Jökull - 01.01.2013, Blaðsíða 1

Reviewed research article
The Stardalur magnetic anomaly, SW-Iceland: a review of
research in 1968–2012
Leó Kristjánsson
Institute of Earth Sciences – Science Institute, University of Iceland, Sturlugata 7, 101 Reykjavík, leo@hi.is
Abstract — Prominent geophysical anomalies of several km extent were noted in the vicinity of the farm
Stardalur in southwestern Iceland in a gravity survey published in 1954, as well as in a 1968 total-field aero-
magnetic survey published in 1970, and a seismic refraction study of the crust published in 1971. Further
low-altitude and ground magnetic surveys were conducted around Stardalur in 1969–1973, and an exploratory
hole was drilled to 200 m depth in 1969–1970 at the site of a very distinct peak in the ground anomaly. Various
studies were carried out on a core (to 143 m depth) and on cuttings recovered by the drilling. From 41 m depth
down, this material consisted of altered olivine tholeiite lava flows, with a mean remanent magnetization in-
tensity of the order of 15 times the average for Icelandic Tertiary lavas. Modelling of the anomaly peak, which
was found to reach at least 27 µT above the main geomagnetic field intensity of 52 µT, indicated that the lava
flows formed a body of dimensions about 200 by 600 m striking NE to NNE, inside a caldera structure. This
structure probably dates from a normal-polarity subchron around 2 m.y. ago during the Matuyama geomag-
netic chron. Further studies on samples from the Stardalur drill core revealed the presence of quite pure and
slightly cation-deficient magnetite, whose percentage in the lavas is more than twice the average for Tertiary
basalts in Iceland. The magnetite has largely been formed by exsolution from titanomagnetite, but it is also
present in small grains which have separated from olivine. A very minor proportion of the magnetite may be
of single-domain size, and it appears not to be a decisive factor in the bulk magnetic properties of the lavas.
For instance, the natural remanence is much less resistant to alternating-field demagnetization than could be
expected for single-domain grains. It is not certain whether it is a primary thermal remanence or of secondary
origin, although the former seems more plausible. In agreement with the conclusions of previous researchers,
it appears likely that the strong magnetization is due to a chance combination of circumstances (such as high
magnetite content, high oxidation state, and strong ambient field) rather than to some unique phenomenon. In
this paper, a new ground magnetic survey at Stardalur is presented, along with a simple model of the source
of the main anomaly peak. The geological reasons for the creation of that source remain unknown, but com-
parisons are made with a magnetic anomaly at Hvanneyri in western Iceland which has similarities with the
Stardalur anomaly.
GEOPHYSICAL AND GEOLOGICAL
RESEARCH RELATED TO THE
STARDALUR ANOMALY, TO 1973
Surface exposures in Iceland consist mostly of lavas
and other extrusives of basaltic composition. They re-
semble the Early Tertiary volcanics in the U.K. (west-
ern Scotland and northern Ireland), the Faeroes and
Greenland. On the basis of this resemblance and
other evidence, the oldest rocks in Iceland were for
decades considered to be of Eocene age and to be-
long to the same volcanic province as the other local-
ities. However, it was established by Moorbath et al.
(1968) that the age of exposures in Iceland does not
JÖKULL No. 63, 2013 1