7
Magnetostratigraphy: From a Million
to a Thousand Years
Carlo Laj, James E. T. Channell, and Catherine Kissel
Since the publication in 1600 of the book De Magnete by
William Gilbert, and the measurements by magnetic observatories progressively obtained from various parts of the
globe, we know that the Earth’s magnetic field is comparable
to one that would be created by a bar-magnet placed at the
center of the Earth and inclined by some 11° with respect to
the axis of rotation (Fig. 7.1).
For each point on the Earth’s surface, the intensity and
direction of the Earth’s magnetic field are defined in terms of
two components: the declination, which is the angle on the
horizontal plane between the magnetic north and the geographic North, and the inclination, which is the angle
between the magnetic field vector and the horizontal plane.
By convention, the declination is zero when the field vector
points to the North (it is 180° if the field vector points to the
South), and the inclination is positive when the field vector
points downwards (which is the case today in the northern
hemisphere).
Measurements by observatories, which began in 1576 in
London and in 1617 in Paris, soon showed that declination
and inclination were not stable throughout history: since
measurements began, the inclination in Paris has changed by
about 10° and the declination by about 30°.
The intensity has decreased by about 5% per century.
This phenomenon is called the ‘secular variation’ of the
geomagnetic field. Changes in declination are plotted on
marine charts, essential tools for navigation by compass,
before the development of GPS.
Secular variation was therefore identified as the first
manifestation of the instability of the geomagnetic field. This
was already surprising, but the biggest surprise was yet to
come, when paleomagnetism methods were able to decipher
the history of the geomagnetic field over prehistoric periods.
In 1906, Bernard Brunhes was the first to measure a
direction of magnetization in rocks which was more or less
opposite to that of the present geomagnetic field. Brunhes
measured this magnetization both in a Miocene lava flow
and in clays that had been baked when covered by this lava
flow, which he called “natural brick”. In doing so, Brunhes
used for the first time a test in the field, now called the
“baked contact test”, which is based on the fact that, when a
lava flow settles on a sedimentary layer, it re-magnetizes it,
either partially or wholly, by heating it up.
If the direction of the magnetic field changed between the
settling of the sediment and the arrival of the lava, the initial
magnetic direction of the sediment will be replaced by the
magnetic direction of the overlying lava. (To quote Bernard
Brunhes “If, in the banks of natural clay, we have a
well-defined magnetic direction and which differs from the
direction of the current terrestrial field, it is reasonable to
assume that the magnetic direction is that of the Earth’s field
at the time when the lava flow transformed the clays into
“natural bricks”). Brunhes’s conclusion that “at the time of
the Miocene, around Saint-Flour, the North Pole was
pointing upwards: it is the Earth’s South Pole that was
closest to Central France” is the first suggestion that the
polarity of the magnetic field of the Earth could have
reversed in the geological past.
Twenty years later, the Japanese scientist Motonori
Matuyama was the first to attribute reverse magnetization of
volcanic rocks in Japan and China to reversals of the Earth’s
geomagnetic field and to differentiate Pleistocene lava from
Pliocene lava on the basis of the polarity of their magnetization. Matuyama was thus the first to use magnetic
stratigraphy as a way to order sequences of rocks in time.
The modern era of magnetostratigraphy began in the
1950s in Iceland with the work of Hospers (1953). Hospers’
C. Laj (&)
Department of Geosciences, Ecole Normale Supérieure, PSL
Research University, 24 rue Lhomond, 75231 Paris Cédex, France
e-mail: carlo.laj@ens.fr
J. E. T. Channell
Department of Geological Sciences, University of Florida, 241
Williamson Hall, P.O. Box 112120 Gainesville, FL 32611, USA
C. Kissel
Laboratoire des Sciences du Climat et de l’Environnement,
LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, 91190
Gif-sur-Yvette, France
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_7
101
Magnetostratigraphy: From a Million
to a Thousand Years
Carlo Laj, James E. T. Channell, and Catherine Kissel
Since the publication in 1600 of the book De Magnete by
William Gilbert, and the measurements by magnetic observatories progressively obtained from various parts of the
globe, we know that the Earth’s magnetic field is comparable
to one that would be created by a bar-magnet placed at the
center of the Earth and inclined by some 11° with respect to
the axis of rotation (Fig. 7.1).
For each point on the Earth’s surface, the intensity and
direction of the Earth’s magnetic field are defined in terms of
two components: the declination, which is the angle on the
horizontal plane between the magnetic north and the geographic North, and the inclination, which is the angle
between the magnetic field vector and the horizontal plane.
By convention, the declination is zero when the field vector
points to the North (it is 180° if the field vector points to the
South), and the inclination is positive when the field vector
points downwards (which is the case today in the northern
hemisphere).
Measurements by observatories, which began in 1576 in
London and in 1617 in Paris, soon showed that declination
and inclination were not stable throughout history: since
measurements began, the inclination in Paris has changed by
about 10° and the declination by about 30°.
The intensity has decreased by about 5% per century.
This phenomenon is called the ‘secular variation’ of the
geomagnetic field. Changes in declination are plotted on
marine charts, essential tools for navigation by compass,
before the development of GPS.
Secular variation was therefore identified as the first
manifestation of the instability of the geomagnetic field. This
was already surprising, but the biggest surprise was yet to
come, when paleomagnetism methods were able to decipher
the history of the geomagnetic field over prehistoric periods.
In 1906, Bernard Brunhes was the first to measure a
direction of magnetization in rocks which was more or less
opposite to that of the present geomagnetic field. Brunhes
measured this magnetization both in a Miocene lava flow
and in clays that had been baked when covered by this lava
flow, which he called “natural brick”. In doing so, Brunhes
used for the first time a test in the field, now called the
“baked contact test”, which is based on the fact that, when a
lava flow settles on a sedimentary layer, it re-magnetizes it,
either partially or wholly, by heating it up.
If the direction of the magnetic field changed between the
settling of the sediment and the arrival of the lava, the initial
magnetic direction of the sediment will be replaced by the
magnetic direction of the overlying lava. (To quote Bernard
Brunhes “If, in the banks of natural clay, we have a
well-defined magnetic direction and which differs from the
direction of the current terrestrial field, it is reasonable to
assume that the magnetic direction is that of the Earth’s field
at the time when the lava flow transformed the clays into
“natural bricks”). Brunhes’s conclusion that “at the time of
the Miocene, around Saint-Flour, the North Pole was
pointing upwards: it is the Earth’s South Pole that was
closest to Central France” is the first suggestion that the
polarity of the magnetic field of the Earth could have
reversed in the geological past.
Twenty years later, the Japanese scientist Motonori
Matuyama was the first to attribute reverse magnetization of
volcanic rocks in Japan and China to reversals of the Earth’s
geomagnetic field and to differentiate Pleistocene lava from
Pliocene lava on the basis of the polarity of their magnetization. Matuyama was thus the first to use magnetic
stratigraphy as a way to order sequences of rocks in time.
The modern era of magnetostratigraphy began in the
1950s in Iceland with the work of Hospers (1953). Hospers’
C. Laj (&)
Department of Geosciences, Ecole Normale Supérieure, PSL
Research University, 24 rue Lhomond, 75231 Paris Cédex, France
e-mail: carlo.laj@ens.fr
J. E. T. Channell
Department of Geological Sciences, University of Florida, 241
Williamson Hall, P.O. Box 112120 Gainesville, FL 32611, USA
C. Kissel
Laboratoire des Sciences du Climat et de l’Environnement,
LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, 91190
Gif-sur-Yvette, France
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_7
101