Ancient rocks hold proof of Earth’s magnetic field
A 3.7-billion-year-old record of our planet’s ancient magnetism has been unearthed, providing evidence that Earth’s magnetic field already existed very early in history. But this discovery is quite surprising. Rocks approaching 4 billion years old are hard to find – most have been recycled through Earth’s tectonic activity, slipping into the mantle through subduction zones before being belched back out via volcanoes.
Yet, somehow, a sequence of rocks in the Isua Greenstone Belt in Greenland has survived the ravages of time thanks to its unique geology, situated on top of a thick continental plate like a life raft amid an ocean of tectonic upheaval.
Researchers from the University of Oxford and the Massachusetts Institute of Technology have dug up some of those Isua rocks, finding that they contain an ironclad record of the early Earth’s magnetic field. According to this, our planet’s magnetic field doesn’t seem to have changed very much in the intervening time – but geologists don’t fully understand how Earth could have produced a magnetic field at all back then.
The existence of a magnetic field is crucial for the development of life on Earth, with field lines warding off the hazardous sleet of charged particles blown towards us via the solar wind. The existence of an early magnetic field could have thus helped life get a foothold on our planet. Previously, estimates and hints of the early Earth’s magnetic field have come from individual mineral crystals called zircons found within ancient rocks from Western Australia. These had suggested the existence of a magnetic field 4.2 billion years ago. However, those results were subsequently doubted as unreliable.
The new results from the Greenland rocks are considered more reliable because, for the first time, they are based on entire iron-bearing rocks to derive the primordial field strength. Therefore, the sample offers the first solid measure of not only the strength of Earth’s ancient magnetic field, but also of the timing of when the magnetic field originally appeared.
“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,” said lead researcher Claire Nichols, a professor of planetary geology at the University of Oxford. “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.”