Earth’s magnetic field extends tens of thousands of kilometres into space
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Two vast, mysterious blobs of hot rock around Earth’s core may have been instrumental in producing Earth’s magnetic field and caused it to be slightly wonky for millions of years.
Scientists have known for decades about two peculiar continent-sized chunks of rock, one beneath Africa and the other under the Pacific Ocean. These blobs, which extend nearly 1000 kilometres from the outer core to the rocky mantle above, must be different from their surroundings because seismic waves travel through them more slowly. But as it is difficult to measure them due to their depth, scientists can’t identify exactly how they differ.
Andrew Biggin at the University of Liverpool, UK, and his colleagues looked to Earth’s magnetic field for clues. This field has been generated for billions of years by the churning of molten iron within our planet’s core. It extends tens of thousands of kilometres into space, protecting us from 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 cooler regions around it. Biggin and his team theorised that by studying how the magnetic field has changed, they could learn about how heat has moved through Earth’s core.
The researchers collated records of ancient volcanic rocks that have preserved the direction of Earth’s magnetic field at several different points over the past tens or hundreds of millions of years, to gather a picture of how Earth’s magnetic field has changed over time. Then, they ran simulations of how heat flowing through the planet’s core and mantle produced a magnetic field, for scenarios both with and without the giant blobs of hot rock, and compared it with the real magnetic field readings.
They found that the simulation with the blobs of rock best matched the ancient magnetic data. “These simulations of the convection that’s happening in the core, that’s generating the magnetic field, can reproduce some of the salient features of the [magnetic] field, but only when you impose this strong heterogeneity in the amount of heat that’s flowing out of the top of the core,” says Biggin.
In other words, these regions have probably been much hotter than the regions around them for hundreds of millions of years, and caused heat flow between the core and the mantle to decrease. This different heat flow would have helped produce and stabilise Earth’s magnetic field, according to the team’s simulations.
Most geologists assume that, over millions of years, Earth’s magnetic field has been essentially symmetrical, similar to a bar magnet used in a compass. But Biggin and his team also found that the ancient magnetic field wasn’t symmetrical, on average, and contained systematic deviations that persisted over millions of years, which also appear to be a result of these blobs of rocks. 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, says Biggin.
If the team’s findings are correct, then the temperature difference found in the blobs might also exist at points in Earth’s uppermost outer core, which could be detectable via seismic waves, says Biggin.
But this would be extremely difficult to capture, says Sanne Cottaar at the University of Cambridge. “I have my doubts,” she says. “It’s very challenging for us to map variations within the core, given we have to look through so much mantle material before we see it.”
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