Huge hot blobs inside Earth may have caused its magnetic field to crackle

Two huge, mysterious blobs of hot rock around Earth’s core may have helped create Earth’s magnetic field, causing it to be slightly unsettled for millions of years.

For decades, scientists have known about two strange continent-sized chunks of rock, one under Africa and the other under the Pacific Ocean. Stretching nearly 1,000 kilometers from the outer core to the rocky mantle above, these spheres must be different from their surroundings because seismic waves travel more slowly through them. But because they’re difficult to measure because of their depth, scientists can’t identify exactly how they differ.

Andrew Biggin at the University of Liverpool in Great Britain and his colleagues looked for clues in the Earth’s magnetic field. This field has been generated over billions of years by the swirling of molten iron at the core of our planet. It stretches tens of thousands of kilometers into space and protects us from the solar wind and cosmic radiation.

The exact shape and form of this magnetic field is determined by the amount of energy in the form of heat that moves from the hot core to the cooler regions around it. Biggin and his team theorized that by studying how the magnetic field changed, they could learn about how heat moved through the Earth’s core.

Scientists have collected records of ancient volcanic rocks that have preserved the direction of the Earth’s magnetic field at several different points over the past tens or hundreds of millions of years to get a picture of how the Earth’s magnetic field has changed over time. They then ran simulations of how heat flowing through the planet’s core and mantle created the magnetic field, for scenarios with and without giant balls of hot rock, and compared them to actual magnetic field values.

They found that the stone ball simulation best matched the ancient magnetic data. “These simulations of the convection that takes place in the core, which generates the magnetic field, can reproduce some of the significant features [magnetic] field, but only if you introduce this strong heterogeneity in the amount of heat that flows from the top of the core,” says Biggin.

In other words, these regions were likely much hotter than the regions around them for hundreds of millions of years, causing the heat flow between the core and the mantle to decrease. This differential heat flow would help create and create Earth’s magnetic field, according to the team’s simulations.

Most geologists assume that for millions of years the Earth’s magnetic field was essentially symmetrical, much like the bar magnet used in a compass. But Biggin and his team also found that the ancient magnetic field was not symmetric on average and contained systematic variations that persisted over millions of years, which also appear to be the result of these rock blobs. This could have implications for how geologists calculate the movement of ancient rocks and tell us about how Earth’s deep structures have changed over time, Biggin says.

If the team’s findings are correct, then the temperature difference found in the droplets may also exist at points in Earth’s uppermost outer core that could be detectable through seismic waves, Biggin says.

But that would be extremely difficult to capture, he says Sanne Cottaar at the University of Cambridge. “I have my doubts,” he says. “It’s very challenging for us to map the variations inside the core because we have to examine so much mantle material before we see it.”

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