The Earth’s oldest rocks bear traces of a global magnetic field that was already present more than 3 billion years ago. Yet at that time, the depths of the Earth’s mantle were still liquid, forming an ocean of magma several hundred kilometers thick. Its gradual solidification (until it became the solid mantle we know today) released a large amount of heat, preventing the metallic core from cooling through convection—a process that is necessary for the generation of a magnetic field.

Conductivity too low to generate a strong field

The results show that, based on current knowledge, magma convection velocities would be too low. To produce a magnetic field comparable to that recorded in ancient rocks (4 billion years old), the magma’s electrical conductivity or heat flux would have to be much higher than those assumed for a realistic magma.

A Mystery That Remains Unsolved

This study reignites the debate over the origin of the Earth’s primitive magnetic field. If the basal magma ocean alone cannot explain the magnetism of the oldest rocks, other scenarios will have to be considered. The mystery of the Earth’s primitive magnetism therefore remains unsolved.