The study provides new, sharper evidence for early plate tectonics and geomagnetic pole flipping

New research analyzing chunks of the oldest rocks on the planet provides some of the strongest evidence yet that the Earth’s crust was being pushed and pulled in ways similar to modern plate tectonics at least 3.25 billion years ago. The study also provides the earliest evidence of when the planet’s north and south magnetic poles swapped places.

The two results offer clues as to how such geological changes may have resulted in an environment more conducive to the development of life on the planet.

The work described in PNAS and led by Harvard geologists Alec Brenner and Roger Fu, focused on part of the Pilbara Craton in Western Australia, one of the oldest and most stable parts of the Earth’s crust. Using novel techniques and devices, the researchers show that some of Earth’s earliest surfaces moved at a rate of 6.1 centimeters per year and 0.55 degrees every million years.

That speed is more than twice the speed at which the ancient crust was moving in a previous study by the same researchers. Both the speed and the direction of this shift in latitude allow plate tectonics to be the most logical and strongest explanation for it.

“There’s a lot of work that seems to suggest that early in Earth’s history, plate tectonics wasn’t the dominant way the planet’s internal heat is released, as plate shifting does today,” said Brenner, a Ph.D. Candidate in the Graduate School of Arts and Sciences and member of the Harvard Paleomagnetics Laboratory. “This evidence allows us to rule out non-plate tectonics explanations much more confidently.”

For example, researchers can now argue against phenomena called “true polar migration” and “stagnant lid tectonics,” both of which can shift the Earth’s surface but are not part of modern plate tectonics. The results lean more toward plate tectonic motion, as the newly discovered higher velocity rate is inconsistent with aspects of the other two processes.

In the paper, the scientists also describe what is believed to be the oldest evidence of when the Earth reversed its geomagnetic fields, meaning the north and south magnetic poles were flipped. This type of flip-flop is a common event in Earth’s geological history, with the pole flipping 183 times in the last 83 million years and perhaps several hundred times in the last 160 million years, according to NASA.

The reversal says a lot about the planet’s magnetic field 3.2 billion years ago. Key to these implications is that the magnetic field was likely stable and strong enough to prevent solar winds from eroding the atmosphere. This finding, combined with the findings on plate tectonics, provides clues to the conditions under which the earliest forms of life evolved.

“It paints this picture of an early Earth that was already really mature geodynamically,” says Brenner. “There have been many of the same types of dynamic processes leading to an Earth that has significantly more stable environmental and surface conditions, making it easier for life to evolve and evolve.”

Today, Earth’s outer shell is made up of about 15 shifting blocks of crust, or plates, that support the planet’s continents and oceans. Over eons, the plates drifted in and out, forming new continents and mountains, and exposing new rocks to the atmosphere, leading to chemical reactions that stabilized Earth’s surface temperature for billions of years.

Evidence of the beginning of plate tectonics is hard to find as the oldest pieces of crust are pushed into the inner mantle, never to resurface. Only 5 percent of all rocks on Earth are older than 2.5 billion years, and no rock is older than about 4 billion years.

Overall, the study adds to the growing body of research that tectonic movements occurred relatively early in Earth’s 4.5-billion-year history and that early life forms emerged in a more temperate environment. In 2018, project members revisited the Pilbara Craton, which spans about 300 miles. They drilled into the original and thick crustal plate there to collect samples, which were analyzed back in Cambridge for their magnetic history.

Using magnetometers, degaussing devices and the quantum diamond microscope, which maps a sample’s magnetic fields and precisely identifies the type of magnetized particles, the researchers developed a number of new techniques to determine the age and the way the samples were magnetized. This allows researchers to determine how, when and in which direction the crust shifted, as well as the magnetic influence emanating from Earth’s geomagnetic poles.

The Quantum Diamond Microscope was developed in a collaboration between Harvard researchers from the Departments of Earth and Planetary Sciences (EPS) and Departments of Physics.

For future studies, Fu and Brenner plan to continue to focus on the Pilbara craton while looking beyond it at other ancient crusts around the world. They hope to find older evidence of modern plate motion, when Earth’s magnetic poles were flipped.

“Finally being able to reliably read these very old rocks opens up so many opportunities to observe a time period that is often known more through theory than solid data,” said Fu, a professor of EPS the Faculty of Arts and Sciences. “Ultimately, we have a good chance of reconstructing not only when tectonic plates started to move, but also how their movements – and thus the deep-seated processes in the Earth’s interior that drive them – changed over time.”