Jökull - 01.01.2010, Blaðsíða 135

Reviewed research article
A gravity study of silicic domes in the Krafla area, N-Iceland
Thorbjorg Agustsdottir, Magnús Tumi Gudmundsson and Páll Einarsson
Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland
thorbag@hi.is, mtg@hi.is, palli@raunvis.hi.is
Abstract – Silicic rocks in Iceland are generally associated with central volcanoes and are often emplaced as
domes on or around caldera rims. Some of these domes were formed subglacially while others were erupted
under ice-free conditions. A gravity survey was carried out in the area of Krafla in 2007 and 2008 to determine
the mean bulk densities of three silicic domes; essential data for meaningful modelling of the emplacement
of cryptodomes and lava domes. Such data are scarce. Profiles were measured over three formations: Hlíð-
arfjall, made of rhyolite, 310 m high, 2 km long and formed under ice 90 000 years BP, Hrafntinnuhryggur,
made of rhyolite, 80 m high, 2.5 km long and formed subglacially 24 000 years BP and Hraunbunga made
of dacite, 125 m high and 1.8 km long, formed under ice-free conditions 10 000 years BP. Mean bulk density
for each formation was obtained by the Nettleton method. Mean bulk density and volumes obtained were;
Hlíðarfjall: 1600-1800 kg m−3, 0.143 ± 0.014 km3; Hrafntinnuhryggur: 1575–1875 kg m−3, 0.021 ± 0.002
km3; Hraunbunga: 1750–1775 kg m−3, 0.040 ± 0.004 km3. The results show that all the domes have low
densities, reflecting both low grain-density and high porosity. The density values are significantly lower than
those of the surroundings, creating a density contrast possibly sufficient to drive the ascent of silicic magma.
Furthermore, results from forward gravity modelling demonstrate that these formations are neither buried by
younger volcanic eruptives nor are any roots detected. The domes studied were therefore emplaced by a dike to
the surface.
INTRODUCTION
Rhyolite magma can rise due to buoyancy forces and
either form a cryptodome in the shallow crust or rise
to the surface, where it erupts. Due to its high vis-
cosity and resistance to flow it often accumulates and
forms a lava dome over the vent. In volcanology, a
dome is defined as an accumulated silicic melt of high
viscosity. The term is used to describe any dome,
whether it is on the surface or not. A dome can ac-
tively deform adjacent strata and break through the
surface. The term coulee is used for a lava dome
where some lateral flow away from the vent has oc-
curred (Fink and Anderson, 2000).
Worldwide, domes are associated with viscous,
silicic magma. The occurrence of domes could there-
fore be expected in areas of silicic volcanism, i.e. sub-
duction zones, rather than in areas where basaltic vol-
canism is dominant, i.e. divergent rift zones and hot
spots. However, dome formations are known in areas
of basaltic volcanism, e.g. in Krafla, Iceland. World-
wide, dikes feed many silicic domes (Fink and Ander-
son, 2000). These domes tend to occur in groups, that
are either in linear or echelon arrays (Fink and Ander-
son, 2000), e.g. in Long Valley, California.
Silicic rocks in oceanic crust are very rare and it
seems likely that the oceanic crust has to reach a cer-
tain thickness before silicic domes can develop. This
can be argued from studies of seismic data for Iceland
(e.g. Menke, 1999) and the geographical distribution
of silicic domes and nunataks in Iceland (Jónasson,
2007). Geological settings in Iceland are very dif-
ferent from those of a subduction zone where domes
are common. Iceland is a ridge-centered hotspot is-
land, situated astride the Mid-Atlantic Ridge. The
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