Earlier than expected: 3.25 billion years ago, the earth had a stable geodynamo and a 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 most important protection against hard cosmic rays and is crucial for the livability of our planet. But since when does this exist? planetary shield? According to current theory, a stable terrestrial magnetic field only developed when the inner core of the earth solidified – and thus created the prerequisites for the geodynamo. So far, however, it is disputed when this happened: some studies speak for a solidification in the early days of the earth, others only 1.3 billion years ago or even first 550 million years ago.
Magnetic field before the geodynamo?
The problem: To prove the existence of a magnetic field in earlier geological eras, you need rocks from that era – and these are very rare on the earth’s surface today. Nevertheless, there were already some measurements that indicated magnetization of rock more than three billion years old. At the beginning of 2020, zirconium crystals even provided evidence that a first magnetic field was already 4.2 billion years ago could have passed.
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 postulate other, exotic forms of magnetic field induction for this time.
Magnetic field already 3.34 billion years ago
New measurement data are now providing more clarity. For their study, Alec Brenner from Harvard University and his colleagues examined rock samples from the eastern Pilbara craton in Australia – 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 revealed 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 drill cores and in individual magnetite grains from the samples.
Oldest proof of a magnetic field reversal
The measurement data also showed that the early earth’s magnetic field must have experienced a polarity reversal around 3.25 billion years ago – an exchange of the magnetic north and south poles. “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 in the past than previous reliable evidence for such events.”
According to the scientists, this demonstrates that, as early as 3.25 billion years ago, Earth possessed a stable dipole field with a geodynamo and largely “modern” behavior – including periodic pole reversals. In the last 80 million years alone, there could have been 183 such polarity reversals and shorter pole jumps. The new data now suggests that the earth’s magnetic field tended towards 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. So far, it has also been disputed when exactly it began and how. Subtle changes in the magnet orientation in the Pilbara rock samples now show that even when the magnetic field was stable in polarity, there were slight shifts in the magnetization of the rocks that increased over time. They point to a gradual drift of the Pilbara craton across 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 around 0.55 degrees per million years,” reported Brenner and his team. This rock formation was therefore moving at around 6.1 centimeters per year – even by the standards of today’s plate tectonics, this is relatively fast. A second phase followed 3.25 billion years ago, in which East Pilbara no longer changed its latitude, but instead rotated counterclockwise.
Geodynamically amazingly “modern”
From these results, the researchers conclude that the earth not only had a magnetic field at that time, but also real plate tectonics. Because alternative hypotheses cannot explain how the crust should have moved so quickly. 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 that is compatible with these findings,” state Brenner and his team.
Taken together, this means: “Our data paint 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 may then provide even more information about when the earthly geodynamo started and when plate tectonics began. (Proceedings of the National Academy of Sciences, 2022; doi: 10.1073/pnas.2210258119)
Source: Harvard University