Oldest undisputed evidence of Earth’s magnetic field found in Greenland

Researchers have recovered a 3.7-billion-year-old record of Earth’s magnetic field from Greenland, providing the oldest estimate of its strength derived from whole rock samples.

“This is a really important step forward as we try and determine the role of the ancient magnetic field when life on Earth was first emerging,” says lead researcher Claire Nichols.

Analysis estimates the planet’s magnetic field at the time appears to have been remarkably similar to the one surrounding Earth today, with a magnetic field strength of 15 microtesla, compared to the modern 30 microtesla.

The whole rock samples, which provide a more accurate and reliable assessment than previous studies which used individual crystals, are detailed in a paper appearing in the Journal of Geophysical Research.

A woman dressed in a yellow top stands in front of a rock formation. The rock is striped with veins of dark grey, orange, and white.
Co-author Athena Eyster standing in front of a large exposure of banded iron formation, the iron rich deposit from which ancient magnetic field signals were extracted. Credit: Claire Nichols.

“Extracting reliable records from rocks this old is extremely challenging, and it was really exciting to see primary magnetic signals begin to emerge when we analysed these samples in the lab,” says Nichols, an associate professor of the geology of planetary processes at the University of Oxford in the UK.

The magnetic field is essential to life on Earth – it’s like an invisible shield that protects us from being bombarded with harmful cosmic radiation by diverting most charged particles around the Earth.

It’s generated by electric currents that form from “geodynamo” convection currents in Earth’s liquid iron outer core. However, it’s believed the solid inner core didn’t’ form until about a billion years after Earth’s early formation, so many questions remain about how the early magnetic field was sustained.

Palaeogeologists can reconstruct information on the magnetic field in the past through some strongly magnetic minerals, particularly iron oxides, as both the field’s strength and direction can become imprinted in iron particles as they crystalise.

A photograph of dozens of small circular drill cores of rock of varying colour
Whole rock drill core samples from Isua, Greenland, were then measured in the MIT Paleomagentism Laboratory to extract their ancient record of Earth’s magnetic field. Credit: Claire Nichols.

However, reconstructing the magnetic field so far back in time comes with significant challenges because successive geological events that heat the rock can subsequently alter these preserved signals.

In this study, the research team examined core samples from the Isua Supracrustal Belt in Greenland, which sits atop a thick continental crust that protects it from extensive tectonic activity and deformation. This allowed the researchers to build evidence supporting the existence of the magnetic field 3.7 billion years ago.

The researchers say the results suggest the mechanism that drove Earth’s early dynamo was similarly efficient to the solidification process that generates Earth’s magnetic field today.

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