Jökull - 01.01.2010, Blaðsíða 142

Agustsdottir et al.
a. The dome is strictly a surface formation without
a root extending down into the crust.
b. Same as a but a narrow dyke extends down-
wards into the crust.
c. The dome is modeled as the top of a partly
buried formation.
All gravity models of type b are shown with the
maximum possible width of the feeder dike, 20–25
m. Insertion of a wider feeder results in a significant
effect on the shape of the calculated anomaly and di-
vergence between model and data.
Hlíðarfjall – profile HF1
The HF1 profile is 3137 meters long from SW-NE,
and measured across the subglacially formed rhyolite
dome Hlíðarfjall (Sæmundsson, 1991).
Hlíðarfjall is clearly a surface structure (Figures 2
and 4a), and is not buried by younger volcanic erup-
tives. This model fits very well to the data. Further-
more, Hlíðarfjall is likely to be a vent-formed struc-
ture (Figure 4a,b), i.e. formed by a dike or vent to the
surface. It is not possible to distinguish between mod-
els a and b for Hlíðarfjall. On the other hand, model c
shows that Hlíðarfjall does not have significant roots
(Figure 4c).
Hrafntinnuhryggur – profile HR2
The HR2 profile is 2264 meters long and is the mid-
dle profile, measured over Hrafntinnuhryggur from
West to East. Models a, b and c are constructed in
the same way as for Hlíðarfjall. Model a in Figure 5
shows Hrafntinnuhryggur and a hyaloclastite ridge to
its east. The model suggests that Hrafntinnuhryggur is
likely to be a surface formation like Hlíðarfjall (Figure
4), i.e. it is not buried by younger volcanic eruptives
(Figure 5), nor does it have gravitationally significant
roots (Figure 5c). Hrafntinnuhryggur is likely to be
emplaced as a vent-forming dome but the vent or any
underlying dike cannot be distinguished, neither by
model a nor model b.
It is difficult to determine the depth of the hyalo-
clastite ridge to the west of Hrafntinnuhryggur due to
the sparsity of gravity stations around it. Furthermore,
the density of the hyaloclastite ridge is quite low com-
pared to values from elsewhere (e.g. Gudmundsson
and Högnadóttir, 2004). The density value chosen
(1500 kg m−3) gives a plausible depth for the ridge,
a higher and more conventional value demands larger
depths of the ridge than the data set allows.
Hraunbunga - profile HB2
The HB2 profile is 2965 meters long and is measured
from NE to SW over the coulee Hraunbunga. The
simplest model is shown in Figure 6a. A slightly bet-
ter fit is obtained when density consistent with a silicic
scoria cone is used for the top of Hraunbunga (Figure
6b). The scoria was observed in the field and is consis-
tent with geological maps (e.g. Sæmundsson, 1991).
Hraunbunga does not have gravitationally signif-
icant roots (Figure 6a), nor is it buried by younger
volcanic eruptives (Figure 6c). Consequently, Hraun-
bunga is a surface formation and most likely a vent-
formed dome/coulee. It is not possible to distinguish
between model b (Figure 6b) and model c (Figure 6c),
but they demonstrate clearly that Hraunbunga could
have formed from a dike to the surface. Consequently,
the results from Hraunbunga suggest similar emplace-
ment mechanisms as for Hrafntinnuhryggur and Hlíð-
arfjall.
The density of the flanks of Hraunbunga is pos-
sibly underestimated by the Nettleton method. They
are dacite lavas, and the models (Figure 6) show devi-
ations from the observed anomaly over the shoulders
of Hraunbunga.
DISCUSSION
Density values
The densities determined by the Nettleton method
are essentially the same for Hlíðarfjall and Hrafn-
tinnuhryggur while there is considerable difference in
the values obtained from samples. Since both for-
mations are subglacially-formed rhyolitic domes and
should have similar density, we argue that the Net-
tleton method is more reliable than the mean of the
samples. It appears that sampling was somewhat bi-
ased towards the more coherent and less vesicular part
of the rocks at Hrafntinnuhryggur. Marginally higher
142 JÖKULL No. 60