Jökull - 01.01.2010, Page 138

Agustsdottir et al.
eruptions (Sæmundsson, 1991), including the forma-
tion of Hraunbunga, a small dacitic dome or coulee.
Geological framework of studied formations
In this study three well-defined domes were surveyed,
Hlíðarfjall, Hrafntinnuhryggur and Hraunbunga (Fig-
ure 1):
1. Hlíðarfjall (rhyolite, 310 m high, 2 km long)
formed 90 000 years BP, parallel to the Krafla
caldera rim on the southwest side (Sæmunds-
son, 1991; Sæmundsson et al., 2000).
2. Hrafntinnuhryggur (rhyolite, 80 m high, 2.5 km
long) formed 24 000 years BP, located 0.5–1 km
inside and sub-parallel with the eastern caldera
rim of Krafla (Sæmundsson, 1991; Sæmunds-
son et al., 2000).
3. Hraunbunga (125 m high, 1.8 km long), a
dacitic coulee erupted from a short fissure
formed 10 000 years BP in the Heiðarsporður
volcanic system (Sæmundsson, 1991).
Hlíðarfjall and Hrafntinnuhryggur are short, steep
ridges that may be regarded as subglacial equivalents
of lava domes. Hlíðarfjall together with the domes
Jörundur and Rani are parallel to the Krafla caldera
rim and may, on the basis of composition and age, be
interpreted to represent a common short-lived phase in
Krafla’s history. It has been suggested that these rhy-
olite eruptions were caused by the emplacement of a
ring dike (Jónasson, 1994) but the activity may have
been more limited and localised, associated with oc-
casional rhyolitic dike intrusions. Hrafntinnuhrygg-
ur has a different composition and age and is paral-
lel to the tectonic trend in the caldera and subparal-
lel with the eastern caldera rim (Sæmundsson, 1991).
Hraunbunga is a dasitic coulee in the Heiðarsporður
volcanic system; one of the few silicic domes in Ice-
land not associated with a central volcano (Jónasson,
2007).
METHODS AND DATA
In general, silicic rocks have lower densities than
those of a more basaltic composition. Measuring a
gravity profile over a rhyolite dome surrounded by
basaltic lava should demonstrate a significant den-
sity contrast. The form of gravity anomalies associ-
ated with outcropping surface formations can often
be used to determine whether they are partly buried
by younger formations or not (Figure 2). Knowl-
edge of past geological processes at the observation
site also gives an idea of what can be expected under
the surface. Data from boreholes that reach to just
over 2 km depth in the Krafla caldera demonstrate
that bedrock in the region consists predominantly of
basalt, with suites of layered piles of alternating lava
and units of hyaloclastite down to 2000 meters, where
granophyre becomes dominant at the bottom (Gud-
mundsson et al., 2008; Arnadottir et al., 2009).
The gravity surveys
In Iceland, gravity studies aimed at determining bulk
density of individual topographic features have been
done on hyaloclastite tindars (hyaloclastite ridges),
tuyas and lava flows but not on silicic formations.
This study will attempt to fill that gap. The Nettle-
ton method has proved to be useful for density
determinations of hyaloclastite and lava formations
(Schleusener et al., 1976; Gudmundsson and Milsom,
1997; Gudmundsson et al., 2001; Gudmundsson and
Högnadóttir, 2004; Schopka et al., 2006), this should
also be the case for rhyolite domes. Density measure-
ments of rock samples have been made for most types
of Icelandic rock (Pálsson et al., 1984; Gudmundsson
and Högnadóttir, 2004). A knowledge of rock den-
sities is necessary for applying the Bouguer anomaly
and the Nettleton method (e. g. Kearey et al., 2002).
The mean bulk density of the surface bedrock in
the Krafla caldera was obtained by Johnsen (1995) by
applying the Parasnis method (Parasnis, 1973) to the
entire Krafla caldera. In the Bouguer reduction we use
his value of 2500 kg m−3 as our background density.
The reason is that this density best represents the sur-
roundings of the domes, i.e. the bedrock in the Krafla
area. A total of 48 gravity stations along nine profiles
were measured (Figure 1) over the three dome forma-
tions. Observations were carried out in the autumn of
2007 and the spring of 2008, during five days.
138 JÖKULL No. 60