Exploring Earth's deep interior is a far bigger challenge than exploring the solar system. While we have travelled 25 billion km into space, the deepest we have ever gone below our feet is just over 12 km.
Consequently, little is known about the conditions at the base of the mantle and the top of the core - the most significant interface in the Earth's interior and the region where new research has now uncovered exciting magnetic activity.
In a study published in Nature Geoscience, research led by the University of Liverpool has identified magnetic evidence that two immense, ultra-hot rock structures located at the base of Earth's mantle, around 2,900 kilometres beneath Africa and the Pacific, affect the underlying liquid outer core.
The study shows that these enormous blobs of solid, superheated material - encircled by a pole-to-pole ring of cooler rock - have been shaping Earth's magnetic field for millions of years.
Both measuring ancient magnetic fields and simulating the processes that generate them are technically demanding.
To investigate these deep-Earth features, the research team combined palaeomagnetic observations with advanced computer simulations of the geodynamo - the flow of liquid iron in the outer core that generates Earth's magnetic field like a wind-turbine generates electricity.
Numerical models enabled them to reconstruct key observations of the behaviour of the magnetic field seen over the past 265 million years. Even with a supercomputer, running such simulations, especially over long timescales, represents an immense computational challenge.
The results revealed that the outer core's upper boundary is far from uniform in temperature. Instead, it displays strong thermal contrasts, with localised hot regions capped by the continent-sized rock structures.
It also showed that some parts of the magnetic field appear to have remained relatively stable for hundreds of millions of years, while others have changed significantly through time.
Andy Biggin, Professor of Geomagnetism at the University of Liverpool, said: "These findings suggest that there are strong temperature contrasts in the rocky mantle just above the core and that, beneath the hotter regions, the liquid iron in the core may stagnate rather than participate in the vigorous flow seen beneath the cooler regions.
"Gaining such insights into the deep Earth on very long timescales strengthens the case for using records of the ancient magnetic field to understand both the dynamic evolution of the deep Earth and its more stable properties.
"These findings also have important implications for questions surrounding ancient continental configurations-such as the formation and breakup of Pangaea-and may help resolve long-standing uncertainties in ancient climate, palaeobiology, and the formation of natural resources. These areas have assumed that Earth's magnetic field, when averaged over long periods, behaved as a perfect bar magnet aligned with the planet's rotational axis. Our findings are that this may not quite be true"
The paper, 'Mantle heterogeneity influenced Earth's ancient magnetic field' is published in Nature Geosciences (DOI: 10.1038/s41561-025-01910-1)
The study was conducted by scientists from the DEEP (Determining Earth Evolution using Palaeomagnetism) research group in the University of Liverpool's School of Environmental Sciences working alongside researchers from the University of Leeds.
Professor Biggin and his team specialise in analysing the magnetic signatures preserved in rocks from around the world to reconstruct the history of Earth's magnetic field and internal dynamics.
DEEP was established in 2017 with support from the Leverhulme Trust and the Natural Environment Research Council (NERC).
Image shows simulated maps of Earth's magnetic field (left) can only be made to look like those of the real field (right) if Earth's core is assumed to have hot blobs of rock sitting directly on top of it.
