Jökull - 01.01.2015, Page 32

Þorsteinsdóttir et al.
isopach line (Figure 2). The volume of compacted
tephra is about 0.12 km3 and of uncompacted tephra
about 0.2 km3. Thickness has only been measured be-
yond ca. 22 km distance, because part of the tephra
fell on the ice cap and was not preserved. Maximum
thickness is therefore not known. The greatest thick-
ness at ca. 22 km distance was about 12 cm (Figure 3).
Figure 3. SILK-LN tephra layer in Loðnugil, 22 km
east of caldera centre. The greyish-green tephra is
bedded and four units are indicated on the photo-
graph. – SILK-LN gjóskulagið í Loðnugili, 22 km aust-
an miðrar öskju. Gjóskan er grá-græn og lagskipt,
fjórar einingar sjást á ljósmyndinni (Ljósm./Photo.
Edda Sóley Þorsteinsdóttir, 2012).
The Hekla Volcanic System
The Hekla volcanic system is situated in the East-
ern Volcanic Zone and is about 40 km long and about
7 km wide as defined by Jakobsson (1979) (Figure 1).
The Vatnafjöll fissure swarm southeast of the Hekla
volcanic system is also thought to belong to the sys-
tem due to similar chemical composition of its basalt
(Jóhannesson and Einarsson, 1992; Jóhannesson and
Sæmundsson, 1998). The Hekla central volcano is
an elongated or ridge-shaped stratovolcano, and no
caldera has developed (Thordarson and Larsen, 2007).
Geophysical and geochemical data indicate the
existence of a magma chamber below Hekla (Thor-
arinsson, 1967; Tómasson, 1967; Sigmarsson et al.,
1992; Soosalu and Einarsson, 2004; Sverrisdóttir,
2007). The chemical composition of the magma
changes with time, the longer the repose between
eruptions the more silicic is the first erupted material
in the next eruption (Thorarinsson, 1967). Holocene
activity of Hekla volcanic system can be divided into
three eruption types:
1. Explosive eruptions of the Plinian type producing
almost solely tephra.
2. Mixed eruptions that have an initial explosive phase
simultaneous with and followed by effusive eruption.
Most of the tephra is emitted in a Plinian/subplinian
explosive phase. The effusive phase produces mostly
lava and moderate tephra.
3. Effusive fissure eruptions producing mostly lava
and tephra production is usually small. The majority
of these eruptions occur on the fissure swarm outside
the Hekla central volcano.
The magma erupted in the explosive- and the
mixed eruptions is rhyolite to basaltic andesites, and
in the effusive fissure eruptions it is basaltic (Þórarins-
son, 1968; Larsen and Thorarinsson, 1977; Jakobs-
son, 1979; Jóhannesson et al., 1990; Larsen et al.,
2013). Of the 18 eruptions in the Hekla volcano
in historical time (last 11 centuries) the first one in
1104 CE was silicic explosive eruption and the others
were silicic/intermediate mixed eruptions (Kjartans-
son, 1945; Thorarinsson, 1967, 1968, 1970; Grönvold
et al., 1983; Larsen et al., 1992; Höskuldsson et al.,
2007). Here the focus is on the intermediate mixed
Hekla eruptions.
Mixed explosive-effusive Hekla eruptions
The recent Hekla eruptions, in 1947 and thereafter,
are well documented (e.g. Thorarinsson, 1954, 1967,
1968, 1970; Grönvold et al., 1983; Larsen et al.,
1992; Höskuldsson et al., 2007). These eruptions
commenced with a Plinian (e.g. Hekla-1947) or sub-
plinian phase (e.g. Hekla 2000) lasting hours. The
1947 and 1991 eruption fissures ran along the Hekla
ridge and into the upper slopes whereas the 1970
fissures opened up on the lower slopes. Most of
the tephra production originates in fissures along the
Hekla ridge. Lava production begins from fissures on
the ridge, later concentrating on the lower shoulders
(Þórarinsson, 1968; Höskuldsson et al., 2007).
32 JÖKULL No. 65, 2015