The magnetic memory of certain rocks makes it possible to reconstruct the evolution of the magnetic field over time. While currently relatively strong, many studies show that in the Devonian, more than 360 million years ago, the magnetic field was, for some still unknown reason, extremely weak.
The magnetic field is one of the key elements of the Earth system. It protects us from dangerous solar radiation.
Despite its constancy, the magnetic field has undergone intense variations in intensity and direction during the history of the planet. This variability is relatedcomposed mainly of iron and which create this geodynamo effect. If we are not yet able to predict and anticipate variations in the magnetic field, which appear to be totally random, scientists are however able to trace its evolution over time, thanks to the magnetic memory of some rocks.
Magmatic rocks, archives of the Earth’s magnetic field
, in particular, are able to record the intensity and the direction of the magnetic field at the time of their cooling and their crystallization. Indeed, when a cools, it passes from a so-called paramagnetic state (without clean), to a state called (acquisition of a proper magnetization). This transition takes place when the magma passes below the Curie temperature, the value of which is intrinsic to each . Thus, as long as is not crystallized, the ferromagnets it contains are oriented according to the ambient magnetic field. When its temperature drops below the Curie temperature, the orientation of these minerals will freeze and thus retain the characteristics of the current magnetic field. Unless the rock undergoes a new episode of fusion which would return it to a paramagnetic state, it will keep this magnetic imprint, even if the ambient field subsequently changes. This is how scientists can go back in time, observing the difference between the remanent magnetization of ancient rocks and that induced by the current field. Over the last 180 million years, we have been able to .
The Devonian Magnetic Anomaly
Paleomagnetic studies are all the more precise as the rocks are young. The more we go back in time, the more the rocks are altered, are more likely to have been moved or to have undergone a new episode of melting which would then have completely erased their previous magnetic memory. Our knowledge of the magnetic field of ancient periods is thus much less precise and the data much less constrained than that of recent periods. A period has long been questioning scientists. It is that of, or between 420 and 360 million years ago. The rocks of this geological period contain virtually no trace of a magnetic field. For a long time, scientists thought that this specificity was linked to an erasure of the magnetic memory of Devonian rocks. One of the reasons put forward is that the magnetic record was lost by heating when the continental masses collided to form . However, more and more studies seem to show that it would not be a loss of magnetic information, but the recording of an exceptionally weak field at that time.
In a new study, published in the journal, a team of researchers therefore took a closer look at these rocks dating from the Devonian. It appears that their paleomagnetic signature is extremely weak, even almost non-existent, despite very good preservation of the rocks. Exit, therefore, the idea of erasing the magnetic memory.
A magnetic field too weak to be properly recorded
Other studies confirm the hypothesis that the Devonian magnetic field was very weak, too weak in any case to be correctly recorded by thecrystallizing at this time. The Devonian is indeed marked by the Plant. However, some studies suggest that these plants would have undergone an abnormally high exposure to UV-B at the end of the Devonian period. This implies that the magnetic field at the time was not strong enough to stop this type of radiation.
Why was the magnetic field so weak at that time and how did this influence the development of life on Earth? Scientists do not yet have answers to these questions.
The study ofvery old magnetic fields, however, provides a great deal of information which makes it possible to better understand how and when the outer core was formed, but also to improve models of the evolution of the magnetic field, with the aim of one day predicting its variations. .