A 3.7-billion-year-old record of our planet’s ancient magnetism has been discovered, providing evidence that Earth’s magnetic field existed very early in history. However, this discovery is quite surprising.
Rocks approaching 4 billion years old are hard to find; most are recycled earthIts tectonic activity, sliding into the mantle through subduction zones before sliding out of volcanoes. Yet somehow, a sequence of rocks in the Isua Supracrustal Belt in Greenland has survived the ravages of time thanks to its unique geology, perched atop a thick continental plate like a life raft amid an ocean of tectonic upheaval.
Now, researchers from the University of Oxford and the Massachusetts Institute of Technology have excavated some of those Isua rocks, revealing that they contain an iron record of the early Earth’s magnetic field. According to this record, our planet’s magnetic field doesn’t seem to have changed much in the intervening time—but geologists don’t fully understand how Earth could have produced a magnetic field at all in that time.
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The existence of a magnetic field is essential to the development of life on Earth, with the field lines deflecting the dangerous shower of charged particles that are blown towards us via the solar wind. The existence of an early magnetic field may have helped give life a foothold on our planet.
Previously, estimates and suggestions of the early Earth’s magnetic field have come from individual mineral crystals called zircon found within ancient rocks from Western Australia. These had suggested the existence of a magnetic field 4.2 billion years ago. However, these results were later suspected as unreliable.
The new results from Greenland rocks are considered more reliable because, for the first time, they are based on whole iron-bearing rocks (rather than individual mineral crystals) to derive the initial field strength. The sample therefore provides the first robust measure not only of the strength of the ancient Earth’s magnetic field, but also of when the magnetic field first appeared.
“Extracting reliable data from rocks this old is extremely challenging, and it was really exciting to see that key magnetic signals started to emerge when we analyzed these samples in the lab,” said lead researcher Claire Nichols, who is professor of planetary geology at . University of Oxford, in a press release. “This is a really important step forward as we try to determine the role of the ancient magnetic field when life on Earth first appeared.”
The iron particles within the Isua rocks can be thought of as tiny magnets, connected to Earth’s magnetic field when the rock around them first crystallized 3.7 billion years ago. Therefore, their alignment holds a record of field strength. This force is measured to have been at least 15 microtesla (mT), which is comparable to Earth’s field strength of 30 mT today.
However, this still leaves that previous puzzle: how did the early Earth produce its magnetic field?
Today, this field is produced by the dynamo effect created by electric currents in Earth’s molten iron outer core, an effect driven by buoyant forces as the planet’s inner core cools and solidifies. However, the inner core cooled enough to begin solidification about a billion years ago; 3.7 billion years ago, it could not have affected a dynamo effect in the same way as it does today. In short, how Earth’s ancient magnetic field was created remains a mystery.
Thankfully, it was indeed created and probably helped primitive microbial life to survive and evolve. The solar wind was stronger in the past than it is today, but over time, Earth’s magnetic field would have been able to resist it, creating conditions for life to leave the oceans, where it was protected from predators. . radiation and on earth.
The findings were published on April 24 in Journal of Geophysical Research.