Look! 3.7 Billion-Year-Old Rocks Reveal The Oldest Evidence of Earth’s Magnetic Field

Our planet was a different place when these rocks formed.

NASA's Scientific Visualization Studio

Rocks in West Greenland resemble cake marbled with chocolate and vanilla. They’re now the oldest evidence of Earth’s magnetic field.

Isua Supracrustal Belt rocks contain an alternating banded iron formation (BIF), which gives them their signature marbled look. About 3.7 billion years ago, the rocks reached a temperature of 550°C. At such a high temperature, iron then had an opportunity to align itself with Earth’s magnetic field. They never encountered such a temperature since, narrowly avoiding getting destroyed by two major subsequent events. In a new paper published Wednesday in the Journal of Geophysical Research, scientists learned that Earth has been protecting us for eons. But, there’s new questions, too.

Why is Earth Magnetic?

Earth’s magnetic field is a precious phenomenon. Liquid iron flowing inside our planet’s outer core interacts with the high temperature surrounding it, creating a powerful magnetic field crucial to life on Earth.

An example of the 3.7 billion year old banded iron formation.

Claire Nichols

The magnetic field shields the surface from dangerous cosmic-ray bombardments from outer space. Like a mega-sized saran wrap, the magnetic field also keeps the atmosphere from evaporating away into the ether.

“The preservation of a temperate climate and liquid water on early Earth depends critically upon the strength of the magnetosphere,” the researchers wrote in the paper.

The new work offers a salve against worry. Their work suggests Earth has sustained a magnetic field since at least 3.7 billion years ago, the age of the cake-looking BIF rocks. This places the rocks in the Eoarchean, when the first records of Earth’s primitive atmosphere and oceans emerge.

An illustration of Earth’s magnetic field. Solar material and energy gets funneled towards the poles where it can create auroras.

NASA's Goddard Space Flight Center

The magnetic field is not steady, however. The poles have reversed several times in Earth’s history. Its strength wanes sometimes, too. These are called excursions. Rock records have revealed that cosmic rays can penetrate down the surface more often during these vulnerable chapters — although fortunately, no extinction or biodiversity drop has correlated with excursions in the fossil record.

But they’re evidence that Earth’s heart is constantly changing. Learning how it has changed depends on how well rocks, called part of the paleomagnetic record, have survived the passage of time.

Often, the magnetization of rocks — when iron forms bands that match the magnetic field — can reset. This can happen when rocks get buried during tectonic activity, and heat up.

Study co-author Athena Eyster stands in front of a large exposure of banded iron formation.

Claire Nichols

The researchers found that the Eoarchean rocks were not reset about a billion years later during the Neoarchean, when intense volcanism churned in the oceans. The rocks also survived the Proterozoic, when crustal recycling increased.

The West Greenland rocks flesh out more of our magnetic field’s long history. In the paper, the researchers suggest a consequence of the work: learning when the magnetic field allowed hydrogen to escape the atmosphere, “eventually culminating” in the Great Oxidation Event. This caused a major extinction event, and eventually, life forms had to evolve to breathe oxygen to survive.

Understanding the inner workings of Earth’s interior exposes just how distinctive and precious our planet is.

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