Simulations indicate that two massive hot rock blobs near Earth's core have played a role in generating and shaping its magnetic field, making it irregular over millions of years. Researchers analyzed ancient volcanic rocks and ran models to reach this conclusion. The findings suggest these blobs created uneven heat flow that affected the field's symmetry.
Earth's magnetic field, which shields the planet from solar wind and cosmic radiation, arises from the churning of molten iron in its core. Scientists have long identified two continent-sized blobs of hot rock—one beneath Africa and the other under the Pacific Ocean—that extend nearly 1000 kilometers from the outer core into the mantle. These structures slow seismic waves, indicating they differ from surrounding material, though their exact composition remains unclear due to their depth.
Andrew Biggin at the University of Liverpool, UK, and his team investigated the magnetic field's history for insights into heat flow. They compiled data from ancient volcanic rocks that recorded the field's direction over tens to hundreds of millions of years. Using this, the researchers simulated core-mantle convection with and without the blobs, comparing results to historical records.
The models including the blobs matched the data best. "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," Biggin explained. The blobs appear to have been hotter than nearby regions for hundreds of millions of years, reducing heat flow between core and mantle and helping stabilize the field.
Contrary to assumptions of a symmetrical field like a bar magnet, the ancient field showed persistent deviations over millions of years, linked to these blobs. This could refine calculations of ancient rock movements and reveal changes in Earth's interior. The team suggests temperature differences from the blobs might extend into the uppermost outer core, potentially detectable by seismic waves.
Sanne Cottaar at the University of Cambridge expressed skepticism about such detection. "I have my doubts," she said. "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." The study appears in Nature Geoscience (DOI: 10.1038/s41561-025-01910-1).