Jökull - 01.01.2010, Blaðsíða 150

L. Kristjánsson
ical interpretation appeared in several of his publica-
tions, e.g., Einarsson (1957, 1962). Einarsson found
that each series (zone) of lavas having "normal" or
"reverse" polarity often consisted of tens of lava flows.
He attempted to set up a scheme for the consecutive
numbering of such polarity zones, especially in the
strata emplaced during the cold climates of the last 3
m.y. Thus, zone N1 corresponded to the present mag-
netic epoch, R1 contained lavas from the last reverse
epoch, and so on. Each of these zones varied in thick-
ness between locations, but N2 and R2 were of the
order of 150 m and 300 m thick respectively, N3 was
only some 50 m, and R3 was 500 m.
Detailed work on worldwide volcanic sites dated
by the potassium-argon (K-Ar) method in the 1960s,
augmented by interpretation of magnetic anomaly lin-
eations parallel to ocean ridges, revealed that on av-
erage at least 3–4 reversals had occurred every mil-
lion years in the Cenozoic era. Several versions of a
"geomagnetic polarity time scale" for these reversals
have been published, with each revision incorporat-
ing additional reversal events named subchrons and
cryptochrons (Ogg and Smith, 2004). It has been con-
cluded from research on sedimentary and volcanic se-
quences that at least ten such short events may have
occurred in the current normal-polarity epoch (now
known as the Brunhes chron) which began at 0.78
Ma, and the situation may be analogous for previous
chrons. The secular variation has also been found to
include many "excursions" of the VGP’s position to
low latitudes.
Due to this increasing complexity in our knowl-
edge of the geomagnetic field behaviour, as well as
to the scarcity of radiometric age determinations so
far obtained from Icelandic lavas, any correspondence
between Einarsson’s polarity zones and the geomag-
netic polarity time scale is still uncertain. Einarsson
(1962, p. 70) was also aware that one or more polarity
zones might be missing in some of his profiles due to
eruption rate variations, hiatuses or erosion. In some
locations, the N2 might be tentatively identified with
the Jaramillo normal-polarity subchron at 0.99–1.07
Ma, i.e., late in the Matuyama reverse magnetic chron
which is supposed to span the interval 0.78–2.58 Ma
ago (Ogg and Smith, 2004). However, in other places
N2 could contain volcanics from an older long sub-
chron in the Matuyama, namely the Olduvai at 1.78–
1.95 Ma. In southwest Iceland, this correlation finds
some support in the K-Ar dating of a presumed N2
unit (Kristjánsson et al., 1980) which yielded an age
of about 1.9 Ma (when recalculated with the decay
constants used by Ogg and Smith). N3 may accord-
ingly be the Reunion subchron which lasted between
2.13–2.15 Ma ago.
In addition to collaborating with Einarsson on po-
larity measurements in lava profiles, Sigurgeirsson
(1957) designed a laboratory instrument for the ac-
curate measurement of the primary remanence direc-
tions in hand samples. Sigurgeirsson carried out a
number of such determinations, mostly on lavas near
the boundaries between polarity zones in Einarsson’s
profiles in southwest Iceland. The best known of Sig-
urgeirsson’s directional measurements are those at the
R3-N3 boundary in various locations south of Hval-
fjörður (H of Figure 1), resampled by Kristjánsson
et al. (1980). Kristjánsson and Sigurgeirsson (1993),
and others. This established that virtual poles formed
an irregular path passing through low latitudes during
the reversals, rather than the dipole simply decreasing
to zero and then growing in the opposite sense while
staying in near-axial orientation. It has also become
clear from Icelandic data that the field intensity is re-
duced during the reversal process, with the dipole field
becoming weaker than non-dipole field components
when the VGP latitude is less than 25◦ or so (Figure 9
of Kristjánsson, 2008).
In 1964 a new stage began in paleomagnetic re-
search on Icelandic lava sequences. That summer, a
large collection of oriented drill cores was acquired
by Doell (1972), mostly in Tjörnes (T of Figure 1)
and the Snæfellsnes peninsula (S of Figure 1). An-
other collection effort was carried out in 1964–1965 in
southwest and east Iceland by a U.K. – Icelandic ex-
pedition. In southwest Iceland the expedition sampled
ten profiles of Pliocene and Pleistocene age, mapped
by T. Einarsson around the Hvalfjörður fjord. The
published account of this study (Wilson et al., 1972)
was somewhat incomplete. Firstly, the altitudes of
lava boundaries mapped by Einarsson were shown in
diagrams, but very little information on the lithology
150 JÖKULL No. 60