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GEOMAGNETIC EXCURSIONS
AND SECULAR VARIATIONS
R M Twyman, University of York, York, UK
ª
2007 Elsevier B.V. All rights reserved.
Introduction
The Earth’s magnetic field is approximately dipolar,
with one pole in the Northern Hemisphere and the
other in the Southern Hemisphere. The magnetic
poles are near the geographic north and south poles
but are not co-aligned. The angle of this east–west
deviation, measured from anywhere on Earth, is
called the declination. The degree of declination is
not fixed, and the poles may drift, independently of
each other, by more than 10 km each year. This
independence, and the fact that the two magnetic
poles are not directly opposite each other, shows
that the similarity between the Earth’s magnetic
field and that of, for example, a bar magnet, is rather
superficial. While the field generated by a bar magnet
represents the coordinated movement of electrons in
a fixed solid structure, the Earth’s magnetic field is
much more complex in origin.
The primary source of the Earth’s magnetic field is
its iron-rich liquid outer core. The movement of this
liquid is continuous, driven by convection currents,
the planet’s rotation (Coriolis effect), and the cycle of
melting and solidification that occurs at the boundary between the outer core and the solid inner core.
Because the liquid outer core is conductive, its movement relative to the inner core induces an electrical
current, which also has a magnetic field. This
reinforces the original magnetic field and results in
a self-sustaining field-generating system known as the
geodynamo (Hollerbach and Jones, 1993; 1995).
The continuous magnetic flux within the Earth’s
core can be recorded as changes in magnetic field
strength and orientation at the surface. The changes
are relatively slow, occurring over historical timescales, and this process of gradual change is defined
as secular variation (Jackson et al., 2000). Secular
variation is punctuated by more dramatic changes,
during which the field strength may fall to 10–20%
of its normal value within a relatively short time
(a few thousand years). Such rapid decreases in
strength are known as geomagnetic excursions and
the intervals between them are measurable on geological timescales (Gubbins, 1999). In most cases, the
field regenerates with the original polarity.
Occasionally, however, the excursion is accompanied
by a reversal of field polarity, in which case it is
known as a geomagnetic reversal. A few such reversals occur every million years on average, but their
occurrence does not fit a predictable pattern and
some periods of Earth’s history are known for the
paucity of such events. The most recent was the
Brunhes-Matuyama reversal approximately 789,000
years ago. Evidence suggests that since the last reversal there have been at least six global excursions (i.e.,
large decreases in magnetic field intensity correlated
at several different sites) as well as 10–15 further
excursions for which evidence exists at a single site
only (Champion et al., 1988; Langereis et al., 1997;
Lund et al., 1998).
Secular Variation
Secular variation describes continuous drift in the
intensity and direction of the Earth’s magnetic field
(Bloxham and Gubbins, 1985). Such changes are
noticeable over relatively short periods of time (tens
of years) and thus magnetic charts need to be updated
periodically to accommodate them. Humans have
observed such changes for around 1000 years, and
have collected data from geographically diverse sites
for nearly 500 years. Such data show a gradual,
monotonic decline in the intensity of the magnetic
field and suggest that it has fallen by approximately
20% during that time (Barraclough, 1982; Jackson
et al., 2000). This downward trend is supported by
archeological evidence, such as mineral-containing
Roman artifacts, which indicate that the field intensity at the time of the Roman Empire was twice as
718
GEOMAGNETIC EXCURSIONS AND SECULAR VARIATIONS
high as it is today. If the field strength continues to
fall at its current rate, the dipole moment will reach
zero by the year 3500. Although the fate of the
magnetic field cannot be predicted with any accuracy, this trend has led some to believe that we are
experiencing an ongoing excursion.
The orientation of the magnetic field also varies in
time. Although the degree of declination differs in
different parts of the world, the overall trend is a
westward drift at about 0.1 per year together with
a decrease in colatitude at a rate of about 0.02 per
year. The degree to which the magnetic axis declines
from the Earth’s axis of rotation is often quoted as
11.3 , but due to the drift described above this is not
an accurate statement. Magnetic data collected regularly at fixed points on the Earth’s surface, i.e., from
ground-based magnetic observatories, demonstrates
both the geographical and temporal secular variation
in the magnetic field (Fig. 1). Geographical maps of
Annual–Mean Declination
5°
BRW
declination have been used to generate models showing how the field at the Earth’s surface varies in
complex patterns in both time and space, with declination contours converging at the geomagnetic poles.
The strength of the field at the Earth’s surface ranges
from less than 30 microteslas (30 mT; 0.3 gauss), for
example, over South America, to more than 60 mT
(0.6 gauss) at the magnetic poles.
Geomagnetic Excursions
Geomagnetic excursions are radical swings in field
direction accompanied by decreases in field strength
which take place over a geologically short period of
time (thousands of years). The distinction between a
geomagnetic excursion and a large secular variation
event is the degree of departure between the virtual
geomagnetic pole (VGP) and the geographic pole,
with the cut-off usually at 45 (Verosub, 1977).
Some of the more widely reported geomagnetic
excursions since the last reversal event are shown in
Figure 2.
While the continuous movement of the Earth’s
molten outer core is sufficient to account for secular variation, it is unclear what causes the much
CMO
SIT
0
2
4
VADM (1022 Am2 )
6
8
10
0
Lake Mungo
Mono Lake
Laschamp
{
NEW
CLH
BOU
Blake
FRD
Jamaica
FRN
TUC
0.3
Biwa II
Calabrian Ridge 1
Levantine
DLR
Biwa III
Age (Ma)
BSL
Biwa I
Pringle Falls
HON
West Eiffel
Calabrian Ridge 2
Emperor
SJG
Big Lost
0.6
Calabrian Ridge 3
{
La Palma
GUA
1900
1920
1940
1960
Year
1980
2000
Figure 1 Stackplot of annual-mean declination (D), measured
in degrees east, at the United States Geological Survey magnetic
observatories over the last century. The field at the surface, and
the declination in particular, has a complex form, resulting in
equally complex secular variation. Also note the occasional offset,
such as in the Honolulu (HON), San Juan (SJG), and
Fredericksburg (FRD)/Cheltenham (CLH) annual-mean data,
due to change of the absolutes pier or moving of the observatory.
Delta
Brunhes
Matuyama
0.9
Figure 2 Approximate times of some of the most commonly
reported geomagnetic excursions of the Brunhes normal polarity
chron. VADM, virtual axial dipole moment.
GEOMAGNETIC EXCURSIONS AND SECULAR VARIATIONS
larger deviations that result in excursions and reversals. One theory is that, occasionally, some aspect
of the geodynamo process results in the creation of
opposing or competing magnetic fields in the core,
such that although the field strength in the core
itself is undiminished, the intensity of the field at
the Earth’s surface falls significantly. This may
involve the temporary dissolution of the typical
dipole organization of the field and its replacement
with multiple north and south poles at different
points on the Earth’s surface, which would be
weaker. Another possibility is that excursions
may be initiated by external phenomena such as
the movement of continental plates or forces generated by interactions between the core and the
mantle. These could disrupt the geodynamo
process resulting in a loss of field intensity not
only at the Earth’s surface, but also within the
core itself.
Detecting Geomagnetic Variation
Variations in the magnetic field are measured with
sensitive devices called magnetometers. There are
two principal types of magnetometer, scalar instruments that measure field intensity and vector instruments that also enable the determination of a
directional component. The use of three orthogonal
vector magnetometers allows the magnetic field
strength, inclination, and declination to be defined
precisely.
To recover evidence of geomagnetic events in the
distant past, it is necessary to identify materials
which preserve the characteristics of the Earth’s magnetic field at earlier points in history. Such data are
obtained from two major sources: volcanic lava and
sedimentary rocks. Both contain iron-rich mineral
domains which provide clues to the magnetic field
strength and orientation at the time they were
deposited. The advantage of sediments is that they
are ubiquitous and can yield almost continuous
records of magnetic variation, while lavas exist only
near volcanic sites and in ocean ridges, and therefore
provide a punctuated history of events. Lava data are
rarely subject to controversy, whereas sediment
data can be unreliable because of chemical and
physical changes occurring after deposition. In this
context, sediments can become magnetized through a
process known as postdepositional remanent
magnetization (PDRM), and there is also filtering
of the geomagnetic signal (Quidelleur and Valet,
1993).
The discovery of similar patterns of magnetic variations at different locations indicates that both
reversals and excursions may be global events.
719
However, reversals are relatively easy to detect
using the above methods due to the change in field
orientation, whereas excursions may be overlooked
because one is essentially searching for the absence of
data (i.e., a sharp decline in the magnetic field intensity). Therefore, while we can be fairly certain of the
number and nature of geomagnetic reversals at different points during the life of the planet, it is possible
that excursions are a much more frequent characteristic of our geomagnetic history than currently
accepted.
Using Magnetic Reversals for Dating
Within the Quaternary
The regular occurrence of polarity reversals and
other events during Earth’s history means that such
events can be used to date rocks and minerals, a
practice known as magnetostratigraphy. Initially,
magnetic-polarity intervals were known as epochs,
with periods of normal polarity (where a compass
needle would point towards the geographical north
pole, as it does today) known as intervals of normal
polarity, and periods of magnetic reversal (where a
compass needle would point towards the geographical south pole) as intervals of reversed polarity. The
epochs were named after famous scientists (e.g.,
Brunhes, Gauss, and Matuyama) with changes of
polarity described thereafter as the boundary
between two epochs (e.g., the Brunhes-Matuyama
reversal; Fig. 3). As more events have been
discovered, the old nomenclature has been replaced
with a simple and universal numbering system, in
which epochs have been replaced by ‘chrons’ and
events are now called ‘subchrons’, each number followed by either n or r to indicate normal or reversed
polarity.
Magnetic excursions during the Quaternary have
been used to date events and to correlate the timing
of events around the world that are otherwise difficult to date, especially events in the early Quaternary.
However, the formal status of the Quaternary both in
terms of its hierarchical rank on the chronostratigraphic scale and the position of its base have been
subject to controversy and revision over the years
(Aubrey et al., 2005; Gibbard et al., 2005). In
September 2005, a meeting of the International
Commission on Stratigraphy recommended that
the Quaternary be formally defined as a Subera of
the Cenozoic Era, and that its base be defined by the
Global Boundary Stratotype Section and Point
(GSSP) for the Gelasian Stage of the Upper Pliocene,
which has an astronomically calibrated age of
2.588 million years. The Gauss-Matuyama reversal
720
GEOMAGNETIC EXCURSIONS AND SECULAR VARIATIONS
2.59
Gauss
normal
0.79
Matuyama
reversed
Bruhnes
normal
Age of epoch
boundary/ocean
crust in Ma
2.59
0.79
Matuyama
reversed
Gauss
normal
Magnetic
polarity
epochs
spreading
direction
reverse
polarity
normal
polarity
ridge axis
oceanic
crust
continental
crust
Figure 3 The formation of normal and reversed magnetic polarity patterns in new oceanic crust, on either side of an oceanic ridge axis.
Alternating dark (normal) and light (reversed) polarity patterns would normally be recorded by shipborne or satellite surveys. Magnetic
reversals are subdivided into major chrons (Bruhnes, normal; Matuyama, reversed; Gauss, normal) peppered with smaller normal and
reversed subchrons, which are used for more precise dating.
provides a close approximation for global correlation
of the base of the Gelasian, which would be equivalent to the base of propos‘ed Quaternary Subera
(Partridge, 1997).
Abbreviations
GSSP
PDRM
USGS
VGP
Global Boundary Stratotype Section and
Point
postdepositional remanent magnetization
United States Geological Survey
virtual geomagnetic pole
See also: Introduction: History of Dating Methods.
Radiocarbon Dating: Variations in Atmospheric 14C;
Causes of Temporal Variations.
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