Huge hot blobs inside the Earth may have dampened its magnetic field

Two huge, mysterious blobs of hot rock around Earth’s core may have been instrumental in producing Earth’s magnetic field and causing it to go a bit naughty for millions of years.

Scientists have known for decades about two distinctive continent-sized chunks of rock, one under Africa and the other under the Pacific Ocean. These spots, which extend nearly 1,000 kilometers from the outer core to the rocky mantle above, must be different from their surroundings because seismic waves travel more slowly through them. But since it’s difficult to measure them because of their depth, scientists can’t identify exactly how they differ.

Andrew Biggin of the University of Liverpool, UK, and his colleagues looked at Earth’s magnetic field for clues. This field has been generated for billions of years by the core of molten iron in our planet’s core. It stretches tens of thousands of kilometers into space, and protects 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 theorized that by studying how the magnetic field has changed, they could learn about how heat has moved through the Earth’s core.

The researchers gathered 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 gather 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 produced a magnetic field, for scenarios both with and without the giant blobs of hot rock, and compared that to the real magnetic field readings.

They found that the simulation with the rock blobs best matched the ancient magnetic data. “These simulations of the convection that happens in the core, which generates 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 flowing out of the top of the core,” says Biggin.

In other words, these areas have probably been much hotter than the areas around them for hundreds of millions of years, causing the heat flow between the core and the mantle to decrease. This different heat flow would have helped produce and stabilize Earth’s magnetic field, according to the team’s simulations.

Most geologists assume that over millions of years the Earth’s magnetic field has been mainly symmetrical, similar to a bar magnet used in a compass. But Biggin and his team also found that the ancient magnetic field was, on average, not symmetric and contained systematic deviations 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 the Earth’s deep structures have changed over time, says Biggin.

If the team’s findings are correct, the temperature difference found in the blobs could also exist at points in the Earth’s uppermost outer core, which could be detected via seismic waves, Biggin says.

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, since we have to see through so much mantle material before we see it.”