Geologists discover rocks with the oldest evidence yet of Earth’s magnetic field on Greenland

Amazing stuff! Why is Greenland called green land? The Vikings discovered a lush green land during the Medieval Warm Period (or global warming before the Industrial Age)!

"Geologists at MIT and Oxford University have uncovered ancient rocks in Greenland that bear the oldest remnants of Earth’s early magnetic field.

The rocks appear to be exceptionally pristine, having preserved their properties for billions of years. The researchers determined that the rocks are about 3.7 billion years old and retain signatures of a magnetic field with a strength of at least 15 microtesla. The ancient field is similar in magnitude to the Earth’s magnetic field today. ..."

From the abstract:

"Recovering ancient records of Earth's magnetic field is essential for determining the role of the magnetosphere in protecting early Earth from cosmic radiation and atmospheric escape. We present paleomagnetic field tests hinting that a record of Earth's 3.7-billion-year (Ga) old magnetic field may be preserved in the northeastern Isua Supracrustal Belt as a chemical remanent magnetization acquired during amphibolite-grade metamorphism in the banded iron formation. Multiple petrological and geochronological lines of evidence indicate that the northernmost part of Isua has not experienced metamorphic temperatures exceeding 380°C since the Eoarchean, suggesting the rocks have not been significantly heated since magnetization was acquired. We use “pseudo” baked contact tests (intrusions emplaced 3.26–3.5 Ga ago) and a fold test (folding 3.6 Ga ago) to demonstrate that some samples preserve a ca. 3.7 Ga record of the magnetic field. We recover a field strength of >15 µT. This suggests that Earth's magnetic field may have been weak enough to enhance atmospheric escape during the Archean."

Geologists discover rocks with the oldest evidence yet of Earth’s magnetic field | MIT News | Massachusetts Institute of Technology The 3.7 billion-year-old rocks may extend the magnetic field’s age by 200 million years.

Possible Eoarchean Records of the Geomagnetic Field Preserved in the Isua Supracrustal Belt, Southern West Greenland (open access)

[Researchers] stand on the outcrop of a banded iron formation containing the oldest records of Earth’s magnetic field. The Greenland ice sheet is in the background.

Fig. 7 Passed pseudo-baked contact tests for site 8A/A.