Jökull - 01.01.2010, Síða 136

Agustsdottir et al.
geology of Iceland is characterised by the interac-
tion of spreading at the mid-ocean plate boundary and
the hotspot, centered in east-central Iceland under the
Vatnajökull icecap (Wolfe et al., 1997). The diver-
gent plate boundary is marked by a chain of active
central volcanoes traversing Iceland, resulting from
plate rifting and volcanism induced by upwelling of
the hotspot. Central volcanoes are defined by fre-
quent eruptions from a central area (Walker, 1963;
Sæmundsson, 1978). Volcanism in the neovolcanic
zone results in elongated volcanic systems, consisting
of a central volcano and a transecting fissure swarm
(Sæmundsson, 1978, 1979). A geothermal field is
often associated with the central volcano. Further-
more, silicic rocks in Iceland are generally associ-
ated with central volcanoes (Jónasson, 2007) and most
central volcanoes have only produced minor amounts
of silicic rock in a few formations (Jóhannesson and
Sæmundsson, 1999). No dome eruption has been
observed with modern monitoring equipment in Ice-
land while the geological record contains many silicic
tephra layers, lavas and dome formations (Jóhannes-
son and Sæmundsson, 1999; Larsen, 2000). Domes in
Iceland generally occur at or near the caldera rims of
an active central volcano. The sides of the underlying
magma chamber are thought to represent the best con-
ditions for the formation of silicic melts due to cool-
ing of the hydrothermal system (Jónasson, 1994). The
central volcano Krafla, situated in the NVZ (Northern
Volcanic Zone), has an ice-free caldera surrounded by
subglacially formed silicic domes Hlíðarfjall, Hrafn-
tinnuhryggur, Jörundur and Rani (not studied here),
all located close to the caldera rim. (Sæmundsson,
1991). The Krafla area is both easily accessible and
one of the most intensely studied volcanic areas in
northern Iceland, due to the geothermal power plant
situated above the caldera’s shallow magma chamber
and the recent volcano-tectonic episode, the Krafla
fires, in 1975–1984 (e.g. Einarsson, 1991; Buck et al.,
2006). For these reasons the Krafla region was cho-
sen as a study area to examine Icelandic domes in an
active central volcano. The determination of the vent
structure can shed light on how these domes were em-
placed, e.g. whether they were formed by a dike-fed
eruption or by a more forceful emplacement displac-
ing surrounding strata in the process. Buoyancy is
important in driving magma through the crust, and
therefore density data are necessary for meaningful
modelling of the emplacement of cryptodomes and
lava domes. The aim of the present work was there-
fore to determine poorly known bulk density values
for domes and to investigate whether the domes have
roots. Rock samples were also collected and their
density measured to further constrain the results.
GEOLOGICAL SETTINGS
The Krafla volcanic system
The Krafla volcanic system (Figure 1) is located in the
Northern Volcanic Zone (NVZ). The NVZ consists
of several elongated closely spaced volcanic systems,
amongst which the Krafla and Askja volcanic systems
are most active. The Krafla volcanic system consists
of a central volcano and a transecting fissure swarm.
The Krafla caldera is approximately 10 km wide, tran-
sected by a 100 km long and N10◦E trending fis-
sure swarm (Sæmundsson, 1991). Around the caldera
a number of concentric fissures have erupted hyalo-
clastites, lavas and rhyolite (Sæmundsson, 1991). A
powerful geothermal system lies inside the caldera,
harnessed by a geothermal power station operated
since 1977. The Krafla caldera began forming about
100 000 years BP (Sæmundsson, 1991). Due to the in-
tensive volcanism, stratigraphic build up has been rel-
atively fast, both during periods of ice-free conditions
and periods when ice covered the land. In the geologi-
cal strata of the Krafla caldera there is evidence of two
main warm periods. The silicic rocks are concentrated
around the caldera rim and away from the central part
of the fissure swarm (Sæmundsson, 1991).
The Heiðarsporður volcanic system
Heiðarsporður is a small volcanic system located
south of Krafla. Its topographic expression is a low
and broad ridge, about 15 km long and 3 km wide,
extending from the Krafla central volcano in the north
towards the Bláfjall ridge and in the south to the
Fremrinámur central volcano. This volcanic system
was active for a relatively short time (1000 years) in
the early Holocene and was built up in several small
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