Jökull - 01.01.2017, Page 49

Leó Kristjánsson and Jónsson
to as a fair indicator of the underlying Rock Mag-
netism. Roughly speaking, the strength of the mag-
netic field will be high (positive anomalies) over „nor-
mal“ bodies and low over „reverse“ bodies. Positive
anomalies tend to be larger than negative ones, due
to so-called Viscous and Induced Magnetizations in
the rocks which add to the positive side. The induced
magnetization is proportional to a property of the rock
called its magnetic susceptibility.
The magnetic structure of underlying formations
cannot be derived unambiguously from the results of
a magnetic survey; an infinity of different geological
formations will give identical anomaly patterns. For
example; a non-magnetic body emplaced inside nor-
mal magnetized lava field will show up in an aeromag-
netic survey in the same way as that body, if reversely
magnetized, would show up in non-magnetized sur-
roundings. Mapping of the magnetic field is, however,
most valuable in finding dominant geological trends
and discovering bodies which have magnetic proper-
ties different from those of the surroundings. Other
information, such as surface geology, gravity, ther-
mal properties and results of drilling will constrain the
range of possible structures.
ÞORBJÖRN SIGURGEIRSSON’S
SURVEYS
An airborne magnetic survey over a part of the
Reykjanes Ridge in 1963 (Heirtzler et al., 1966), re-
vealed a now famous pattern of alternating positive
and negative anomaly „stripes“ aligned with the axis
of the ridge. Subsequently, Þorbjörn Sigurgeirsson
measured the total magnetic field from a helicopter in
1965 on seven profiles, crossing the Reykjanes penin-
sula NW-SE at 600 m altitude and 5 km spacing. A
field-strength profile from one of these lines was pub-
lished by Sigurgeirsson (1967) who concluded that
the field deviations were due to some deep sources
rather than being correlated with the landscape. Ac-
cording to the available literature, all volcanic units
having surface outcrops in the peninsula (west of
Reykjavík) are thought to have been emplaced in the
Brunhes geomagnetic chron, i.e. less than 0.78 M.y.
ago.
In 1968 Sigurgeirsson initiated a 12-year project
of aeromagnetic surveys over Iceland. In this first
year he covered Southwest Iceland with lines oriented
around 60◦ west of north, spaced 4 km apart. The
flight altitude was 900–1200 m. Positioning relied
on observations of geographical features seen directly
underneath the plane. He published the measurements
as a profile map of area no. 3 of the Iceland Geodetic
Survey’s series in 1:250,000 scale (see Sigurgeirsson,
1970 a,b).
A positive anomaly turned out to be present along
the southern and central parts of the peninsula. Model
calculations by Kristjánsson (1972) indicated that the
normally magnetized crustal layer causing it could
be up to 2.5 km thick. In most of the profiles, the
anomaly had two main peaks separated by a promi-
nent dip. It was suggested that these dips which did
not coincide with active geothermal manifestations,
might be due to occurrences of non-magnetic tuffs or
reversely magnetized rocks.
FISSURE SWARMS AND VOLCANIC
SYSTEMS
Charting of the largely SW-NE running faults, fissures
and crater rows on the Reykjanes peninsula started
in the late 1920s. Results up to 1960, reviewed by
Sæmundsson (1983), indicated that these volcano-
tectonic phenomena were more concentrated in some
areas of the peninsula than others. The earliest map
available to us which explicitly delineates such group-
ings is in Fig. 1 of Tryggvason’s (1974) paper. Each
of his three „zones of most dense faults and fissures“
passes through an active high-temperature area. So
do zones outlined in Figs. 1a, 2 of Walker (1975)
and Fig. 1 of Klein et al. (1977). Jakobsson et al.
(1978, Fig. 2) presented a map of five „volcanic fis-
sure swarms“ named after Reykjanes, Grindavík, Krí-
suvík, Bláfjöll and Hengill respectively, see our Fig-
ure 1. Their paper was also the first (and possibly
so far the only one) to note that magnetic field highs
on Sigurgeirsson’s (1970b) contour map occurred not
far from some of the swarms. Although the accuracy
of the contours was limited by the long distances be-
tween flight lines, it became likely that a new interpre-
44 JÖKULL No. 67, 2017