Reviewed research article
Paleomagnetic research on Icelandic lava ows
Leó Kristjánsson
Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík
leo@raunvis.hi.is
Abstract — Following a short review of the main efforts in paleomagnetic eld work on basement rocks in
Iceland since 1950 and a description of their main magnetic characteristics, some examples of paleomagnetic
research on these rocks are described. This research has had two major aims. One has concerned the stratigraphy of the lava pile, where reversals of the remanence polarity are useful in correlating between mapped
proles of similar age some kilometers apart. The other aim has been to obtain information on various aspects
of the geomagnetic eld conguration and its changes through the last 15 M.y. Additionally, knowledge of the
magnetic properties of basement rocks is essential in the geological interpretation of magnetic anomalies.
INTRODUCTION – HISTORICAL
The area of research known as “Paleomagnetism”
originated as the study of the Earth’s past magnetic
eld, by means of measurements on stable components of remanent magnetization in rocks. For general properties of the geomagnetic eld, the reader is
referred to books such as Lanza and Meloni (2006).
The subject of paleomagnetism has also come to include other applications of the stable magnetic remanence directions e.g. in age estimates, regional tectonics, or rock deformation. Therefore, research which is
concerned solely with actual aspects of geomagnetic
elds has in recent years been referred to by the more
appropriate name “Paleo-geomagnetism”. A complementary research area called “Rock magnetism” embraces the magnetic properties of rocks and minerals
in general, including the mechanism of remanence acquisition.
Although remanence measurements on samples of
igneous rocks from Iceland (at 64–66◦N) were initiated before 1930, their potential for paleomagnetic
purposes was rst appreciated through the work of J.
Hospers in the early 1950’s (e.g. Hospers, 1953). A
few years later, Sigurgeirsson (1957) and others improved the techniques of Hospers and demonstrated
additional applications of lava remanence directions,
both in local geological research and in a global context. Subsequently, a U.K.-Icelandic expedition in
1964–65 carried out major sampling in East Iceland
(Dagley et al., 1967; Watkins and Walker, 1977).
Their collection included over 850 distinct lava ows
and additionally about 200 thin ow units within those
lavas that were classied as compound ows. The
samples were used for a variety of paleomagnetic,
rock-magnetic, and dating studies.
Among the accomplishments of the above pioneers we may mention:
• Early applications of some key concepts, e.g.
“virtual geomagnetic pole” (VGP) and “Fisher
statistics” (Fisher, 1953).
• Development of the alternating-eld (AF) technique for eliminating secondary viscous remanent magnetization (VRM)
• Conrmation of the occurrence of geomagnetic
reversals at variable intervals through the period
2–13 M.y. ago, at a rate of at least 5–6 per M.y.
• Application of polarity reversals in stratigraphy
across 10 km or more (Figure 1)
• Demonstration of the long-term dependence of
the (virtual) geomagnetic dipole moment upon
VGP latitude
JÖKULL No. 58, 2008
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L. Kristjánsson
Research on Icelandic rocks in the 1960’s also
contributed to an improved understanding of the
physico-chemical factors inuencing remanence stability (see Ade-Hall et al., 1971, and references
therein), and helped in establishing the reality of short
polarity-reversal events (Wensink, 1964).
From the work in the 1950’s–1960’s it became
evident that basalts in Iceland are characterized by
highly stable primary remanence directions and constitute some of the best material available anywhere
for paleo-geomagnetic research. This statement however, applies without qualication only to relatively
fresh lavas, i.e., those that have not suffered secondary
regional hydrothermal alteration to a degree characterized by the presence of the zeolites mesolite and
scolecite in the lava pile. It may indicate a maximum
temperature of around 150◦C although both higher
and lower temperature estimates have appeared in the
literature. Only limited studies have been carried out
on more altered rocks (in deeply eroded localities, in
drillholes, and in the vicinity of extinct or active central volcanoes). They indicate that in lava ows heated
beyond some 200◦ C (characterized by laumontite) serious decay of the original remanence may have taken
place (Watkins and Walker, 1977). In lavas suffering more than 250–300◦C reheating (epidote zone),
secondary magnetite has appeared. Presumably, that
magnetite has acquired a thermal remanence while
buried within a large body cooling slowly from the
maximum temperature reached, the geomagnetic eld
even reversing one or more times during that time.
These observations have a bearing on the signicance
of some paleomagnetic directions reported from igneous rocks in much older localities abroad, where
Figure 1. The south-facing side of mt. Kistufell, part of the Esja central volcano complex near Reykjavík, rises
from an altitude of 100 m to 750 m. Above a hyaloclastite that reaches to about 300 m, 65 lava ows are found.
They presumably date from the Matuyama geomagnetic chron (2.6–0.8 M.y. ago) and are reversely magnetized
except for 13 ows at around 500 m altitude. This thin “N3” polarity zone (Sigurgeirsson, 1957; Kristjánsson
et al., 1980; Goguitchaichvili et al., 1999) can be followed in outcrops for at least 20 km to the northeast and
is also found 4–5 km to the south-southeast. – Suðurhlið Kistufells í Esju rís úr 100 m í 750 m hæð, og er að
mestu samsett úr hraunlögum ofan við um 300 m. Sú syrpa 65 laga er með “öfuga” segulstefnu fyrir utan 13
lög um miðbikið sem má rekja bæði inn eftir Hvalrði og til suðurs.
102 JÖKULL No. 58, 2008
Paleomagnetic research on Icelandic lava ows
the rocks have been subjected to even more severe
thermal and chemical inuences as well as mechanical deformation.
Paleomagnetic eld work in Iceland in 1972–78,
especially a large collection of lava samples acquired
by N.D. Watkins and collaborators, aided progress
in paleomagnetism in various ways. Their work included radiometric dating of some of the geomagnetic reversals occurring in the last several M.y. (McDougall et al., 1977), and statistical processing of
remanence vectors from over 2100 stable lava ows
(Kristjánsson and McDougall, 1982).
ROCK TYPES PRESENT IN ICELAND,
AND THEIR GENERAL MAGNETIC
PROPERTIES
Volcanism in Iceland (as exposed now above sea
level) has been essentially continuous for the past 15
M.y., concurrent with steady east-west spreading of
the crust away from active zones. Those rock formations which have not been erupted under Pleistocene
glaciers or lakes consist mostly of basalt lava ows
with thin intervening sediments. It is thought that the
lavas were generated in eruptions which chiey took
place in so-called central-volcano complexes. Each
such complex may have had a lifetime of some 0.5
M.y. Their main features include a caldera structure
of several km size accompanied by intrusions and a
zone of relatively intense hydrothermal alteration, as
well as a dike swarm of tens of km in length. The
volcanic activity built up a plateau which was subsequently eroded and dissected by fjords, creating excellent exposures of the lava pile. Due to a continuous
process of subsidence of the areas around the volcanic
zones, the lava sequences tend to tilt towards these by
a few to several degrees, but that situation is complicated by eastwards jumps of these zones which appear
to have taken place a few times in the past 15 M.y.
The dominant magnetic mineral in Icelandic
basalts is titanomagnetite in various stages of oxidation. The titanium content is reected in the Curie
point (Tc ) which in fresh rapidly cooled material such
as pillow basalt may even be below 100◦ C. Most subaerial lava ows exhibit nal Curie points exceeding
500◦C when heated in air, but the thermomagnetic
curves have a very varied appearance. In many of
these cases, a homogeneous component with a Curie
point of the order of 300◦ C or less is apparently being converted to a magnetite-ilmenite intergrowth as
the heating proceeds. Lava samples that have been
oxidized during their initial cooling, often have Curie
points (or at least tails on the thermomagnetic curves)
exceeding the 578◦ C value for pure magnetite, which
points to the presence of a stable maghemite. In those
having a reddish color, as well as in red interbasaltic
sediments, Tc -values may reach 620◦ C. The thermomagnetic curves are irreversible in most cases, cf.
typical examples illustrated in Goguitchaichvili et al.
(1999) and Kristjánsson et al. (2003). An increase
of susceptibility upon return to room temperature after heating to 300◦ C or more often occurs in connection with the conversion of a low-Tc component;
the author has even observed a consistent increase
(averaging 5%) in this property after heating 23 stable lava samples from Northwest Iceland to 160◦ C
(L. Kristjánsson, unpublished data 1968, 2007). A
decrease in susceptibility of a specimen with a high
Curie point after heating, which is also often noted,
is probably associated with the formation of minerals
like hematite and pseudobrookite. No clear correlation between magnetic properties and silicate lithology has been noted. Typical values for the initial
susceptibilities of lava samples are of the order of
0.025 SI units, indicating roughly a titanomagnetite +
maghemite content of 1% by volume. Overall remanence intensities in large collections of Tertiary lavas
after 10 mT AF treatment are around 3–4 A/m. Lavas
with a weak and unstable remanence tend to have high
room-temperature magnetic susceptibility and somewhat complex thermomagnetic curves.
SAMPLING AND MEASUREMENT
PROCEDURES, WITH EMPHASIS ON
LAVA FLOWS
Since 1980 paleomagnetic eld work has been carried
out every summer, in various regions of Iceland. Usually, local geologists have been involved in the initial
stages of these projects. Their task has consisted in
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103
L. Kristjánsson
choosing an interesting and favorable area, surveying
it to nd proles that are suitable for sampling, mapping these proles in detail, establishing correlations
between them, and numbering the lava ows. The
sampling has generally included 4 core samples of
4–8 cm length per ow: an extra core is sometimes
added e.g. in outcrops that are crumbly or are suspected of having been disturbed in some way.
The standard procedure of measurement at the
Science Institute, University of Iceland employs an
“Institut Dr. Förster” four-probe static uxgate meter, calibrated at intervals with a 19-turn coil of the
same shape as the standard core specimens. It has
been checked against other instruments and found to
give very satisfactory results. AF demagnetization is
done with a 2-axis tumbler: a bulky system of current
coils was replaced by a compact “Molspin” instrument in 1989. In the majority of the lava samples, a
normal-polarity viscous magnetization has been completely eliminated at 10 mT AF treatment. In order
to ascertain this, measurements are also made after
15 and 20 mT. In the infrequent cases when the directional change between these two is 2◦ or more (including alignment errors in replacing a specimen in its
holder), the treatment is repeated at 20 mT and the results averaged. If the remanence direction appears to
be changing systematically, the treatment is extended
as necessary. Very few samples are rejected due to
e.g. lightning strikes, suspected baking, or gross orientation errors. In some surveys there have been opportunities to resample problematic lavas, usually resulting in improved condence parameters of mean
remanence directions. Rotational remanence (RRM)
which apparently may be acquired by some types of
igneous rocks during AF treatment at high eld intensities, is hardly ever noticed in Icelandic lavas. A
problem which is fairly common but easily dealt with,
is the rapid acquisition of new VRM while specimens
are being transferred from the demagnetizing equipment to the measuring device.
The high stability of primary remanence directions in Icelandic basalts, evident from the above description, is also reected in excellent agreement of
directions within sampling sites after simple AF treatment. Typical values for the 95%-condence radii of
104 JÖKULL No. 58, 2008
average directions from four samples spread over a
few to several meters, are 5◦ or less after demagnetization at 20 mT peak eld. The median destructive
elds (MDF) at which the treatment has reduced the
remanence intensity to half its initial value, lie commonly in the range 10 to 40 mT, as shown by examples
in several papers including Kristjánsson (2002) and
Kristjánsson et al. (2003). However, directions even
from samples where the MDF may be of the order of
5 mT or less, usually agree with each other or with
more stable samples from the same ow after careful AF treatment. Tests (Kristjánsson and Auðunsson,
2007; Kristjánsson and Jónsson, 2007) on widely distributed sampling sites in several lava ows or owunit series have conrmed that the sites yield uniform
directions (within a few degrees) even when spread
over distances of 1 km or more. All these successful demonstrations of consistency have enabled the
acquisition of reliable directions from a large number of lavas, without need for spending time on excessive sampling, extended demagnetization treatments,
or sophisticated statistical processing. The circumstances have also resulted in an emphasis on magnetic
studies of lava ows; much less information has been
gathered on the paleomagnetism of Icelandic dikes
and other intrusions, subglacial extrusives, or sediments, where consistency checks cannot be made as
thoroughly as in lava ows.
STRATIGRAPHIC APPLICATIONS OF
PALEOMAGNETISM IN ICELAND
Stratigraphic mapping of the lava pile and central
volcanoes in East and Southeast Iceland was initiated by G.P.L. Walker in the mid-1950’s (e.g. Walker,
1959). For correlation he employed occasional extensive clastic sediments as well as fairly distinct groups
of feldspar-porphyritic lavas and of olivine tholeiites
which are found within the predominantly tholeiitic
lava pile. The mapping efforts of Walker, his students and other successors have formed a very useful foundation for paleomagnetic studies in Iceland.
These studies have concentrated on the regularly layered Tertiary to Early Pleistocene lava pile where a
large number of well-exposed mountainside or stream
Paleomagnetic research on Icelandic lava ows
proles are accessible. Much less has been done in
the late Pleistocene to Recent active volcanic zones
(cf. the section “Conclusions and Discussion” below),
although paleomagnetic work has e.g. been demonstrated to provide quite useful results on the extent
of certain contemporaneous sequences in these areas
(Levi et al., 1990).
A unique aspect of the lava sequences in Iceland is
that they cover in a fairly continuous fashion the time
interval from 0 to 15 M.y. ago, during which the geomagnetic eld reversed its polarity at least 120 times
(Kristjánsson and McDougall, 1982) including shortlived events. Along with the easy accessibility, stability and reliability of the lavas as already mentioned,
this allows the charting of some average properties of
the geomagnetic eld in the above period.
Due to the unknown and variable but usually
rather long time interval (thousands of years, cf. below) inferred to have passed between eruption of any
two successive lavas in the Icelandic lava pile, details such as paths of the VGP during individual geomagnetic polarity transitions and excursions are very
rarely recorded.
It should be noted here that the average thickness
of a lava ow in the older parts of Iceland is of the
order of 10 m. The duration between two successive
ows appears to vary widely, with an overall mean of
the order of 5–10000 years. That estimate does not
include the above-mentioned ow units, which (judging e.g. from the similarity of their remanence directions) may be separated in time by only years to centuries. Several sampling proles, usually composed
of 20–80 lavas each, some km distant from each other
in the down-dip direction, and partly overlapping in
age, may be connected together into a composite section. In the last few decades, several sections were
studied in this way (Figure 2). Each such collection
usually contains over 300 lava ows (including prole overlaps), covering an interval of millions of years
(e.g. Figure 3). The number of lava ows in a zone
of uniform polarity within such a stratigraphic column can vary between 1 and 70, but an average value
is 15–20 lava ows. Paleomagnetic polarity results
on the lava ows have therefore often been helpful
for geologists in their stratigraphic work in Iceland.
The polarity of the natural remanence (NRM) in hand
samples can generally be measured in the eld with
a portable uxgate magnetometer. However, caution
is needed in such measurements and their interpretation because the total magnetization of a hand sample
in this situation may include signicant induced and
viscous (VRM) components in addition to its primary
thermal (TRM) or thermo-chemical remanence.
SOME PROPERTIES OF THE
PALEO-GEOMAGNETIC FIELD IN
ICELAND
A number of long-term properties of the paleomagnetic eld can be studied when large collections of
data from basalt lava sequences spanning many geomagnetic reversals are available. The results from
such studies in a relatively small area like Iceland can
provide important constraints on models of the general conguration of the eld and theories of its generation in the Earth’s core. A few simple studies of
this type will be addressed in separate sub-sections
below. Other properties which have long been discussed among paleomagnetists and which have been
treated in recent papers on Icelandic lava collections,
include:
• Normal/reverse asymmetry as regards remanence intensity, chron lengths, secular variation
parameters and so on (see Kristjánsson, 1999,
2002)
• The frequency distribution of virtual geomagnetic poles as a function of geographic latitude, and the relative frequency of occurrence
of excursions and transitions (see Kristjánsson,
1999; Kristjánsson and Jónsson, 2007). These
distributions tend to have a long tail towards the
Equator, compared to e.g. a Fisher’s (1953) distribution peaking at similar VGP latitude.
• The overall frequency of reversals and major
eld excursions (Kristjánsson and McDougall,
1982). Progress regarding estimates of this kind
has been hampered by the fact that few reliable
radiometric dates on Icelandic Tertiary lavas
have been published for the last two decades.
JÖKULL No. 58, 2008
105
L. Kristjánsson
It would be valuable to have access to another set
of lava ows of comparable age range, size and reliability from a site at a different latitude, but such
sets do not seem to be existing. Possibly, some of
the above-mentioned results from Iceland can be compared to carefully compiled results from many small
surveys on magnetically stable lavas around the globe.
However, statistical results from the latter as presented in publications, have often been unnecessarily
modied by procedures such as arbitrary rejection of
low- and mid-latitude VGPs.
Figure 2. Sampling of lava ows in Iceland, 1964–2007. Circles refer to surveys where two samples were collected per ow (mostly in 1964–65), triangles three (mostly in 1972–78) and squares four (from 1979). Each
symbol stands for 60–100 lava ows. Crosses indicate smaller isolated studies on 35–60 lavas each. The approximate positions of surface exposures of 0.8, 3.3 and 10 M.y. age are shown. Some sampling expeditions
from which direction results have not been published in detail, are not included. The map is revised and simplied from Kristjánsson and Jónsson (2007) where the relevant publications are also listed apart from the recent
ones by Kristjánsson et al. (2006, two valleys in N-Iceland) and Kristjánsson (in preparation 2008, southwest
part of the Northwestern peninsula). – Yrlitskort af mestallri sýnasöfnun til bergsegulmælinga á Íslandi frá
1964, ef niðurstöður um segulstefnur hraunanna hafa birst eða verða það jótlega. Yrleitt hafa verið tekin
fyrir fáein hundruð lög á hverju svæði. Hvert tákn á við 60–100 hraunlög nema krossar, sem sýna smærri
verkefni.
106 JÖKULL No. 58, 2008
Paleomagnetic research on Icelandic lava ows
Figure 3. Left: The positions of sampling proles in one typical local survey of paleomagnetic directions in the
lava pile of Iceland. This survey encompassed two valleys in the Skagafjörður district of North Iceland. As the
tectonic tilt here is generally to the south, ages are expected to decrease southwards, from some 9 M.y. at the
base of TD to around 5 M.y. at the top of PO. Center: Altitudes of the proles, and some suggested correlations
between them. Right: A tentative polarity column in the composite section derived from the proles (180 lavas,
after elimination of prole overlaps). Black: normal polarity. Dotted: sedimentary beds. Ages are somewhat
uncertain, as radiometric dates are only available from locations west of TD and east of PO, and unconformities
have been noted within the area. Simplied from Kristjánsson et al. (2006). – T.v.: Kort af sýnasöfnun í Skagafjarðardölum, í nýlegri dæmigerðri jarðlaga-kortlagningu hraunastaa. Í miðið: Hæðir sniðanna sem safnað
var úr, og nokkrar tengingar milli þeirra. T.h.: Segulstefnurnar í samsettu sniði, að skörun slepptri.
Mean geomagnetic pole positions
It is generally considered by paleomagnetists that
the geomagnetic eld direction when sampled many
times at a single location during a long time interval
and averaged, approximates that of an axial central
dipole (cf. Hospers, 1953). This has been a foundation for common applications of paleomagnetic studies worldwide, in the estimation of geological ages,
continental drift, and apparent polar wandering. The
number of independent “spot readings” of the eld
required for obtaining an average direction with less
than 5◦ (say) condence radius, has often been given
in the literature as 20 or less, which however (even
in the absence of uncertainties in tilt corrections, etc.)
is in fact far too small, see Section 5 of Kristjánsson
(2002).
It is also obvious that a set of remanence directions to be averaged should preferably span a few to
several polarity reversals. Figure 4 shows mean VGPs
from twelve such large (> 300) collections of lava
ows in Iceland. Using Fisher’s (1953) statistical parameters the 95% condence radii (α95 ) are around 3◦
for each mean pole, which would indicate that at least
four of those in the Figure are signicantly different
from the geographic pole. However, the α95 -values
derived in this way are probably underestimated, both
because the VGPs do not in fact follow Fisher’s distriJÖKULL No. 58, 2008
107
L. Kristjánsson
bution and because of serial correlation, i.e. clustering
of VGPs which often is seen in two or more successive
lavas. See a discussion of the latter point in particular cases by Kristjánsson et al. (1980, 2003, 2006).
Those poles which are most “right-handed” (i.e. nos.
7/7a, 10, 12/12a and 14) include some of the oldest
rocks in the country, but otherwise their positions do
not show a systematic variation with age. Divergence
of these means from the geographic pole may in some
cases be in part due to e.g. incorrect estimates of the
local tectonic tilt. Tectonic rotations about a vertical
axis have not been considered here, although it is for
instance possible that they occur through “bookshelf
faulting” in some tectonic situations in Iceland (see
LaFemina et al., 2005).
Paleo-secular variation: eld and dipole strength
as function of VGP latitude
Sigurgeirsson (1957) noted that the remanence intensity in hand samples from lavas with intermediate
(transitional) virtual geomagnetic poles was in general lower than in lavas with high-latitude poles.
These intensities are of course inuenced by a
large number of rock properties which vary greatly
both within and between lava ows, as well as by
the ambient eld intensity. As the other properties
vary irregularly with time in an independent fashion,
it may be assumed that effects of the eld dominate in
suitable comparisons between sufciently large collections of results. Long-term trends in the eld intensity with time cannot be extracted from such collections due to the possible effects of long-term variations in the composition of erupted magma, and alteration effects including viscous decay of the primary
remanence. On the other hand, variations in the eld
intensity with for instance VGP latitude should show
up in large collections. Dagley and Wilson (1971)
made the rst attempt at quantifying this relationship
by plotting average remanence intensities (after a selected AF demagnetization step) against VGP latitude
in two data sets from Icelandic lava ows. Similar
attempts involving considerably larger sample collections were published by Kristjánsson and McDougall
(1982) and Kristjánsson (1995, 1999). In the lastnamed publication, data from 3514 lava ows were
processed.
108 JÖKULL No. 58, 2008
Figure 4. Mean VGPs from 12 collections of lava
ows, varying in size from 310 to 580 ows. Those
nos. 1–12 are taken directly from the compilation in
Kristjánsson and Jóhannesson (1989) as extended by
Kristjánsson (1995). Their mean poles no. 2, 4 and
8 which were derived from collections with mostly
two samples per lava, have been omitted here. The
rejection criterion for within-ow consistency was
α95 = 20.5◦ . In three more recent collections where
four samples were collected per ow, very few withinow α95 values exceed 15◦ . Of these, nos. 13 and
14 are from Kristjánsson et al. (2003, 2004); the
mean pole no. 15 combines results from Kristjánsson and Guðmundsson (2001, 2005) and Kristjánsson et al. (2006). The total number of ows is 5029;
note that low-latitude VGPs are not excluded from
these averages. One typical α95 -circle of radius 3◦
is shown. Due to the non-linear relation between virtual geomagnetic paleolatitude and eld inclination,
poles corresponding to the mean eld direction in
each collection would lie some 2◦ farther away from
Iceland. – Meðalstaðsetning sýndar-jarðsegulskauts
í 12 syrpum hraunlaga á Íslandi sem bergsegulmælingar hafa verið gerðar á; í hverri eru yr 300 lög.
Paleomagnetic research on Icelandic lava ows
Figure 5 shows the results from a new analysis
based on the data set of Kristjánsson (1999) and a
large number of data from Iceland (almost all with 4
samples per lava) obtained subsequently.
Figure 5. Bottom points: results from remanence intensity measurements (after 10 mT AF treatment) in
4970 Icelandic lavas of 1–15 M.y. age; geometric averages at 10◦ intervals in VGP latitude. In the upper two
sets of points, the intensity values have been converted
for each lava (i.e. multiplied by a factor between 1 and
2 depending on paleo-geomagnetic latitude, assuming
a central dipole eld) to the value that would have
been observed if that lava had been erupted at its VGP.
Middle points: geometric averages, with a curve showing one possible simple interpretation. Upper points:
arithmetic averages, with standard errors indicated in
two cases. – Meðalstyrkur segulmögnunar í íslenskum
hraunlögum; efri tveir punktahóparnir eru úr gildum
reiknuðum fyrir hvert hraunlag um sig ef það hefði
verið statt á viðkomandi sýndarsegulskauti. Þeim hefur verið skipt í hópa á 10◦ - bili í breiddargráðu sýndarsegulskautsins. Neðri tveir punktahóparnir eru geometrisk meðaltöl, sá efsti eru venjuleg meðaltöl fyrir
sömu hraunlögin.
The acceptance criterion has been tightened considerably from the within-lava directional α95 value
of 20.5◦ used by Kristjánsson (1995, 1999). In the
present case we reject those lavas which have α95 values greater than 16◦ if the VGP latitude is less than
40◦ N or S, greater than 14◦ if the VGP latitude is between 40◦ and 50◦ , and greater than 12◦ if the VGP
latitude is greater than 50◦ . This sliding cutoff criterion is an attempt to allow for the fact that remanence
direction measurements on lavas yielding low-latitude
VGPs tend to be less accurate than others. The results
are shown in Figure 5 for averaged remanence intensities (after 10 mT AF treatment) in 10◦ - intervals of
VGP latitude. The total number of acceptable lavas
is 4970, of which 284 are in the three groups with
VGP latitudes between 0◦ and 30◦ and have mean
α95 -values of 7◦ , while the average α95 -value in the
remaining lavas is about 5◦ . The bottom set of points
shows the geometric average remanence intensities by
themselves. The central set shows how the geometric average intensity of remanence would vary if each
lava ow had been erupted at its VGP instead of in
Iceland. These values are clearly proportional to the
virtual geomagnetic dipole moment (VDM). The standard deviation of logarithm values around each point
in this set corresponds approximately to a factor of 2.0
up or down. The top set shows arithmetic averages of
the intensities as they would have been recorded at the
VGPs. The standard error of each of these 10◦ -means
is about 0.2 A/m for the three points on the right, 0.1–
0.15 A/m for the others.
These results form a useful constraint on theoretical predictions and models of the long-term variations of the eld. Kristjánsson and McDougall (1982)
and Kristjánsson (1995) assumed that the VDM might
decrease in a linear fashion all the way as the VGP
moves from the geographic poles to the Equator. This
rst-order approximation indicated a reduction in the
VDM by a factor of the order of 4. The results from
the improved data sets point to the possibility, already
hinted at in the previous publications, that the curve
of inferred VDM vs. VGP latitude is fairly at above
75–80◦ VGP latitude (in part probably due to effects
of non-dipole elds and various error sources) as well
as below 25–30◦ VGP latitude. Consequently, the
above factor is around 2.5 rather than 4. Possibly, the
geomagnetic dipole eld is becoming weak in comparison with irregular non-dipole elds at around 30◦
VGP latitude. Furthermore, the VGPs in Figure 5 may
consist of two populations, one from times of “ordiJÖKULL No. 58, 2008
109
L. Kristjánsson
nary” secular variation of the VGP in latitudes above
30◦ and another one (comprising of order 10% of the
total) of considerably weaker poles distributed at random over the globe (Harrison, 1980). This is supported by the observation of a fairly at tail of the
frequency distribution of Icelandic VGPs in latitude,
below 25–30◦ (Kristjánsson and Jónsson, 2007).
It should be noted that a few published studies
have described absolute paleointensity determinations on individual Icelandic lava samples, usually
from heating experiments. These include Levi et al.
(1990), Tanaka et al. (1995), and most recently Goguitchaichvili et al. (1999) yielding results from a
single reversal that are qualitatively similar to our
Figure 5. However, as Icelandic rocks do often alter during laboratory heating (cf. above) and some of
their original remanence may also have decayed with
time, especially in lavas with low MDFs (Kristjánsson, 1999, Sect. 4), further research in this eld is
needed. There is still no consensus on the optimum
procedures for paleointensity measurements on igneous materials, and internally inconsistent results
continue to be obtained from lava units abroad where
the samples selected for the intensity determinations
have passed conventional quality tests (cf. Biggin et
al., 2007).
sions. Kristjánsson (1999; 2002, Sect. 8.2) concluded
that such preferences are not evident in the distribution of low-latitude VGPs from Icelandic lavas or
in the corresponding mean dipole moments. Lowand mid-latitude VGPs have now been regrouped in
36◦ - longitude intervals, using the same data set and
acceptance criteria as in the section on secular variation above. The results are indicated in Table 1,
where 525 lava ows with VGP latitudes between
40◦ S and 40◦ N, 419 lavas with VGP latitudes 41–
50◦ and 425 lavas with latitudes 51–56◦S or N are
grouped separately. The rms value of within-ow α95
radii in the ows in the Table is 7◦ . Iceland itself lies
around the middle of the last longitude interval. It is
seen from Table 1 that the preference is at any rate
not very strong. For poles at 50◦ latitudes or less,
the highest numbers are at 36–71◦E and 216–251◦E,
passing through the Middle East countries and the
Western U.S. respectively. These two antipodal longitude zones lie west of those generally suggested in
the literature to be preferred by transitional VGPs, i.e.
through Western Australia and Eastern U.S.
Paleo-secular variation: changes in its overall amplitude with time
Kristjánsson and Jóhannesson (1989) studied the observed scatter of VGPs around their mean positions in
the various large collections of lava ows then published from Iceland. They pointed out that this scatter (as measured for instance by the angular standard
deviation of poles) appeared to have been decreasing signicantly since 15 M.y. ago. This observation was followed up by Kristjánsson (1995, 2002)
The distribution of low- and mid-latitude VGPs in
longitude
A question which has been debated since suggested
by Laj et al. (1991) concerns the possibility that the
geomagnetic pole travels preferentially along certain
longitude intervals in polarity transitions and excur-
Table 1: Number of low- and mid-latitude geomagnetic virtual geomagnetic poles in various longitude intervals
on the globe. – Dreing sýndarsegulskauta á lengdargráður.
Long. deg. E
0–35
36–71
72–107
108–143
144–179
180–215
216–251
252–287
288–323
324–359
40◦ S to 40◦ N
54
69
47
39
42
52
60
50
60
52
51–56 S and N
45
45
49
44
30
38
43
59
38
34
41–50◦ S and N
◦
39
55
110 JÖKULL No. 58, 2008
44
43
38
30
51
41
38
40
Paleomagnetic research on Icelandic lava ows
and Kristjánsson et al. (2003) with augmented data
sets, conrming the original conclusion. One manifestation of this change is a signicant reduction in
the proportion of low-latitude VGPs (Kristjánsson and
Jónsson, 2007). As emphasized in these papers, the
observed variations are very unlikely to be due to artifacts such as effects of alteration or tectonics. Figure 6 is based on recomputation of the lava data set
of Kristjánsson (2002) after inclusion of most of the
comparable data from Iceland published subsequently
and some unpublished results from Northwest Iceland (L. Kristjánsson, in preparation 2008). This is
the same set as in the previous section, plus 56 lavas
for which intensity information was not available; the
same acceptance criteria have been applied. An age
estimate between 1.0 and 15.0 M.y. was assigned to
each lava, to the nearest 0.5 M.y., although it must be
kept in mind that some may be uncertain by 1 M.y. or
more due to the scarcity of radiometric dates already
mentioned. These lavas have been split into seven
groups, each covering three to ve such age slots. The
angular standard deviation of VGPs in each group is
then computed. Results are plotted in the top graph of
Figure 6. The number of accepted lava ows in each
group is between 510 and 950, with a total of 5025
compared to 4230 in Figure 4 of Kristjánsson (2002).
In the bottom graph of Figure 6, all lavas with VGPs
less than 40◦ have been rejected, leaving 4523 lavas.
In the middle graph, only those 4392 which have a
within-ow α95 less than or equal to 8◦ are included.
The graphs are very similar to those already presented by Kristjánsson (2002). Although they may
only be considered as semi-quantitative, they indicate
that the a.s.d. of all reliably recorded VGPs in Iceland
was around 34◦ 12–15 M.y. ago, around 31◦ 6–12
M.y. ago and 27◦ or less 1–5 M.y. ago. As most paleomagnetists seem prepared to accept the occurrence of
long-term variations in the rate of geomagnetic reversals and in the average geomagnetic dipole moment,
there is no reason to doubt that the secular variation
can also undergo long-term changes. The current state
of knowledge from about all these global properties is,
however, still quite insufcient to allow speculations
about possible relationships between them.
Figure 6. The angular standard deviation (a.s.d.) of
VGPs in 1–15 M.y. old Icelandic lava ows, approximated as cos−1 (R/N) for a vector sum R of N unit
vectors. The age of each ow was estimated to the
nearest 0.5 M.y., and then they were split into seven
non-overlapping age groups. Each group covers 3,
4 or 5 of the 0.5 M.y.- slots. The groups in the top
curve contain N=510 to 950 lavas. Corrections for
within-ow scatter (which are of the order of 0.2◦ ,
similar for all the points of each graph) have not
been applied. The vertical bar indicates 95% condence limits for 500 Fisher-distributed poles (Cox,
1971). – Staðalfrávik horna milli sýndar-segulskauta
og meðalstaðsetningar þeirra á jörðinni, í sjö stórum
hópum íslenskra hrauna. Hver hópur er áætlað að
nái yr 1.5–2.5 milljón ára tímabil í aldri.
AEROMAGNETIC ANOMALIES
A subject closely related to the high intensity of magnetization in oceanic basalts in general, is the appearance of magnetic anomalies in their vicinity. The major effort in aeromagnetic surveying of Iceland was
carried out by T. Sigurgeirsson in 1968–80, mostly
at 900–1200 m altitude above sea level and 3 km
spacing. Additional measurements were made by the
present author in 1972–92 both over Iceland and parts
of the surrounding shelf, partly in collaboration with
G. Jónsson. All the maps were then converted to a digJÖKULL No. 58, 2008
111
L. Kristjánsson
Figure 7. A composite total-eld magnetic map of Iceland and parts of the surrounding area, mostly acquired
from aircraft at around 1000 m altitude in 1968–92. Yellow and red: positive anomalies, green and blue: negative anomalies. – Kort af niðurstöðum segulsviðsmælinga yr Íslandi, ásamt mælingum frá ugvélum og skipum
yr ýmsum svæðum kringum landið.
112 JÖKULL No. 58, 2008
Paleomagnetic research on Icelandic lava ows
ital format suitable for viewing larger areas, cf. Figure 7 (G. Jónsson and L. Kristjánsson, unpublished
data). Sigurgeirsson’s (1970–85) prole maps in scale
1:250,000 on nine sheets are available from the author, as well as copies of reports on these surveys,
other maps and reprints of papers (e.g. Jónsson et al.,
1991). No further surveys have been carried out after 1993, when a detailed aeromagnetic map of the
Reykjavík area was acquired (Jónsson and Kristjánsson, 2002). The information available from such magnetic maps is of many kinds but it is often of a qualitative nature only. The very distinct magnetic lineations
which are seen to lie parallel to many oceanic spreading ridges, are not much in evidence here. Among
interesting features that can be studied in the anomaly
structure over Iceland and its surroundings are the following (in order of decreasing dimensions):
• Major tectonic trends, especially at the volcanic
zones
• Approximate location of boundaries between
large areas with a single remanence polarity dominating in each (e.g. the BrunhesMatuyama boundary at 0.8 M.y.)
• Extensive topographic or bathymetric features,
like the shelf edge south and southeast of Iceland
• Central volcano complexes, both active and extinct
• Strongly magnetized Quaternary volcanoes,
probably formed under water or glacier ice
• Geothermal areas, several of which were surveyed by T. Sigurgeirsson (unpublished contract reports and maps) in connection with exploration efforts.
For a recent review of various aspects of the aeromagnetic anomalies over Iceland and the shelf, see
Kristjánsson and Jónsson (2007).
CONCLUSIONS AND DISCUSSION;
FUTURE TASKS
The geomagnetic reversals and excursions recorded in
the Tertiary and early Quaternary lava pile in Iceland
have rst and foremost served as a tool in local stratigraphic mapping. While we may expect that these
and other variations of the eld will continue to be
of use for correlation in such work, it has become
increasingly clear that the distances between neighboring mapped proles must be reduced from 10 km
or more as often happened in the past, to something
like 2 km when feasible. This is because the rate
of reversals is higher than previously anticipated, and
the rate of eruptions has also been more variable in
time and space than expected. The well-known correlation methods based largely on lava lithology and
extended sediment layers will have to be expanded
to include e.g. lava chemistry. Furthermore, an increased number of radiometric age determinations is
needed, in particular in order to date major sediment
horizons and other locations where unconformities
are suspected. Accurate dating of the boundaries of
thick polarity zones (McDougall et al., 1977, 1984) is
also important for long-distance and possibly global
correlations.
As is evident from Figure 2, many parts in Iceland
have still not been sampled for paleomagnetic studies.
The reasons for this incomplete coverage are manifold:
• In some highland areas: lack of exposures, difculty of access
• In the Quaternary regions: dominating presence
of unsuitable formations such as tuffs and breccias; correlation difculties due to the landscapes formed by glaciation
• In the vicinity of central volcanoes and in areas
where these are most numerous (e.g. Southeast
and Central West Iceland): alteration, local tectonic disturbances
• Elsewhere: limited nancial resources, lack of
interest among geologists, etc.
It is evident that just the basic mapping of paleomagnetic reversals in the Tertiary formations will take several decades yet at the current rate. Concurrent with
the mapping of new areas, it would also be in some
cases desirable to perform further mapping within the
already sampled ones and to carry out sampling in additional proles there, in order to make correlations
JÖKULL No. 58, 2008
113
L. Kristjánsson
more certain.
Paleomagnetic studies on basement rocks abroad
have often concentrated on the acquisition of evidence
about major episodes of alteration and tectonic upheavals to which these rocks have been subjected.
These studies depend on knowledge of the overall
properties of the geomagnetic eld, but due to the
many factors inuencing the remanence directions in
complex ways, they are not in a position to contribute
much to that knowledge. In Iceland on the other hand,
most of the formations that have been selected for paleomagnetic research have suffered only minor disturbances (< 150◦ C heating, < 8◦ tilt) and can therefore
deliver much information about overall properties of
the geomagnetic eld in the last 15 M.y. In this paper,
examples have been presented regarding, for instance:
• Mean VGP positions, for which perhaps as
many as 300 lavas have to be averaged in order
to bring 95% condence limits below 5◦ .
• A decrease of the long-term average virtual
dipole strength as a function of VGP latitude,
from a value applying to VGP latitudes above
75–80◦ N or S, by a factor of about 2.5 to a
fairly constant value for VGP latitudes less than
25–30◦.
• A signicant decrease with time in the longterm scatter of VGPs around their long-term (1–
2 M.y.) mean positions. This decrease may
have been most rapid near 12 M.y. ago and at
5 to 6 M.y. ago.
Aeromagnetic surveys have since the late 1960’s
been helpful in studying the tectonics and ages of various areas in Iceland and on the shelf. They are of especially great value where the bedrock is covered by
water, ice caps, tuffs or detritus. Due to the large spacing between survey lines, many structures such as central volcanoes are only intersected by one or two such
lines. Very little other geological/geophysical mapping of the central volcanoes has taken place thus far.
In the future, detailed low-altitude surveys of volcanic
centers may be expected to become an important part
of integrated studies on these volcanoes.
Extensive bibliographical lists of publications on
paleomagnetism in Iceland, as well as on related
114 JÖKULL No. 58, 2008
stratigraphic and rock-magnetic research, are accessible at www.raunvis.hi.is/∼leo/vef−rit.html
Acknowledgements
Rósa Ólafsdóttir and Geirnnur Jónsson drafted Figures 2-7.
ÁGRIP
Mælingar á styrk og stefnu varanlegrar segulmögnunar í íslensku gosbergi voru fyrst gerðar um 1930.
Á sjötta áratugnum varð ljóst, að slíkar athuganir
gátu veitt jarðvísindamönnum ýmsar mjög gagnlegar upplýsingar, meðal annars vegna þess að segulmögnunarstefnan endurspeglar allnákvæmlega stefnu
jarðsegulsviðsins þegar bergið myndaðist. Mælingar
hafa nú verið gerðar hérlendis og erlendis á hátt í átta
þúsund hraunlögum, að miklu leyti í þykkum syrpum
(um og yr 300 lög) þar sem nokkur snið sem skarast
í aldri, eru tengd saman. Segulstefnurnar hafa nýtst
vel við slíka kortlagningu, einkum þeir umsnúningar
jarðsegulsviðsins sem orðið hafa með óreglulegu millibili mörgum sinnum á hverri ármilljón. Ljóst er þó
orðið nú, að til þess að tengja umsnúningana með
nokkurri vissu milli sniða (með hjálp annarra atriða
s.s. setlaga eða efnasamsetningar bergsins), þurfa fjarlægðir milli sniðanna yrleitt að vera minni en áður
var talið duga, helst af stærðargráðu 2 km fremur
en t.d. 10 km. Kortlagningu á uppruna, byggingu
og aldri hraunlagastaans í landshlutum utan gosbeltanna hefur miðað hægt síðustu áratugina, og eru
mörg svæði enn alveg ókönnuð að þessu leyti. Einnig
hefur lítið verið gert af segulmælingum t.d. á setlögum
hér, innskotum, gosbergi mynduðu í vatni, og hraunlögum ummynduðum af jarðhitavirkni.
Vegna áreiðanleika og stöðugleika segulstefna
í íslensku blágrýti, og hins mikla fjölda mældra
hraunlaga, hafa segulmælingar á þeim jafnframt
skipt verulegu máli í almennri öun þekkingar á
alþjóðavettvangi um sögu jarðsegulsviðsins síðustu
15 milljón ár. Þar má nefna m.a. þróun tækni
við mælingar, niðurstöður úr einstökum kortlagningarverkefnum, athuganir á venslum seguleiginleika bergsins við samsetningu steinda í því, og
úrvinnslu mikils fjölda stefnu- og styrkleikamælinga
Paleomagnetic research on Icelandic lava ows
til að kanna langtímahegðun jarðsegulsviðsins. Fáein
dæmi um slíka úrvinnslu eru sýnd og rædd í greininni.
Að síðustu er þess getið, að bergsegulmælingar séu nauðsynlegar við jarðfræðilega túlkun á
niðurstöðum nákvæmrar kortlagningar á frávikum
jarðsegulsviðsins yr landinu.
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