Jökull - 01.01.2013, Síða 2

L. Kristjánsson
greatly exceed 15 million years (m.y.). Occurrences
of gabbro, andesitic and rhyolitic rocks are common
in Iceland; Walker (1959) showed that these as well as
dike swarms are associated with central-volcano com-
plexes of a similar kind as those in the U.K. The vol-
canic centers in Iceland are however smaller in size,
often 5–10 km across as compared to 15–20 km for
some Scottish centers.
Early geophysical studies
In the early 1950s, T. Einarsson (1954) carried out a
gravity survey of Iceland. Where his stations were
sufficiently dense, several localized anomalies were
noted, one being the Stardalur gravity anomaly near
a farm of that name 20 km northeast of Reykjavik.
In Einarsson’s maps this anomaly is assumed to be of
about 8 km size and +10 milligal (mgal) amplitude,
but its northern part could not be measured due to
mountainous terrain.
An aeromagnetic survey of total-field intensity at
900 m altitude above sea level (a.s.l.) over south-
western Iceland, made in 1968–1969 by Þ. Sigurgeirs-
son (1970a,b,c), revealed several elongated or roughly
circular positive or negative magnetic anomalies of
typical amplitudes 0.5–2 micro-Tesla (µT). However,
three prominent positive anomalies stood out in the
map, see Figure 1. One of these (at St in Figure 1)
coincided with the above-mentioned gravity anomaly.
As the flight-line spacing was 4 km, the dimensions of
the magnetic anomaly are not known with certainty.
They may be estimated from Sigurgeirsson’s maps as
being 8–10 km (Friðleifsson and Kristjánsson, 1972).
The Stardalur positive magnetic anomaly lies inside
a negative anomaly lineation of 20 km width and -
1.5 µT amplitude (relative to the local International
Geomagnetic Reference Field intensity F which was
51.75 µT in 1968; it has increased by about 0.65 µT
since then), striking ENE to NE. The amplitude of the
Stardalur anomaly at this survey altitude may reach
4.5 µT above the expected regional field value within
the negative lineation.
The two other prominent positive anomalies were
at mt. Skálafell in Hellisheiði (Sk in Figure 1) and
at Ferstikla (F in Figure 1). The former was later
surveyed in detail by Þ. Sigurgeirsson at 800 m alti-
tude, and a contour map of his results was published
by Pálmason (1987). The Skálafell anomaly which
is elongated in a direction 30◦ east of north, is as-
sociated with Late Quaternary volcanics erupted sub-
glacially (Sæmundsson et al., 2010). The Ferstikla
anomaly is clearly related to a volcanic center (Jóhan-
nesson and Sæmundsson, 2009; Kristinsson, 2009),
active 3.3–2.6 m.y. ago during the Gauss geomagnetic
chron. No detailed magnetic results are available on
this anomaly which has its peak within a 2–4 km wide
fjord. Apart from negative magnetic lineations run-
ning sub-parallel to the volcanic zones, the main neg-
ative anomalies in Sigurgeirsson’s (1970a) map occur
just east of Reykjavík (R in Figure 1).
Sigurgeirsson’s discovery of the Stardalur ano-
maly sparked considerable additional research at that
locality. He carried out a total-field aeromagnetic sur-
vey in 1969 on six short lines at altitudes of 300–350
m a.s.l., intersecting over the Stardalur farm buildings
at 64◦12.7’N, 21◦29.0’W, alt. 190 m a.s.l. Judging
from a contour map of his results (Kristjánsson, 1987)
the maximum intensity exceeded 59 µT. Field mea-
surements made on the ground in the Stardalur area in
1969 indicated that the magnetic anomaly was com-
posed of two parts: a wide anomaly of mean dimen-
sions 7 km and amplitude 5 µT, and a few peaks su-
perimposed on the wider anomaly. In the most promi-
nent of these peaks, field values of up to 79 µT were
found. From the ground survey data, Kristjánsson
(1970) provisionally estimated that the cause of this
peak could be a single body of 200×600 m size strik-
ing NE, with an upper surface at a depth of 50–70
m, and a total magnetization intensity of 50–60 Am-
peres per meter (Am−1). Analysis of more detailed
ground-survey data (Búason, 1971) yielded a magne-
tization estimate of 80 Am−1 and slightly different di-
mensions.
General information on rock magnetism in Iceland
Magnetization (i.e., dipole moment per unit volume)
of a material is a sum of two vector quantities. One
of these vectors is induced magnetization Mi, which
is proportional to both the ambient field F at any time
and a dimensionless material quantity called the vol-
ume magnetic susceptibility. Mi = F·χ/µo where the
constant µo is 4π×10−7 TmA−1 in SI units. In Ter-
2 JÖKULL No. 63, 2013