A 3.7 billion-year-old record of our planet’s ancient magnetism has been unearthed, providing evidence that The Earth’s magnetic field it already existed very early in history. This finding, however, is quite surprising.
Rocks approaching 4 billion years old are difficult to find; most were recycled Earthof tectonic activity, sliding into the mantle through subduction zones before being erupted again through volcanoes. Yet somehow, a sequence of rocks in Greenland’s Isua supracrustal belt has survived the ravages of time thanks to its unique geology, sitting atop a thick continental plate like a life raft amid an ocean of tectonic upheaval .
Now, researchers at the University of Oxford and the Massachusetts Institute of Technology have unearthed some of those Isua rocks, finding that they contain an ironclad record of the early Earth’s magnetic field. According to this data, our planet’s magnetic field does not appear to have changed much in the meantime, but geologists do not fully understand how the Earth could have produced a magnetic field at that time.
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The existence of a magnetic field is fundamental to the development of life on Earth, since field lines protect against the dangerous sleet of charged particles blown towards us through the solar wind. The existence of a primordial magnetic field may therefore have helped life gain a foothold on our planet.
Previously, estimates and clues to the early Earth’s magnetic field came from single mineral crystals called zircons found within ancient rocks in Western Australia. These had suggested the existence of a magnetic field 4.2 billion years ago. However, these results were later questioned as unreliable.
The new results from Greenland rocks are considered more reliable because, for the first time, they rely on whole iron-bearing rocks (rather than individual mineral crystals) to derive the strength of the primordial field. Thus, the sample offers the first solid measurement of not only the strength of Earth’s ancient magnetic field, but also when the magnetic field originally appeared.
“Extracting reliable data from such ancient rocks is extremely challenging, and it was really exciting to see primary magnetic signals start to emerge when we analyzed these samples in the lab,” said lead researcher Claire Nichols, professor of planetary geology at the University of Los Angeles. University of Oxford, in Press release. “This is a really important breakthrough as we try to determine the role of the ancient magnetic field when life on Earth was first emerging.”
The iron particles within the Isua rocks can be thought of as tiny magnets, aligning with Earth’s magnetic field when the rock around them first crystallized 3.7 billion years ago. Their alignment therefore holds a record for field strength. The intensity is estimated to have been at least 15 microtesla (mT), which is comparable to the Earth’s field strength of 30 mT today.
However, this still leaves the old puzzle unanswered: how did the primordial Earth produce its magnetic field?
Today, that field is produced by the dynamo effect generated by electric currents in Earth’s molten iron outer core, an effect stimulated by buoyant forces as the planet’s inner core cools and solidifies. However, the inner core only cooled enough to begin solidifying about a billion years ago; 3.7 billion years ago, it could not have affected the dynamo effect in the same way it does today. In short, how Earth’s ancient magnetic field was generated remains a mystery.
Luckily, it was indeed generated and certainly helped primitive microbial life survive and evolve. The solar wind was stronger in the past than it is today, but over time the Earth’s magnetic field would have been able to resist it, creating the conditions for life to move out of the oceans, where it was protected from harmful aggression. radiation and on land.
The results were published on April 24 in Journal of Geophysical Research.