Jökull - 01.01.2010, Page 146

Agustsdottir et al.
The main results from gravity models are the con-
straints placed on the basal structure of the studied
formations; Hlíðarfjall, Hraunbunga and Hrafntinnu-
hryggur. The models show that they are neither buried
by younger volcanic eruptives nor are roots with a
density contrast with the surroundings detected. The
formations studied were therefore emplaced as dike-
fed domes, i.e. by the extrusion from a dike onto
the surface or the ice-bedrock interface. This idea
is in agreement with Jónasson (1994, 2005, 2007)
who states that Icelandic rhyolites are rather liquid
and therefore erupt more quickly than a more vis-
cous magma. This rules out processes such as stop-
ing, where a magma body forces its way upwards as
a massive block, displacing and deforming the sur-
rounding country rock in the process. When reaching
the surface, such a forcefully emplaced body is likely
to leave rotated and disrupted country rock with the
rhyolites extending some distance into the bedrock.
Thus, the models, and as far as can be seen also the
surface geology, indicate emplacement through dike
intrusion to the surface followed by eruption. How-
ever, the high viscosity of the magma and confine-
ment in a glacier leads to the formation of a steep
dome (Hlíðarfjall and Hrafntinnuhryggur). No struc-
tural difference is seen between the domes formed
during the last glaciation and in the Holocene dome
Hraunbunga. We conclude therefore that the three
domes were formed in a smilar way. The domes have
been erupted through a narrow dike (on the order 5–
10 m) reaching to the surface. Maximum width of
a dike before it starts to register a gravity anomaly
of its own that can be distinguished from that of the
overlying dome is 20–25 m. Our results are in broad
agreement with that of Tuffen and Castro (2008) who
studied Hrafntinnuhryggur and found evidence of a
feeder dike at the surface with a thickness ranging
from 2 m to slightly more than 10 m. However, our
results suggest that the implied structure shown on
their Figure 13 (p. 365, Tuffen and Castro, 2008)
should be modified with a narrow dike and flat or
semi-flat dome-bedrock contact, as indicated in the
gravity model on Figure 5b.
CONCLUSIONS
• All the studied formations, Hlíðarfjall, Hraun-
bunga and Hrafntinnuhryggur, are neither
buried by younger volcanic eruptives nor were
any roots detected. The results are consistent
with a flat or semi-flat dome-bedrock contact.
• All dome formations studied are emplaced by
a dike to the surface. A dike width of 5–10 m
is likely and maximum possible dike width is
20–25 m.
• All the domes have low densities (1600–1800
kg m−3), reflecting both low grain-density and
high porosity.
• The domes display a significant density differ-
ence between the formations studied and the
surroundings.
Acknowledgements
We thank the University of Iceland Science Fund
(Rannsóknarsjóður Háskólans) for financing the work
of this paper. Furthermore we thank Arnar Már Vil-
hjálmsson and Hjalti Nönnuson for assistance in the
filed and Iceland Geosurvey (ÍSOR) for financing
most of the field work. Leó Kristjánsson, Kristján
Sæmundsson and an anonymous reviewer read the
manuscript critically and made valuable suggestions
for improvements.
Þyngdarmælingar á súrum gúlum á Kröflusvæðinu
Súrt berg á Íslandi tengist allajafnan megineldstöðv-
um og mynda gúla á eða í kringum öskjurima. Sumir
þessara gúla hafa myndast á jökulskeiðum en aðrir á
hlýskeiðum. Þyngdarmælingar voru gerðar á Kröflu-
svæðinu 2007 og 2008, til þess að ákvarða meðal-
eðlismassa þriggja súrra gúla. Eðlismassagögn eru
forsenda líkanagerðar sem eykur skilning á mynd-
un hraungúla og leynigúla. Snið voru mæld yfir
þrjá gúla. (1) Hlíðarfjall er líparítgúll, 310 m hár,
2 km langur og myndaður undir jökli fyrir um 90
þúsund árum. (2) Hrafntinnuhryggur er líparít- og
hrafntinnugúll, 80 m hár og 2,5 km langur mynd-
aður fyrir um 24 þúsund árum. (3) Hraunbunga er
146 JÖKULL No. 60