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Updated January 27, 2024



Image: The geographic north pole of the Earth corresponds magnetically to the south pole of the Earth's magnet, and vice versa. Magnetic field lines exit from the magnetic north pole and enter through the magnetic south pole.
Over the course of geological time, the Earth's magnetic field has reversed several times. We talk about normal periods and inverse periods where the north pole was in place of the current south pole. This information has been preserved in the basalts. Image credit: Public domain.

What is paleomagnetism?

The Earth's magnetic field is generated by the Earth's outer core, which is composed of liquid iron.
Heat from the inner core, composed of solid iron, is transferred to the liquid outer core by convection currents. Convection currents cause the liquid outer core to rotate irregularly but slightly faster than the Earth's rotation speed, creating an electric current. This electric current, in turn, creates a magnetic field which is characterized by direction, meaning and intensity.

On the Earth's surface, ferromagnetic minerals, such as magnetite (Fe3O4) and hematite (Fe2O3), contained in the rocks of the Earth's crust, have the capacity to become magnetized. It is when basalt is formed by rapid cooling of magma emitted by volcanoes, that the ferromagnetic minerals present in the rock align with the ambient Earth's magnetic field.

This magnetization of ferromagnetic minerals creates a magnetic imprint in the basalt that can be studied later to understand the geomagnetic history of the region. That is, when rocks form, they record the orientation of the Earth's magnetic field at a specific time. Over time, due to the movements of tectonic plates, rocks may be moved, but their magnetic footprint remains unchanged.

Geophysicists and geologists can take rock samples from anywhere in the world, date them, and analyze their orientation and magnetic intensity. By comparing this information with the current position of the rocks, they can reconstruct the past movements of the continents.

How to rediscover the magnetism of the past?

Ferromagnetic minerals preserve the alignment of iron ions in the direction of the Earth's magnetic field when they solidified, but also the intensity of the magnetization. Thus, by studying numerous sites around the world (geomagnetic observatories), scientists can reconstruct the history of the Earth's magnetic field.

All matter (plants, metals, animals, human bodies, etc.) has induced magnetization, in other words, it has a response to the Earth's magnetic field. Remanent magnetization is what matter can retain, store in memory, when the ambient magnetic field is reduced to zero. The paleomagnetician therefore seeks to identify this remanent magnetization.
The paleomagnetician is especially interested in the direction of the magnetization, in relation to the geographical marker of the sampling location. The sample taken (long cylinder of rock) is located using a system which allows the exact direction of the magnetic arrow in space to be noted at the sampling location.
The intensity and direction of the retained magnetization are measured with a magnetometer.
The sample is positioned in its benchmark, that is to say in the same position as that of its sampling location. A machine will drive it into rapid rotation inside shielded coils, isolated from the external magnetic field. A deviation is observed in the measuring instrument. The 3 components of magnetization are recorded (direction, direction, intensity).
Then, the scientists heat the sample to make all the so-called natural thermoremanent magnetization disappear. Indeed, a ferromagnetic material loses its permanent magnetization at a temperature called the "Curie temperature", this phenomenon was discovered by the French physicist Pierre Curie (1859-1906) in 1895.
Then they will produce a new magnetization in a known field (that of the Earth). Without moving the sample, they will compare it with the old natural magnetization that they recorded.
This manipulation allows us to know the 3 parameters of the magnetic field at the moment when the rock has cooled.

Reversal of the Earth's magnetic field over time

Normal and reverse Earth magnetic polarity

Image: The reversal of the Earth's magnetic field is recorded in the oceanic crust in the form of magnetic bands parallel to the mid-ocean ridge. New oceanic crust is magnetized as it forms, then moves away from the ridge in both directions. In this model appears, a ridge (a) about 5 million years ago, a ridge (b) about 2 million years ago and a ridge (c) today. Image credit: Public domain.

Using paleomagnetism, scientists have discovered that there have been numerous reversals of the Earth's magnetic field throughout Earth's history.

The reversal of the Earth's magnetic field is recorded at mid-oceanic ridges. At these locations, molten mantle rises to the surface and solidifies to form new oceanic crust, preserving the strength and direction of the contemporary ambient magnetic field. As new material is extruded, the existing crust is pushed to either side of the ridge, the direction of the ambient magnetic field at the time of formation is kept. In other words, when magma cools and solidifies to form oceanic crust, ferromagnetic minerals align with the ambient Earth's magnetic field as they solidify. This magnetizes the ocean crust, recording the orientation of the magnetic field at that precise location and at that precise time.

Thus, parallel bands of magnetized rocks form on either side of the central ridge of the ridge. These bands have opposite magnetic polarities, forming what are called magnetic anomalies. These are interpreted as alternating blocks of normal and inversely magnetized oceanic crust. These anomalies observed at mid-ocean ridges are particularly interesting because by analyzing these bands, scientists can reconstruct the history of magnetic reversals over time.

By analyzing these anomalies, scientists were able to trace the history of reversals of the Earth's magnetic field over the last 800 million years. They found that the frequency of reversals is irregular, but that it has a certain periodicity. On average, the Earth's magnetic field reverses every 250,000 to 300,000 years. However, there are periods of time when reversals are more frequent, such as 80 million years ago, when the Earth's magnetic field reversed every 100,000 years.

There are also periods of time when reversals are rarer, such as 100 million years ago, when the Earth's magnetic field remained stable for more than 10 million years.

Magnetic field reversal in progress

Scientists are carefully monitoring Earth's magnetic field for any signs of significant change. Modern observations are made using satellites and geophysical instruments to map the magnetic field precisely.

However, even with these advanced technologies, predicting exactly when a reversal will occur remains a challenge due to the complexity of the dynamic processes taking place in the Earth's core.

Currently, we are seeing a decrease in the dipole moment of 6% per century. The decrease in the dipole moment indicates a reduction in the intensity of the Earth's magnetic field.
The decrease in the dipole moment of the Earth's magnetic field does not necessarily imply an immediate reversal of the magnetic field. Although the decrease in the dipole moment can be associated with changes in the Earth's magnetic field, magnetic reversal is a complex process and the evolution of the magnetic field can follow various unpredictable scenarios.

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