3.5-Billion-Year-Old Rocks Rewrite the Story of Plate Tectonics

Ancient rocks are revealing that early Earth may not have been geologically still at all.

Earth’s story is recorded in its tectonic plates. As these plates moved, they reshaped continents, opened oceans, and helped produce the climates and environments that made evolution and Earth’s rich variety of life possible.

That raises a major question: when did continental and oceanic plates first start to move? Did the lithosphere begin shifting soon after Earth formed 4.5 billion years ago, or did that happen only within the past billion years?

A new study from Harvard geoscientists reports the oldest direct evidence yet of plate motion, dating to 3.5 billion years ago. In a study published March 19 in Science, the researchers found that moving plates, even if they did not behave exactly like modern ones, were already influencing Earth’s early development.

“There has been a huge range of ages suggested for timing,” said lead author Alec Brenner, PhD ’24, who conducted the research in the Department of Earth and Planetary Sciences (EPS) in the Harvard University Kenneth C. Griffin Graduate School of Arts and Sciences. “With this study, we’re able to say three and a half billion years ago, we can see plates moving around on the Earth’s surface.”

A Window Into the Archean

The findings came from the Pilbara Craton in western Australia, home to some of the world’s oldest well-preserved rocks. These formations date to the Archean Eon, when early microbial life existed on Earth and the planet was still being heavily struck by astronomical objects. The Pilbara region also preserves some of the earliest known signs of life, including stromatolites and microbialite rocks formed by single-celled organisms such as cyanobacteria.

A team led by Roger Fu, Professor of Earth and Planetary Sciences at Harvard University, has been working in East Pilbara since 2017. Fu studies paleomagnetism, a branch of geophysics that uses changes in Earth’s magnetic field to reconstruct the planet’s early history. Last year, the group published a paper on an ancient meteor impact at the same location.

Paleomagnetism does more than reveal features of Earth’s magnetic field. It can also help scientists trace plate motion. By studying magnetic signals locked inside ancient mineral grains, researchers can estimate the orientation and latitude of rocks when they formed, essentially turning those rocks into ancient GPS markers.

“Almost everything unique about the Earth has something to do with plate tectonics at some level,” said Fu. “At some point, the Earth went from something not that special, just another planet in the solar system with similar materials, to something very special. A very strong suspicion is that plate tectonics started Earth down this divergent track.”

For the new study, the researchers examined more than 900 rock samples from over 100 sites across an area known as the North Pole Dome.

They collected cylindrical rock samples, or “cores,” with an electric drill fitted with a hollow bit and diamond teeth, cooled by a hand-pump garden sprayer. Afterward, they carefully recorded each sample’s position using an instrument placed in the hole that included a compass and goniometer (a device for measuring angles).

Reading Ancient Magnetism

Back at Harvard, the cores were sliced into sections like cookies, lined up on trays, and placed in a magnetometer, a machine that can measure magnetic signals 100,000 times more faint than a compass needle. The samples were repeatedly measured while being heated to progressively hotter temperatures up to 590 degrees Celsius until the magnetite minerals lost their magnetization. The step-by-step heating allows researchers to isolate magnetic signals from different periods in the rock’s history. All told, the analysis took about two years.

“We took a really big gamble,” said Brenner, now a postdoc at Yale. “Demagnetizing thousands of cores takes years. And boy, did it pay off! These results were beyond our beyond our wildest dreams.”

In ferromagnetic minerals, the orientation of the electrons serves like a compass needle pointing towards the magnetic pole. The electron orientation also provides hints about the position on the three-dimensional globe relative to the magnetic pole when the rock formed—thus providing an indication of latitude.

By analyzing a series of rocks spanning 30 million years just after 3.5 billion years ago, they found that part of the East Pilbara formation shifted in latitude from 53 degrees to 77 degrees—a drift of tens of centimeters annually over several million years—and rotated clockwise by more than 90 degrees. (Because the magnetic pole occasional reverses, it remains uncertain whether this motion occurred in the northern or southern hemisphere.) Within about 10 million years, the motion slowed and followed by a period of little motion.

Comparing Ancient Regions

To compare this motion with Archaean sites elsewhere, the researchers examined a contemporary site in South Africa, the Barberton Greenstone Belt. Previous paleomagnetic studies showed that the latter was located near the equator and nearly stationary during the same time interval. Apparently, the two distant regions had different patterns of drift.

In the modern world, the North American and Eurasian plates are now moving away from each other by about 2.5 centimeters, or 1 inch, per year.

It remains an open question about when and how the Earth took on its current form of plate tectonics, which geophysicists call an “active lid.” Various theories posit that the early Earth had a “stagnant lid” (a single unbroken global plate), a “sluggish lid” (slowly moving plates), or “episodic lid” (plates moving sporadically). The new study rules out a stagnant lid but cannot distinguish which model of plate movement was most likely; the Fu team is pursuing additional studies to answer this question.

“We’re seeing motion of tectonic plates, which requires that there were boundaries between those plates and that the lithosphere wasn’t some big, unbroken shell across the globe, as a lot of people have argued before,” said Brenner. “Instead, it was segmented into different pieces that could move with respect to each other.”

An Ancient Magnetic Flip

The team also discovered the oldest-known case of a geomagnetic reversal—a phenomenon in which the magnetic field of the planet occasionally flips. After a reversal, a compass needle would point south instead of north.

This phenomenon is believed to be governed by the “dynamo action” involving the convection of molten iron in the Earth’s core that produces electrical currents and magnetic fields. The last reversal occurred about 780,000 years ago.

Fu said the new evidence suggests that 3.5 billion years ago, reversals occurred less frequently than in more recent history. “It’s not by itself conclusive, but it suggests that maybe the dynamo was in a slightly different regime than today,” he said.

Reference: “Paleomagnetic detection of relative plate motions and an infrequently reversing core dynamo at 3.5 Ga” by Alec R. Brenner, Roger R. Fu, Bradford J. Foley, Diogo L. Lourenço, Jasmine Palma-Gomez, Zheng Gong, Sarah C. Steele, Joanna Li, David T. Flannery, Adrian J. Brown and Eben B. Hodgin, 19 March 2026, Science.
DOI: 10.1126/science.adw9250

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