Jökull - 01.01.2013, Blaðsíða 7

The Stardalur magnetic anomaly, SW-Iceland
the magnitudes of its remanent and induced magne-
tization vectors. This so-called Königsberger ratio in
the Stardalur core, computed for the average proper-
ties given above in µT field, is about 23 as compared
to 4 in typical Tertiary lavas in Iceland. This may
indicate a significant contribution to the remanence
from single-domain material. However, when pieces
of the four low-MDF samples were heated to 610◦C
in air and cooled in a field of 50 µT, the Königsberger
ratios of their new artificial remanence averaged 14,
which is in the lower part of a range obtained for other
similarly reheated rock material from Iceland (Table
3 of Friðleifsson and Kristjánsson, 1972; Fig. 4 of
Kristjánsson, 1972).
Past intensity of the geomagnetic field
The advanced alteration state of the Stardalur lavas
as well as the chemical changes taking place in their
magnetic minerals upon laboratory heating have so
far precluded any detailed experiments to find the
paleo-intensity of the local geomagnetic field during
emplacement of the lava pile drilled through. Judg-
ing from worldwide research results (Tauxe, 2010,
Fig. 14.15; Goguitchaichvili et al., 1999), it is pos-
sible that this intensity could occasionally have been
at least 50% higher than the intensity of the present
field in Iceland. The present field in turn appears
to be roughly similar to that of the long-term aver-
age field during times when the geomagnetic dynamo
is in a stable state. It should also be kept in mind
that during buildup of that pile, the ambient field seen
by each new lava was considerably strengthened by
fields caused by the underlying lavas. Such locally
strong fields could be a partial explanation of the high
Königsberger ratios of the NRM.
DEVELOPMENTS TO 1990
Little research directly relevant to the Stardalur mag-
netic anomaly was carried out between 1973 and
1990. In that period however, magnetic surveys of
all of Iceland and parts of the surrounding shelf were
completed by Þ. Sigurgeirsson and others (see Krist-
jánsson et al., 1989; Jónsson et al., 1991). Sev-
eral localized anomalies were found, some of which
lie within the Quaternary areas (perhaps mostly con-
nected with occurrences of pillow lavas and other such
rapidly cooled material). Anomalies also commonly
occur at active or extinct volcanic complexes on shore,
as well as off central western and eastern Iceland.
These two categories of magnetic anomalies contain a
few examples which are comparable to the Stardalur
anomaly in both size and amplitude. In Figure 1, the
locations of three prominent magnetic anomalies of
the latter type onshore in western Iceland (Sigurgeirs-
son, 1979) are shown. A broad negative anomaly oc-
curs over the central volcano mt. Hafnarfjall (Ha). A
positive anomaly occurs at Hvanneyri (Hv, see below)
south of the Hvítá river, accompanied by a small neg-
ative anomaly to the north across that river. A positive
and a negative anomaly are also found over a caldera
in high terrain at Breiðakinnarsandur (B) within the
Reykjadalur central volcano (Jóhannesson, 1975; Jó-
hannesson and Sæmundsson, 2009).
No gravity data have been published from south-
western Iceland in recent decades, only a Bouguer
gravity map of Iceland in scale 1:1,000,000 (Þorbergs-
son et al., 1990). This map is based on low-pass fil-
tered data, so that it does not show details of 5 km size
or less. A gravity data base of Iceland at the National
Energy Authority only has two points in the general
vicinity of Stardalur (Þ. Högnadóttir, pers. comm.,
2012). To our knowledge, no attempt has been made
at a joint interpretation of the available geological and
geophysical results in the Stardalur area in terms of a
crustal structure.
The general conclusions from the Stardalur inves-
tigations and other knowledge up to 1990, indicated
that the causes of the very large amplitude of the main
magnetic anomaly peak did not involve any single
unique phenomenon. Rather, the anomaly might be
due to a chance combination of somewhat unusual cir-
cumstances:
a) Rapid buildup of a pile (200×600 m in extent, at
least 160 m high) of lavas of a uniform kind, having
relatively high iron content
b) This iron residing in magnetite to a greater extent
than is common in basalt lavas
c) The magnetite being pure (i.e. titanium-free) or
nearly so, both in small grains and where slightly
JÖKULL No. 63, 2013 7