The Earth's Magnetic Field Is At Least 3.7-Billion-Years Old, New Evidence Shows

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The imprint of the field from that time looks a lot like the one we know today.

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Banded iron formations in 3.7-billion-year-old rocks from the Isua Supracrustal Belt in Greenland have been distorted but not destroyed by the passage of time.

Image Credit: Claire Nichols

The oldest clear evidence for the Earth's magnetic field shows that it had formed well within the first billion years of the planet's existence. The new finding takes us closer to answering the important question of whether the Earth's magnetic field dates to its formation and if not, how soon afterward it appeared.

The magnetic field that surrounds Earth, and partially protects us from solar radiation, is thought to have been essential to life's formation and survival. Although we know giant planets like Jupiter can have mighty magnetic fields, there is much debate about how common they are for rocky planets. If planet-wide magnetic fields are rare or typically weak, it might represent a limitation to the widespread appearance of life, or at least advanced life, in the galaxy.

This has made the quest to discover the origins of Earth's magnetic field a priority for many geologists, but it's not an easy question to answer. Rocks can maintain evidence of the magnetic field in which they formed, but this can be lost or altered if they get hot enough to reorder iron particles within them. Now, however, clues have been found in some of the planet's oldest unaltered rocks.

The Isua Supracrustal Belt represents some of the first continental crust material to form, and parts of it have survived relatively unaffected by geologic processing ever since. Unfortunately for geologists, it lies in West Greenland, one of the hardest places on the planet for field research.

Lead author Claire Nichols and Tim Greenfield using rock-core drills to collect samples for analysis.

Image credit: Claire Nichols

As with many younger iron-rich rocks, banded iron formations in the Isua Belt reveal the direction, and to some extent the strength, of the magnetic field in which they were formed. Magnetite particles line up in the direction of the field like iron filings around a modern bar magnet. Lead author Professor Claire Nichols of the University of Oxford is confident these are original, rather than having been orientated later while cooling from an intense heat event.

"Extracting reliable records from rocks this old is extremely challenging, and it was really exciting to see primary magnetic signals begin to emerge when we analyzed these samples in the lab," Nichols said in an emailed statement to IFLScience. "This is a really important step forward as we try and determine the role of the ancient magnetic field when life on Earth was first emerging."

The banded iron formations ripple across the landscape, with study co-author Athena Eyster standing in front.

Image Credit: Claire Nichols

The age of the Earth's magnetic field remains under question in part because we don't fully understand what causes it today. We know it is a product of movements in the molten outer core, whose high iron content turns convection currents into a dynamo, and these currents in turn are produced by the solidification of the inner core. 

However, there are enough uncertain details that we can't be confident whether such movements took place before the solid core formed. The rocks Nichols and colleagues studied suggest that they did, since the inner core is almost certainly much younger, probably more recent than multicellular life. The findings have implications for how much heat escaped the Earth's core early on, which would have driven upwellings in the mantle and contributed to volcanic activity.

The northeastern part of the Isua Belt is unusual, perhaps unique, for rocks of this age in that it sits above continental crust thick enough to protect it from the sort of activity that has heated many other counterparts. Nichols and co-authors concluded the segment they studied reached 550°C (980°F) 3.69 billion years ago, during formation, but has not been above 380°C (720°F) since, making the magnetic particle alignment original.

The study is published in the Journal of Geophysical Research: Solid Earth.