Ancient Greenland rocks preserve traces of the Earth’s magnetic field | Polarjournal
The scientists behind the recently published study standing on a banded iron formation in the Isua region of Greenland. This geological site, known as the Isua supracrustal belt, is 35 kilometers long and is bordered by the ice cap. Photo : Claire Nichols

3.7 billion years ago, the Earth may already have had a magnetic field, according to a study published this week based on ancient rocks from Greenland.

Scientists have discovered the oldest evidence of the Earth’s magnetic field in rocks from Greenland. These 3.7-billion-year-old rocks, located in the Isua region near Nuuk in south-western Greenland, have preserved the signature of a magnetic field with a strength of at least 15 microtesla, a magnitude comparable to that of the Earth’s current magnetic field.

This discovery, made by scientists from Oxford University and the Massachusetts Institute of Technology (MIT) and published on Wednesday in Journal of Geophysical Research is one of the first proofs of our planet’s magnetic field, and could help determine the conditions that enabled life on Earth to emerge.

Without a magnetic field, no life could have arisen on our planet. In fact, the magnetic field is a veritable shield that protects us from solar winds and cosmic rays. “The magnetic field is, in theory, one of the reasons we think Earth is really unique as a habitable planet,” explains Claire Nichols, associate professor of the geology of planetary processes at Oxford University, in a press release published by the MIT on Wednesday. “It’s thought our magnetic field protects us from harmful radiation from space, and also helps us to have oceans and atmospheres that can be stable for long periods of time.”

Banded iron formation. This type of rock appears as bands rich in iron and silica, and contains iron oxide minerals. These can act as tiny magnets that orient themselves with any external magnetic field. Thanks to uranium-lead dating carried out at MIT, the age of the rocks collected was estimated at 3.7 billion years. Then, a step-by-step process of demagnetizing and remagnetizing the rocks determined that they harbored traces of a 15 microtesla magnetic field 3.7 billion years old. Photo : Claire Nichols

Until now, our magnetic field was thought to have existed for less than 3.5 billion years. The present discovery adds another 200 million years. “That’s important because that’s the time when we think life was emerging,” explains Benjamin P. Weiss, professor at MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “If the Earth’s magnetic field was around a few hundred million years earlier, it could have played a critical role in making the planet habitable.”

Our planet’s magnetic field is powered by its central core essentially composed of iron. In a convection movement, the core generates electric currents, which in turn generate the Earth’s magnetic field.

But how can we determine when this magnetic shield appeared? That’s where the rocks come in. During their formation over millions of years, grains of sediment and minerals accumulated, compacted and buried beneath subsequent deposits. If the deposits contain magnetic minerals, such as iron oxides, they will follow the attraction of the Earth’s magnetic field during their formation, leaving the imprint of the magnetic field in the rocks.

All that is left to do is to find preserved rocks that have not been altered by heat. A difficult exercise: “Recovering ancient records of Earth’s magnetic field is challenging because the magnetization in rocks is often reset by heating during tectonic burial over their long and complex geological histories.”, note the authors in their study published this Wednesday.

However, there are places on Earth where preserved rocks offer crucial information about our planet and its development. This is precisely the case for Greenland and its 35km-long, 3.7 billion-year-old Isua supracrustal belt. It’s a place well known to geologists, and has already yielded information about what our planet looked like in its youth. In 2021, a study revealed that Isua rocks had established that the Earth was covered, several billion years ago, by a vast ocean of molten magma.

Link to the study: Claire I. O. Nichols, Benjamin P. Weiss, Athena Eyster, Craig R. Martin, Adam C. Maloof, Nigel M. Kelly, Mike J. Zawaski, Stephen J. Mojzsis, E. Bruce Watson, Daniele J. Cherniak. Possible Eoarchean Records of the Geomagnetic Field Preserved in the Isua Supracrustal Belt, Southern West Greenland. Journal of Geophysical Research: Solid Earth, 2024; 129 (4) DOI: 10.1029/2023JB027706

Mirjana Binggeli, Polar Journal AG

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