Jökull - 01.01.2017, Side 48

Reviewed research article
A total-field magnetic anomaly map of the Reykjanes
peninsula, Southwest Iceland
Leó Kristjánsson and Geirfinnur Jónsson
Institute of Earth Sciences – Science Institute, University of Iceland, Sturlugata 7, 101 Reykjavík,
Correspondence to: leo@hi.is
Abstract — Aeromagnetic surveys of the Reykjanes peninsula in Southwest Iceland were carried out in 1965
and 1968. They revealed the presence of positive magnetic anomalies in the southern and central parts of the
peninsula, whereas a broad negative anomaly followed its northern coast. These variations were caused only
to a small extent by the local topography. A map of SW-NE trending fissure swarms in the peninsula, published
in 1978, indicated that they were correlated with the positive anomalies as well as with centers of recent
volcanism and high-temperature geothermal activity. Processing of the results from the 1968 survey combined
with further surveys in 1973, 1985–1986 and 1991–1992 has resulted in a new multicolor map of field residuals
in the Reykjanes peninsula and its surroundings. The most prominent feature in the map is lineations of positive
anomalies partially overlapping with the volcanic fissure swarms. Susceptibility measurements on drill cuttings
indicate that their sources may reach to more than 1.5 km depth.
INTRODUCTION
The Geomagnetic Field of the Earth is mostly caused
by slowly varying electric currents in the Earth’s liq-
uid core. For practical purposes, it can be replaced
with a bar magnet in the center of the Earth. This
hypothetical magnet tends to align with the rotational
axis, either parallel or antiparallel to it (at present tilt-
ing 11◦ from it). The geological history is divided
into periods of Normal and Reverse episodes – nor-
mal with respect to current field. The last complete
turnover of the field occurred 780 thousand years ago.
At any location, the geomagnetic field is a vector
quantity, which in Southwest Iceland points at about
76◦down from the horizontal and has a strength of
around 52 microTesla (µT).
In volcanic areas like Iceland, the measured mag-
netic field may vary considerably from place to place
because of the Rock Magnetic Field, which is due to
magnetic iron oxides such as magnetite in the nearby
rocks. It decreases rapidly with distance, small-scale
features faster than larger ones. The attenuation is
the same, whether these crustal fields pass through
air, water or sediments. At the normal flight altitude
of our onshore aeromagnetic surveys, this field may
reach 2 µT or more. Only rocks in the uppermost part
of the crust, at temperatures lower than the Curie tem-
perature of the magnetic minerals (578◦C for mag-
netite), contribute to the magnetic field.
The Rock Magnetism is a vector, usually either
parallel or antiparallel to the current Geomagnetic
Field, depending on whether the lava or dike cooled
down during a period of „normal“ or „reverse“ geo-
magnetic field. These bodies are said to be normally
or reversely magnetized.
In airborne and marine magnetic surveys, the field
is usually measured at short intervals, on a set of par-
allel lines. In our case, only the strength of the field
is measured, omitting its direction. Subtracting the
Geomagnetic Field from the measured field, and cor-
recting for temporal variations of ionospheric origin,
results in a „residual field“ which is often referred
JÖKULL No. 67, 2017 43