Jökull - 01.01.2017, Side 50

Magnetic anomaly map of the Reykjanes peninsula, SW Iceland
Figure 1. Outlines of volcanic fissure swarms in the Reykjanes peninsula, from Fig. 2 of Jakobsson et al. (1978);
Reykjanes (R), Grindavík (G), Krísuvík (K), Bláfjöll (B), and Hengill (H). Among places mentioned in the text
are Kollafjörður (Ko), Stardalur anomaly (St), Reykjavík (Re), Skálafell (Sk), and Ingólfsfjall (I). – Útlínur
eldvirkra sprungusveima á Reykjanesskaga samkvæmt Jakobsson o.fl. (1978), ásamt staðsetningu nokkurra
örnefna sem minnst er á í greininni.
tation of the magnetic anomalies was needed in view
of these observations. Thus, the two positive peaks on
most of Sigurgeirsson’s flight lines could be caused
by the flights crossing over two sub-parallel swarms
of fissures, each swarm harboring an extended mag-
netic source.
Several different attempts at grouping the fissure
swarms and associated volcanic systems have been
presented since the 1970s; reviewing these efforts is
beyond the scope of the present note. Among the
maps published most recently is one by Sæmundsson
and Sigurgeirsson (2013).
POSSIBLE CAUSES OF THE MAGNETIC
ANOMALIES
Magnetic susceptibility measurements on rock cut-
tings from deep drill holes in the Reykjanes peninsula
and elsewhere (e.g. Kristjánsson and Watkins, 1977;
Pálmason et al., 1979) showed in most cases a fairly
constant susceptibility of the crystalline basalts pene-
trated, i.e. reflecting a uniform magnetite content. In
some cases an increase in this parameter was seen be-
low about 1.5 km depth, in lavas and intrusions. That
observation was in contrast to views at the time on the
thickness of the magnetic crustal layer at ocean ridges,
commonly assumed to be 0.5 km or even less.
One possibility coming to mind for the type of
magnetic sources associated with the volcanic sys-
tems, would be that they represent clusters of dike
intrusions. Dike swarms emanating from extinct vol-
canic centers are indeed well documented, especially
from studies in East Iceland (Walker, 1975). The em-
placement of dikes has also been monitored during
some eruption events in recent decades (e.g. Buck et
al., 2006). The average intensity of magnetization in
Icelandic dikes which we have sampled is not much
greater than that in lava flows (Kristjánsson and Jóns-
son, 2007, Table 4, and unpublished data by L.K.).
However, they would generate positive anomalies if
intruded here either into tuffs or into subsided se-
quences of reversely magnetized lava flows erupted
during the Matuyama geomagnetic chron (0.78–2.6
M.y. ago). If such reverse lavas had low Curie points
and/or were at elevated temperatures when the intru-
sions were emplaced, these intrusions would also be
likely to cause significant remagnetization of the lavas
in their vicinity (cf. Kristjánsson, 1985; Hall, 1985).
Findings from the 1.9 km drill core recovered in
1978 in the lava pile of East Iceland point to another
possibility of deep magnetic sources related to high-
temperature geothermal areas. In the core, a new
component of magnetization was observed due to the
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