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Quartz Could Be The Solution to Why Our Magnetic Field Is So Old

Solving the magnetic field paradox.

MIKE MCRAE
23 FEB 2017
 

A mystery concerning the age of our planet's magnetic field could now be solved, thanks to evidence that Earth's core produces crystals of silicon dioxide – better known as quartz.

This suggests that shifting chemistry within the outer core's liquid rock mixture is affecting how it flows, and if the evidence holds up, it could give us a better understanding of the conditions of our infant Solar System, and the inner workings of our planet in its earliest moments.

 

Our planet's surrounding geomagnetic field is the result of charged particles moving with the flow of molten rock rising away from a heated core – a core that's likely to be just a billion or so years old.

Yet previous studies have also found evidence of a magnetic field as far back as 4 billion years ago - creating something of a paradox.

According to research by Kei Hirose from the Tokyo Institute of Technology in Japan, the formation of tiny crystals of quartz deep beneath our feet could help resolve this conflict by changing how we model the buoyancy of our planet's deep, molten rock.

So what's wrong with Earth's core as it is? The problem is known as the New Core Paradox, which describes a mismatch between what we think we know about our planet's guts, and the apparent age of its magnetic field.

Part of the problem is based on how heat from the inner core drives currents of gooey rock up from the outer core towards the crust.

Computer models developed in 2012 indicated that heat is conducted through the inner core's iron at about 150 watts per metre per kelvin, dissipating too quickly to have kick-started convection currents in the surrounding molten rock.

 

Something's gotta give – either our magnetic field is in fact fairly new; the heat-generating inner core is a lot more than a billion years old; or something else helped get the necessary convection currents moving 4 billion years ago.

Some solutions to the New Core Paradox have tried to explain how Earth's core could have conducted heat in ways we we didn't expect. But Hirose took a different approach. Instead, he looked to the core's chemistry.

While the inner core is thought to be mostly made of iron, there's probably a scattering of other elements, including about 5 percent nickel, 2 percent silicon, and 5 percent oxygen.

"In the past, most research on iron alloys in the core has focused only on the iron and a single alloy," says Hirose. "But in these experiments we decided to combine two different alloys containing silicon and oxygen, which we strongly believe exist in the core."

It's not clear how such elements might come together to form alloys and other compounds under such conditions, so Hirose and his team used precision cut diamonds to squeeze microscopic amounts of iron, silicon, and oxygen to core-like pressures.

They then hit the samples with lasers to achieve temperatures of up to 4,000 degrees Celsius (7,232 Fahrenheit), which caused the material to melt in some parts, while crystallising into quartz in others.

If this process takes place in the core as hypothesised, crystallisation of silicon and oxygen would change the overall composition of the core material, giving it more buoyancy and allowing it to rise.

In other words, the slow rise and fall of molten rock in a young Earth could have been assisted by quartz pulling silicon and oxygen from the mixture billions of years ago.

"This result proved important for understanding the energetics and evolution of the core," says team member John Hernlund.

"We were excited because our calculations showed that crystallisation of silicon dioxide crystals from the core could provide an immense new energy source for powering the Earth's magnetic field."

But silicon dioxide isn't the only contender for this chemically aided model of convection - a contending model argues that it isn't quartz that settled out of the molten mix of an ancient core, but magnesium oxide.

According to geophysicist David Stevenson from the California Institute of Technology in Pasadena, magnesium oxide would precipitate out of the molten solution before silicon dioxide, therefore it "makes much more sense".

Hirose and his team maintain that silicon dioxide is more likely, however, stating in their recently published paper that the temperatures inside a newly formed planet Earth would need to be a lot hotter for magnesium to be inside the core.

Whoever is right, understanding the role chemistry plays in driving geomagnetic processes can help us better understand how an early Earth generated a magnetic field billions of years before expected.

This paper was published in Nature.

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