Jökull - 01.01.2016, Blaðsíða 83

Reviewed research article
Extension of the Middle Miocene Kleifakot geomagnetic
instability event in Ísafjörður, Northwest Iceland
Leó Kristjánsson
Institute of Earth Sciences – Science Institute, University of Iceland, Sturlugata 7, 101 Reykjavík, leo@hi.is
Abstract — Detailed records of past geomagnetic polarity transitions and excursions are seldom found in
extrusive volcanics. One type of such events seems to involve very irregular variations of the magnetic field
direction. Several examples of that type have been discovered in paleomagnetic surveys on the Neogene lava
pile of Iceland. Lava sequences spanning the most notable event are accessible in the two tributary fjords
Mjóifjörður and Ísafjörður south of Ísafjarðardjúp, Northwest Iceland. Paleofield direction results from about
80 sampling sites in these sequences have been described in previous publications. The present paper adds 20
sampled sites in Ísafjörður, extending the area where parts of this „Kleifakot instability event“ of the geomag-
netic field are recorded, to 5–6 kilometers along the fjord. Intermediate paleofield directions in the collection
often agree closely with each other in correlated strata, even more than 4 kilometers apart. Events like this
can provide valuable correlation tools for future stratigraphic mapping and various studies on volcanological
features in the relatively uncharted region around Ísafjarðardjúp and beyond. Globally, recognition of the exis-
tence of such events will aid in the interpretation of results in other kinds of paleomagnetic studies. The role of
geomagnetic paleo-intensity determinations is discussed briefly in this context.
INTRODUCTION
Paleomagnetic research and some of its methods
The existence of the Earth’s magnetic field has fasci-
nated both scientists and the general public for cen-
turies, but many aspects of its sources, its long-term
history and its characteristics are still not well known.
The configuration of this vector field at the Earth’s
surface resembles a field that would be created by a
magnetic „dipole“ (i.e. a short bar magnet or a current
coil close to its center). Gradual changes observed
worldwide in directions and intensities of the field on
time scales of years to millennia, are termed its „sec-
ular variation“.
Paleomagnetism is the branch of geoscience
which involves research on the remanent magnetiza-
tion (remanence) vectors in geological formations. It
is primarily based on the fact that in many rocks the
remanence carries information about the direction of
the geomagnetic field prevailing at certain times in
their history. A convenient way of presenting and
comparing such directional results from different sites
makes use of the so-called virtual geomagnetic pole
(VGP) corresponding to the known paleomagnetic
field direction at a site. A VGP position is calculated
from a primary paleomagnetic direction (corrected for
tectonic tilt of the strata) and the site coordinates, as-
suming the geomagnetic field to be caused by only a
central dipole magnet pointing away from the position
of that VGP. In addition to the secular variation, the
field alternates at irregular intervals between opposite
polarities, termed „normal“ (N, with VGPs mostly in
high northern latitudes) and „reverse“ (R).
The natural remanence (NRM) in a rock sample
may consist of components of different ages, being
due to different processes, and having different stabil-
ity when subjected to heating or magnetic fields. Two
types of remanence are most common in Icelandic
lava flows and hence of interest to the present study.
They are thermal remanence (primary TRM) acquired
JÖKULL No. 66, 2016 83