Jökull - 01.07.2003, Side 23

Reviewed research article
Paleomagnetic observations on Late Quaternary basalts around
Reykjavík and on the Reykjanes peninsula, SW-Iceland
Leó Kristjánsson
Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland; leo@raunvis.hi.is
Abstract – This paper reports paleomagnetic remanence directions from Late Quaternary lava units in the
Reykjavík area and on the Reykjanes peninsula. In Reykjavík, samples were collected at about 30 sites in the
interglacial “gray basalts” in the hope of finding unusual directions which might help in stratigraphic mapping
in this region as well as documenting the behaviour of the geomagnetic field. The directions in these lavas are
tightly grouped which indicates that the lavas were emplaced in a short time interval (<50 kyr?). One instance
was found of a geomagnetic excursion, which probably occurred early in this volcanic phase. On Reykjanes,
emphasis was on sampling lavas which might exhibit directions similar to the “Skálamælifell geomagnetic
excursion”. Lava sequences erupted during this excursion, of about 43 kyr age, were previously found to cover
a few isolated areas in the region northeast of Grindavík. Their presence has already been of value in studies of
local geomorphologic activity. The current work has since 1990 resulted in minor modifications in a map of the
main areas covered by the excursion sequences, and one new area was found. Several lava flows erupted during
an excursion of the geomagnetic field (possibly the Skálamælifell one) were sampled in mt. Fagradalsfjall.
GENERAL INTRODUCTION
The geomagnetic field undergoes slow changes called
the “secular variation”. As a result, the field direc-
tion at any location such as Iceland, moves irregularly
around the direction corresponding to that of the mean
field. The angular standard deviation (a.s.d.) of this
movement in Iceland was of the order of 20–24 Æ in the
last several million years (see e.g. Kristjánsson et al.,
1980; Kristjánsson and McDougall, 1982). Large de-
viations (excursions) also occur, as well as complete
reversals of the field.
Until about 1960, reversals were thought to take
place once per million years or so, and few in-
stances were noted of other major field deviations. In
1963–1964 short-lived reversal “events” were first de-
scribed, and in 1967 evidence from lava flows for the
presence of such an event in the Uppermost Quater-
nary was published. This was the Laschamp event
discovered in the south of France, now estimated to
have occurred about 40 kyr ago (see Levi et al., 1990).
Kristjánsson and McDougall (1982) deduced from
work on Icelandic lavas that on average the main geo-
magnetic field reverses at least eight times per million
years if short events (subchrons) are included. Few
of these short reversal events have been incorporated
into published geomagnetic polarity time scales.
In the last 20 years, reports on short-lived Late
Quaternary geomagnetic reversals and major excur-
sions of the field have proliferated (e.g., Lund et al.,
1998; Guyodo and Valet, 1999). Most are based on
evidence from wet marine and lake sediments. Ex-
perience has shown that such observations need inde-
pendent confirmation, preferably from igneous rocks,
before they can be taken seriously.
Outside Iceland, the average direction of the ge-
omagnetic field in rock formations has been widely
used in tectonic studies, on the assumption that it co-
incides with the direction of a central axial dipole
field. An important question in that regard concerns
the minimum number of independent field determina-
JÖKULL No. 52, 2003 21