An ocean of magma at the origin of the Earth’s ancient magnetic field ?

Published in PNAS, a study involving scientists from the Institute of Earth Sciences (ISTerre-OSUG, CNRS / IRD / UGA / Univ. Savoie Mont-Blanc / Univ. Gustave Eiffel) explores the hypothesis that the Earth’s primitive magnetic field may have originated in the subterranean magma ocean that surrounded the core at the base of the mantle. Using high-resolution numerical simulations carried out on French supercomputers, the researchers show that the electrical conductivity of such an ocean would be insufficient to explain the magnetic field recorded in ancient rocks. The mystery of the origin of Earth’s magnetism therefore remains unsolved.

The Earth’s oldest rocks preserve evidence of a global magnetic field that was already present more than 3 billion years ago. Yet at that time, the deep mantle was still molten, forming a magma ocean several hundred kilometers thick. As it gradually solidified (eventually becoming the solid mantle we know today), it released a large amount of heat, preventing the metallic core from cooling by convection, a process that is nevertheless necessary for generating a magnetic field.

A numerical dive into the primordial magma ocean

To test the idea that the magnetic field could have formed directly in this basal magma ocean, the researchers carried out several high-resolution numerical simulations using the latest French GPU-based supercomputers. These studies make it possible to estimate the electrical conductivity required for a magma ocean to generate a strong dipolar field.

Conductivity too low to generate a strong field

The results reveal that, according to current knowledge, the convective velocities of the magma would be too low. To produce a magnetic field comparable to that recorded in ancient rocks (4 billion years ago), the electrical conductivity of the magma or the heat flux would need to be much higher than what is considered realistic for magma.

A mystery still unsolved

This study reignites the debate on the origin of the Earth’s primitive magnetic field. If the basal magma ocean alone cannot account for the magnetism recorded in the oldest rocks, other scenarios will need to be considered. The mystery of Earth’s early magnetism therefore remains unsolved.

Reference :

Energetically expensive dynamo action in Earth’s basal magma ocean. Nathanaël Schaeffer , Stéphane Labrosse, and Jonathan M. Aurnou. PNAS. November 3, 2025, 122 (45) e2507575122
DOI : 10.1073/pnas.2507575122

This news was originally published by CNRS-INSU.