Jökull - 01.01.2010, Blaðsíða 149

Reviewed research article
Paleomagnetic observations at three locations in the Pleistocene
lava sequences of southwest and south Iceland
Leó Kristjánsson
Institute of Earth Sciences, Science Institute, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland
leo@raunvis.hi.is
Abstract — Magnetic polarity measurements on lava samples were introduced in the 1950s as an aid in strati-
graphic research in Iceland, and applied by T. Einarsson especially on Pleistocene sequences in southwest
and south Iceland. However, the stratigraphy of these sequences is often complex, and Einarsson’s mapping
has only been followed up to a limited extent. The present detailed laboratory study on magnetic remanence
directions in lava flows is focused on three locations within the above region. In two of these locations (in
southwest Iceland), profiles spanning the boundary between the polarity zones named R2 and N2 by Einarsson
were sampled. The potential of using this boundary for correlation over short distances was confirmed. In a
third location (in south Iceland), three normal-polarity zones are present in a 500 m thick lava and sediment
sequence of dominantly reverse polarity. The stability and within-lava agreement of primary remanence vectors
is generally excellent. Remanence directions in successive lava units are in many cases very similar, indicat-
ing that the volcanism was episodic. In comparison to geomagnetic poles obtained in previous paleomagnetic
collections of older lava series in Iceland, the poles derived from these and other Pleistocene lavas are rarely
situated in low latitudes.
INTRODUCTION – PREVIOUS WORK
In Iceland, research on the age and stratigraphy of
lava sequences is fundamental to many other earth
science studies of local and regional interest. Of-
ten it is also relevant to global processes such as
climate changes. One of the methods employed in
stratigraphic research in Iceland since the early 1950s
(Hospers, 1953) is the measurement of the direc-
tion vectors of remanent magnetization (remanence)
in lava flows. The remanence acquired by the rock
on cooling, which has a direction more or less identi-
cal to that of the ambient geomagnetic field, may re-
main stable for tens of millions of years (m.y.). An
important aspect of this research concerns complete
reversals of the geomagnetic field which have taken
place at irregular intervals. In the first half of the 20th
century, these intervals were thought to be millions of
years in duration. Hospers (1953) introduced a useful
concept called a "virtual geomagnetic pole" (VGP).
This is the magnetic (south) pole corresponding to a
field direction observed somewhere on the Earth, on
the assumption that the geomagnetic field is due to a
dipole (resembling a short bar magnet) at its center.
The slow "secular variation" of the geomagnetic field
was thought of as a semi-regular wobbling of the VGP
around the geographic pole between abrupt reversals
of polarity.
Following the work of Hospers, application of the
paleomagnetic method in Iceland was taken up by
Einarsson and Sigurgeirsson (1955). Einarsson car-
ried out measurements of the polarity of basalt lava
flows and other bodies with a field compass, mostly in
hillside profiles in many parts of the country. Descrip-
tions of the strata, their magnetic polarity, and geolog-
JÖKULL No. 60 149