Jökull - 01.01.2016, Page 70

Þorsteinsdóttir et al.
crease in a caldera lake. Deposits of pre-settlement
(pre-870 CE) jökulhlaups towards south and east are,
however, poorly, or inaccessible, in surface sections
and only a few pre-settlement jökulhlaups have been
documented (Dugmore, 1989; Maizels, 1994; Larsen,
2010).
Figure 1. The Katla volcanic system (Larsen, 2000).
– Eldstöðvakerfi Kötlu (Larsen, 2000).
Grain size and grain morphology can provide im-
portant information about an eruption because dif-
ferent eruption conditions influence both size and
shape of the grains. Both can help to decide whether
the grains are a product of fragmentation by hydro-
magmatic activity or due to exsolution of magmatic
gases (e.g. Sparks et al., 1981; Eiríksson and Wigum,
1989; Dellino and Volpe, 1995, 1996; Dellino and
Liotino, 2002; Büttner et al., 2002; Bonadonna and
Haughton, 2005; Rose and Durant, 2009; Cioni et al.,
2014). These parameters have been studied in indi-
vidual basaltic and silicic tephra layers (Guðmunds-
dóttir, 1998; Óladóttir, 2003; Thorsteinsdóttir, 2012;
Thorsteinsdóttir et al., 2015), but no systematic inves-
tigation of tephra from one volcano, covering an ex-
tended period of time has been done until a small pi-
lot study was carried out as a part of a masters project
(Thorsteinsdóttir, 2015). Here we investigate whether
changes in the grain characteristics of the silicic Katla
tephra layers erupted between 2800 and 8100 years
ago (Table 1) could throw some further light on the
conditions at the volcanic vents and potential changes
of the environment during this period.
The Katla volcanic system
The partly ice-covered Katla volcanic system is lo-
cated in the southern part of the Eastern Volcanic
Zone, and is one of the most active Holocene sys-
tems in Iceland (Larsen, 2000, 2010; Björnsson et al.,
2000; Óladóttir et al., 2005, 2008; Thordarson and
Höskuldsson, 2008). It is about 80 km long, trending
southwest-northeast (Figure 1), and consists of a cen-
tral volcano to the southwest, largely covered by the
600 km2 Mýrdalsjökull ice cap, and a fissure swarm
located mostly outside and to the northeast of the
glacier. The central volcano has an ice-filled caldera,
about 100 km2 and 700 m deep (Jakobsson, 1979;
Björnsson et al., 2000; Larsen, 2000, 2010; Óladóttir
et al., 2005, 2008).
Holocene volcanic activity within Katla volcanic
system has been divided into three categories:
1. Explosive hydromagmatic/phreatomagmatic basal-
tic eruptions from short volcanic fissures below the
Mýrdalsjökull ice cap are the most frequent type of
Katla eruptions and mostly take place within the Mýr-
dalsjökull caldera.
2. Explosive silicic eruptions are the second most
common type during the Holocene. These eruptions
originate from vents beneath the ice cap, either inside
the caldera or on the caldera fracture and are generally
less voluminous than the basaltic eruptions.
3. Effusive basaltic eruptions are less common, occur-
ring outside the glacier along the margins of the cen-
tral volcano or on the fissure swarm to the northeast,
where the two most voluminous eruptions occurred.
Where fissures extend under the ice the eruptions are
partly explosive.
The eruption history of Katla during the last
∼8400 years is fairly well known, especially the
basaltic eruptions. The majority of both basaltic and
silicic eruptions begin below the ice cap. In this paper,
the focus is on the explosive silicic eruptions.
70 JÖKULL No. 66, 2016