasse
et al., 1995; Hildebrand et al., 1998; Larsen et al.,
2001; Thordarson et al., 2001).
Katla volcanic system is the second most active
system in Iceland during historical time, with 21
recorded eruptions since 870 AD. The established
chronology reveals 1–3 eruptions per century, with an
average repose time of 48 years (range, 13–95 years)
between events (Larsen, 2000). Twenty of the histor-
ical eruptions were explosive subglacial basalt erup-
tions that took place on fissures within or along the pe-
riphery of the caldera. These eruptions have produced
widespread tephra layers with volume ranging from
0.02 to 1.5 km3 (Thorarinsson, 1975; Larsen, 2000).
54 JÖKULL No. 55