Earlier than expected: 3.25 billion years ago, Earth had a stable geodynamo and magnetic field – and experienced the first polarity reversal. Evidence of this is provided by rock samples from the Pilbara craton in Australia that are 3.34 to 3.18 billion years old. They also suggest that there was already active plate tectonics at that time, with relatively rapid shifts in the Earth’s plates. Both together indicate that our planet was geophysically relatively modern more than three billion years ago.
The Earth’s magnetic field is our main protection against harsh cosmic rays and is crucial for the habitability of our planet. But how long has this planetary protective screen been around? According to current theory, a stable Earth’s magnetic field did not develop until the Earth’s inner core solidified—creating the conditions for geodynamo. However, until now it is disputed when this happened: some studies speak of a solidification in the early days of the Earth, others only 1.3 billion years ago or even 550 million years ago.
Magnetic field for the geodynamo?
The problem: To prove the existence of a magnetic field in earlier geologic eras, you need rocks from that era – and these are very rare on the Earth’s surface today. Yet there were already some measurements that indicated magnetization of rocks more than three billion years old. In fact, in early 2020, zirconium crystals provided evidence that the first magnetic field could have existed 4.2 billion years ago.
But that would mean that this first, primordial magnetic field must have existed before the geodynamo. Because more than four billion years ago, the inner core of the Earth could not have been solid – the interior of the young Earth was still much too hot for that. Some scientists therefore assume other, exotic forms of magnetic field induction for this time.
Magnetic field as early as 3.34 billion years ago
New measurement data now provide more clarity. For their study, Alec Brenner of Harvard University and his colleagues examined rock samples from Australia’s eastern Pilbara craton — one of the oldest and most stable rock formations on Earth — that are 3.43 to 3.18 billion years old. The team extracted drill cores from the rock once formed by volcanic eruptions and determined the strength and direction of the magnetization in the different layers.
The analyzes showed that the Earth must have had a pronounced dipole magnetic field more than three billion years ago. The researchers were able to detect remnants of a corresponding magnetization both in the rock cores and in individual magnetite grains from the samples.
Oldest evidence of a reversal of a magnetic field
The measurement data also showed that the early Earth’s magnetic field must have undergone a polarity reversal – an exchange of the north and south magnetic poles – about 3.25 billion years ago. “This is the oldest evidence of geomagnetic polarity reversal and the oldest direct test of Earth’s magnetic field geometry,” Brenner and his team report. “This polar reversal is 480 million years further into the past than previously reliable evidence for such events.”
According to the scientists, this shows that as early as 3.25 billion years ago, Earth possessed a stable dipole field with a geodynamo and largely “modern” behavior – including periodic polar reversals. There could have been 183 such polarity reversals and shorter pole shifts in the last 80 million years alone. The new data now suggests that Earth’s magnetic field tended toward such polarity reversals even in its early days.
Evidence of early plate tectonics
But the magnetic measurements of the Pilbara samples also provide new information about a second geodynamic process: plate tectonics. Until now it has also been disputed when exactly it started and how. Subtle changes in magnet orientation in the Pilbara rock samples now show that even when the magnetic field was stable in polarity, there were small shifts in the magnetization of the rocks that increased over time. They indicate a gradual drift of the Pilbara craton over the Earth’s surface – early plate tectonics.
Specifically, the measurements revealed: “First, 3.34 to 3.35 billion years ago, East Pilbara drifted north at about 0.55 degrees per million years,” Brenner and his team reported. This rock formation was therefore moving at about 6.1 centimeters per year – even by the standards of current plate tectonics, this is relatively fast. A second phase followed 3.25 billion years ago, during which East Pilbara no longer changed latitude, but rotated counterclockwise.
Geodynamically amazing “modern”
From these results, the researchers conclude that the Earth at that time not only had a magnetic field, but also real plate tectonics. Because alternative hypotheses can’t explain how the crust could have moved so fast. This is only possible if there were already tectonic plates and mantle convection based on the modern model. “The differential motion within a mobile crustal lid is the only mechanism compatible with these findings,” Brenner and his team say.
Taken together, this means, “Our data paints a picture of an early Earth that was already geodynamically mature,” says Brenner. “It already had the same dynamic processes that give our planet its stable conditions to this day — and that once allowed life to arise and evolve.”
The researchers now want to look for other, even older rock samples in the Pilbara craton. Their analyzes could then provide even more information about when Earth’s geodynamo began and when plate tectonics began. (Proceedings of the National Academy of Sciences, 2022; doi:10.1073/pnas.2210258119)
Source: Harvard University