The day the earth’s magnetic field almost disappeared!


The magnetic memory of some rocks allows us to reconstruct the evolution of the magnetic field over time. Although currently relatively strong, many studies show that in the Devonian, more than 360 million years ago, the magnetic field was, for some reason as yet, extremely weak.

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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 throughout the history of the planet. This variability is correlated to the instabilities that drive the outer core composed mainly of iron liquid and that create this geodynamic effect. If we are not yet able to predict and anticipate variations in the magnetic field, which appear completely random, scientists are still able to trace their evolution over time, thanks to the magnetic memory of some rocks.

Magmatic rocks, archives of the Earth’s magnetic field

Magmatic rocksin particular, they are able to record the intensity and direction of the magnetic field at the moment of their cooling and crystallization. In fact, when a magma cools down, goes from a so-called paramagnetic state (without magnetization clean), in a named state ferromagnetic (acquisition of correct magnetization). This transition occurs when the magma passes below the Curie temperature, the value of which is intrinsic to each mineral. So, as long as the magma is not crystallized, the minerals the ferromagnets contained in it are oriented according to the environmental magnetic field. When its temperature drops below the Curie temperature, the orientation of these minerals will freeze and thus maintain the characteristics of the current magnetic field. Unless the rock undergoes a new episode of melting that returns it to a paramagnetic state, it will retain this magnetic footprint, even if the environmental field subsequently changes. This is how scientists can go back in time, looking at the difference between the residual magnetization of ancient rocks and that induced by the current field. Over the past 180 million years we have been able to do this count that the Earth’s magnetic field has reversed about 300 times, without regularity.

The magnetic anomaly of the Devonian

Paleomagnetic studies are all the more precise when the rocks are young. The further you go back in time, the more the rocks are altered, the more likely they have been moved or have undergone a new episode of melting that would have then completely erased their previous magnetic memory. Our knowledge of the magnetic field of ancient times is therefore much less precise and the data much less constrained than those of recent times. A period questioned scientists for a long time. Is that of Devonian, or between 420 and 360 million years ago. The rocks of this geological period contain practically no trace of a magnetic field. Scientists have long thought that this specificity was linked to an erasure of the magnetic memory of the Devonian rocks. One of the reasons advanced is that the magnetic record was lost due to warming when the continental masses collided to form. the supercontinent Pangea. However, more and more studies seem to show that this 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 Earth Science Reviews, a team of researchers then took a closer look at these Devonian rocks. Their paleomagnetic signature appears to be extremely weak, even almost non-existent, despite the excellent conservation of the rocks. Therefore, the idea of ​​erasing the magnetic memory comes out.

A magnetic field that is too weak to be registered correctly

Other studies support the hypothesis that the Devonian magnetic field was very weak, in any case too weak to be correctly registered by the igneous rocks crystallizing in this moment. The Devonian is in fact marked by theemergency Plant. However, some studies suggest that these plants would have experienced abnormally high UV-B exposure 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 affect the development of life on Earth? Scientists still have no answers to these questions.

The study of anomalies very ancient magnetic fields, however, provide a large amount of information that allows us to better understand how and when the outer core was formed, but also to improve the models of the evolution of the magnetic field, with the aim of predicting variations one day . .

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