Paleomagnetism: the magnetic memory of rocks

What is paleomagnetism?

Earth's magnetic field and the core

The Earth's magnetic field is generated by 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. The convection currents drive the liquid outer core into an irregular rotation, slightly faster than Earth's rotation speed, creating an electric current. This electric current, in turn, creates a magnetic field characterized by a direction, a sense, and an intensity.

Magnetization of ferromagnetic minerals

At Earth's surface, ferromagnetic minerals, such as magnetite (Fe3O4) and hematite (Fe2O3), contained in rocks of the Earth's crust, have the ability to become magnetized. It is when basalt forms 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. In other words, when rocks form, they record the orientation of Earth's magnetic field at a precise moment. Over time, due to the movements of tectonic plates, rocks may be displaced, but their magnetic imprint remains unchanged.

A magnetic memory of rocks

Geophysicists and geologists can collect rock samples from all over 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. This magnetic memory thus constitutes a precious natural archive for tracing the geological history of our planet.

How can we retrieve the magnetism of the past?

Natural remanent magnetization

Ferromagnetic minerals preserve the alignment of iron ions in the direction of Earth's magnetic field at the moment they solidified, as well as the intensity of the magnetization. Thus, by studying a great many sites around the world (geomagnetic observatories), scientists can reconstruct the history of Earth's magnetic field.

All matter (plants, metals, animals, human bodies, etc.) has an induced magnetization, in other words, it has a response to Earth's magnetic field. Remanent magnetization is what matter can retain, store in memory, when the ambient magnetic field is reduced to zero. The paleomagnetist therefore seeks to identify this remanent magnetization.

Measurement with a magnetometer

The paleomagnetist is mainly interested in the direction of the magnetization, relative to the geographic reference frame of the sampling site. The sample collected (a long cylinder of rock) is marked using a system that allows the exact direction of the magnetic arrow in space at the sampling site to be recorded.
The intensity and sense of the preserved magnetization are measured with a magnetometer.
The sample is positioned in its reference frame, that is, in the same position as its sampling site. A machine will spin it rapidly inside shielded coils, isolated from the external magnetic field. A deviation is observed in the measuring instrument. The 3 components of the magnetization are recorded (direction, sense, intensity).

The Curie temperature

Next, scientists heat the sample to erase all the natural thermoremanent magnetization. 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 they recorded.
This manipulation makes it possible to know the 3 parameters of the magnetic field at the moment the rock cooled.

Reversal of Earth's magnetic field over time

Recording in the oceanic crust

Thanks to paleomagnetism, scientists have discovered that many reversals of Earth's magnetic field have succeeded one another over the course of Earth's history.

The reversal of Earth's magnetic field is recorded at mid-ocean 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 away from the ridge on each side, and the direction of the ambient magnetic field at the time of formation is retained. In other words, when magma cools and solidifies to form oceanic crust, ferromagnetic minerals align with the ambient Earth's magnetic field at the moment of their solidification. This magnetizes the oceanic crust, recording the orientation of the magnetic field at that precise place and at that precise moment.

Magnetic anomalies

Thus, parallel bands of magnetized rocks form on each side of the central ridge of the mid-ocean ridge. These bands exhibit opposite magnetic polarities, forming what are called magnetic anomalies. These are interpreted as an alternation of blocks of normally and reversely 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.

Frequency of reversals

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

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

Reversal of the magnetic field in progress

Decrease in the dipole moment

Scientists carefully monitor Earth's magnetic field to detect any sign of significant change. Modern observations are made using satellites and geophysical instruments to map the magnetic field with precision.

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 Earth's core.

An imminent reversal?

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