Magnetism in ancient crystals reveals when Earth’s inner core emerged

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Researchers were able to use an analysis of ancient rock crystals – and the records of magnetism locked inside them – to trace the history of Earth’s inner core over hundreds of millions of years.

The Earth’s core, a dense, hot mass of iron and nickel, is actually made up of two layers – the solid inner core packed inside a molten outer core. Next comes the rocky mantle (the thickest of all layers) and the crust on which we all live. We’re talking about 2,900 kilometers (about 1,800 miles) underground.

Based on the researchers’ findings, it appears that Earth’s inner core was crystallizing into a significantly large mass around 550 million years ago. This crystallization provided enough heat to restore the magnetic field – which had been depleted about 15 million years earlier – and set the stage for a major burst of life.

Earth’s magnetic field, protecting life from harmful solar winds, is actually governed by the swirling liquid iron in the outer core. However, as this new study clearly shows, the solid iron-nickel alloy at the center also has a key role to play as an energy source.

“The inner core is extremely important,” says geophysicist John Tarduno of the University of Rochester in New York. “Just before the inner core started growing, the magnetic field was about to collapse, but as soon as the inner core started growing, the field regenerated.”

“This research really highlights the need for something like a growing inner core that maintains a magnetic field for the lifetime – several billion years – of a planet.”

Huge distances and hot temperatures make it nearly impossible to measure the Earth’s core, so scientists rely on crystals in the rock – in this case, feldspar crystals in anorthosite. These crystals act as very precise recorders of magnetism.

By comparing rocks dated 565 million years ago to rocks dated 532 million years ago, the team was able to see the change in magnetic strength – a dramatic return of the Earth’s magnetic field. The change took tens of millions of years, but it’s relatively fast in terms of geological time scales.

Research-based thermal models suggest that the structure of the inner core changed around 450 million years ago, creating a boundary between the innermost and outermost inner core. Changes in the coat also correspond to these times.

“Because we limited the age of the inner core more precisely, we were able to explore the fact that the current inner core is actually made up of two parts,” says Tarduno.

“Plate tectonic movements on the Earth’s surface have indirectly affected the inner core, and the history of these movements is imprinted deep within the Earth in the structure of the inner core.”

Learning more about how the inner core evolved to its current state can also teach us how it might change again in the future – as well as give us a point of comparison to use when studying other planets.

Just take a look at Mars to see what would have happened if the inner core hadn’t grown and provided the necessary impetus for Earth’s magnetic field to be strong enough to fend off harmful solar radiation from the surface.

Without a global magnetic field to protect it, the Martian atmosphere has been stripped away by solar winds over billions of years – and at the same time taking away the water and oxygen necessary for life to thrive properly.

“The Earth would certainly have lost a lot more water if the Earth’s magnetic field had not been regenerated,” says Tarduno. “The planet would be much drier and very different from today’s planet.”

The research has been published in Nature Communication.

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