Geomagnetic Excursions and the Scale
of Magnetic Instabilities (GITS)
Discovery of Geomagnetic Excursions
Throughout the process of establishing the scale of magnetic
polarities, the presence of very short excursions of the field
polarity was observed in lavas and sedimentary sequences.
These excursions were often characterized by intermediate
magnetization directions between direct and reverse polarities, and did not correlate to polarity intervals recognizable
in the polarity scale well established on other sites. Therefore, these excursions were initially regarded with some
skepticism by paleomagnetic researchers and were sometimes attributed to episodes of extreme amplitude of the
secular variation or even to artifacts due to sampling,
re-magnetization or sedimentological processes. The short
duration of these excursions also meant that their registration
in volcanic or sedimentary sequences was often random, and
until very recently, no excursion was observed in more than
one site.
A change of attitude in the community occurred about
fifteen years ago, and can be largely attributed to a combination of two main factors: firstly, considerable progress in
the dating methods of both volcanic rocks and sedimentary
sequences allowed the unambiguous correlation of some of
these excursions recorded in these two media, and, on the
other hand, the discovery of rapid climate changes recorded
in ice cores encouraged paleoceanographers to collect marine sedimentary sequences with a very high sedimentation
rate, and with the longest possible series (IMAGES programs, ODP, IODP). This was made possible by the
development of new corers as previously mentioned. These
high rates of accumulation significantly increased the probability of recording rapid changes in magnetization, characteristic of an excursion.
Despite this progress, detailed records of excursions
remain largely limited to the last two million years. The best
documented excursions occur as double changes in polarity
which define a very short polarity interval in the opposite
direction to the initial and final polarities. The total duration
of an excursion is very short: the most recent (and most
accurate) estimates converge around a duration of around
1500–2000 years (Laj et al. 2000, 2004, 2014; Laj and
Channell 2007). In addition, directional changes related to
the best documented excursions like the Laschamp (LE) excursion or the Iceland Basin (IBE) excursion, seem to have
occurred more or less simultaneously in the various places
where they were recorded. Therefore, geomagnetic excursions, as well as being of fundamental interest as a probe into
the phenomena of magnetic instability in the Earth’s core,
also provide very precise points of correlation with a global
significance.
It is by now well established that excursions are accompanied by a decrease in the intensity of the geomagnetic
Fig. 7.6 Magnetostratigraphic
results obtained by Johnson et al.
(1985) on two sections from Chita
Parwala and Gabhir Kas. Some
levels of hard sandstone
(represented in gray) clearly
connect the two cuts. The
magnetic data is shown as the
virtual geomagnetic pole
(VGP) latitude, which is located
either in the southern hemisphere
(reverse polarity) or in the
northern hemisphere (normal
polarity). The filled-in circles
denote the most reliable
estimates, the open circles
represent the estimates of lower,
but acceptable, “quality”. The
column on the right shows the
sequence of polarities obtained
along both cuts and their
correlation
108
C. Laj et al.
of Magnetic Instabilities (GITS)
Discovery of Geomagnetic Excursions
Throughout the process of establishing the scale of magnetic
polarities, the presence of very short excursions of the field
polarity was observed in lavas and sedimentary sequences.
These excursions were often characterized by intermediate
magnetization directions between direct and reverse polarities, and did not correlate to polarity intervals recognizable
in the polarity scale well established on other sites. Therefore, these excursions were initially regarded with some
skepticism by paleomagnetic researchers and were sometimes attributed to episodes of extreme amplitude of the
secular variation or even to artifacts due to sampling,
re-magnetization or sedimentological processes. The short
duration of these excursions also meant that their registration
in volcanic or sedimentary sequences was often random, and
until very recently, no excursion was observed in more than
one site.
A change of attitude in the community occurred about
fifteen years ago, and can be largely attributed to a combination of two main factors: firstly, considerable progress in
the dating methods of both volcanic rocks and sedimentary
sequences allowed the unambiguous correlation of some of
these excursions recorded in these two media, and, on the
other hand, the discovery of rapid climate changes recorded
in ice cores encouraged paleoceanographers to collect marine sedimentary sequences with a very high sedimentation
rate, and with the longest possible series (IMAGES programs, ODP, IODP). This was made possible by the
development of new corers as previously mentioned. These
high rates of accumulation significantly increased the probability of recording rapid changes in magnetization, characteristic of an excursion.
Despite this progress, detailed records of excursions
remain largely limited to the last two million years. The best
documented excursions occur as double changes in polarity
which define a very short polarity interval in the opposite
direction to the initial and final polarities. The total duration
of an excursion is very short: the most recent (and most
accurate) estimates converge around a duration of around
1500–2000 years (Laj et al. 2000, 2004, 2014; Laj and
Channell 2007). In addition, directional changes related to
the best documented excursions like the Laschamp (LE) excursion or the Iceland Basin (IBE) excursion, seem to have
occurred more or less simultaneously in the various places
where they were recorded. Therefore, geomagnetic excursions, as well as being of fundamental interest as a probe into
the phenomena of magnetic instability in the Earth’s core,
also provide very precise points of correlation with a global
significance.
It is by now well established that excursions are accompanied by a decrease in the intensity of the geomagnetic
Fig. 7.6 Magnetostratigraphic
results obtained by Johnson et al.
(1985) on two sections from Chita
Parwala and Gabhir Kas. Some
levels of hard sandstone
(represented in gray) clearly
connect the two cuts. The
magnetic data is shown as the
virtual geomagnetic pole
(VGP) latitude, which is located
either in the southern hemisphere
(reverse polarity) or in the
northern hemisphere (normal
polarity). The filled-in circles
denote the most reliable
estimates, the open circles
represent the estimates of lower,
but acceptable, “quality”. The
column on the right shows the
sequence of polarities obtained
along both cuts and their
correlation
108
C. Laj et al.