Jökull - 01.01.2013, Síða 12

L. Kristjánsson
Figure 5. a) Contours (in µT) of a positive total-field aeromagnetic anomaly near the Agricultural University at
Hvanneyri on the southeastern shore of the Borgarfjörður fjord. It was surveyed in 1970 by Þ. Sigurgeirsson
(Kristjánsson et al., 1989) on three flight lines indicated by arrows. The small box encloses the area of Fig-
ure 5b. b) Total-field intensity contours drawn from a ground magnetic survey on meadows northwest of the
University buildings in 2012. – a) Jafnstyrkslínur fyrir jákvætt segulfrávik innst á suðurströnd Borgarfjarðar.
Mælingar voru gerðar úr flugvél af Þorbirni Sigurgeirssyni 1970, á þrem línum sem sýndar eru með örvum.
Litli kassinn sýnir staðsetningu Myndar 5b. b) Jafnstyrkslínur fyrir segulsviðsmælingar í 2 m hæð yfir jörð,
gerðar 2012 á engjum við fjörðinn. Háskólabyggðin á Hvanneyri stendur um 20 m ofar, nálægt suðausturhorni
kortsins.
surements in 552 points on the flat lower ground in
this area, using the same instruments as in the recent
Stardalur ground survey. A contour map of the ma-
jor peak at ground level (Figure 5b) shows consid-
erable similarity with the Stardalur ground anomaly.
The field intensity F reaches up to 18.6 µT above the
current IGRF value (52.45 µT), as compared to the 29
µT maximum deviation at Stardalur. This anomaly is
very steep on its northwest side, suggesting that here
the source rocks reach within tens of m of the surface.
The results fully confirm the inference from the air-
borne survey regarding the trend of the anomaly con-
tours, which is 30–40◦ east in Fig. 5b as in Figure 5a.
A broad anomaly peak to the northeast (with fields up
to about 63 µT) has not been surveyed in detail and
is not shown. It makes the axis of the 56 µT contour
about 3 km long. Many hillocks with lava outcrops
lie north and northeast of the map of Figure 5b; four
normally magnetized samples taken from two of these
about 100 m north of the map however had a mean re-
manence intensity of 4 Am−1, which is equal to the
mean for Tertiary Icelandic lavas.
The tectonic map of Iceland by Jóhannesson and
Sæmundsson (2009) shows two calderas at the Hafn-
arfjall central volcano (Figure 1), one of which is only
partly exposed. Sigurgeirsson’s magnetic anomaly at
Hvanneyri appears to lie outside the Hafnarfjall vol-
cano.
DISCUSSION AND CONCLUSIONS
The discovery of the Stardalur magnetic anomaly
in 1968 occurred at the beginning of considerable
growth of research activity in the geosciences in Ice-
land. Among tasks undertaken through the 1970s was
the stratigraphic, structural and geochemical mapping
of several volcanic centers (e.g. Friðleifsson, 1973;
Jóhannesson, 1975). Observations at extinct centers
demonstrated various common traits in these, includ-
ing in particular the presence of magnetic anomalies
12 JÖKULL No. 63, 2013