Rocks hold records of Earth’s magnetic history

The history of the Earth’s magnetic field is indelibly written in vortex-like structures inside grains of the iron oxide magnetite, a new study has shown.

Researchers in the UK and Germany used electron holography to map the magnetic properties of individual grains of the material.

They say it preserves magnetic information unaltered even when exposed to large temperature fluctuations.

The imaging technique, for the first time, maps the magnetic vortices while they are heated to 550 °C – just short of the rock’s Curie point, the temperature at which it loses its permanent magnetic properties.

In effect, this is a record of the Earth’s magnetic field at the time of the rock’s formation.

That could give us a detailed picture of the evolution of the Earth’s magnetic field and improve our understanding of the planet’s core and plate tectonics.

By being able to see the orientation of magnetic poles within rocks at the time of their formation, it will show how continents moved relative to each other, the researchers say.

“These remarkable images show that vortex structures in natural magnetic systems can hold information about how the Earth’s inner structure evolved,” says Wyn Williams from the University of Edinburgh, who led the study.

“This is a game-changer in our understanding of rocks’ ability to act as reliable magnetic recorders, and helps us see a little clearer into Earth’s history.”

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A visualisation of the thermomagnetic behaviour of a small grain of magnetite. Image A shows an electron microscopic image of the grain, while the other images are Magnetic induction maps reconstructed from holograms taken at various temperatures. – Trevor P. Almeida et al/Science Advances

As the name suggests, magnetite is the most magnetic natural mineral there is. It is a commonly occurring oxide of iron with the formula Fe3O4.

The geomagnetic imprint is formed as molten lava cools and magnetite grains align with the Earth’s magnetic field.

Another author, Trevor Almeida, of Imperial College London, says only a small portion of naturally occurring magnetite held stable magnetic structures.

“However, far more common are tiny magnetic vortices, and their stability could not be demonstrated until now,” he says.

“Magnetite rocks, which carry signs of temperature fluctuations, are indeed a reliable source of information about the history of the Earth.”

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