Earth’s history is recorded on plates. Over billions of years, their migrations have formed continents, opened oceans, and created climates and environments that allow life to emerge and evolve.
But one fundamental question remains unresolved. When did these plates actually start moving? Did Earth’s outer shell start changing as soon as the planet formed 4.5 billion years ago, or did this process begin much later?
A new study by Harvard University geoscientists provides the clearest answer yet. Published on March 19th sciencethis study provides the oldest direct evidence of plate movement, dating back 3.5 billion years. The findings show that early plate motions played a role in the formation of young planets, even if they differed from today’s systems.
“A very wide range of ages has been proposed for timing,” said lead author Alec Brenner, Ph.D. 24, who conducted the study in the Department of Earth and Planetary Sciences (EPS) at Harvard University’s Kenneth C. Griffin School of Arts and Sciences. “Through this research, we can confirm that plates were moving around on the Earth’s surface 3.5 billion years ago.”
Ancient rocks reveal early Earth’s movements
This groundbreaking discovery comes from some of the oldest well-preserved rocks on Earth, discovered in the Pilbara Craton in western Australia. These rocks formed during the Archean era, when early microorganisms existed and the Earth was frequently experiencing impacts from space.
The area also preserves some of the earliest evidence of life, including stromatolite and microbial stone formations made by single-celled organisms such as cyanobacteria.
A research team led by Roger Hu, a professor of earth and planetary science at Harvard University, has been studying the East Pilbara since 2017. Hu specializes in paleomagnetism, which uses records of the Earth’s magnetic field preserved in rocks to reconstruct the planet’s past. Previous research by the group also identified signs of an ancient meteorite impact at the same location.
Using ancient magnetism as geological GPS
Paleomagnetism allows scientists to not only study the Earth’s magnetic field, but also to track how parts of the Earth’s crust have moved over time. Tiny magnetic signals trapped within mineral grains act like a record of where rocks on Earth were formed.
By analyzing these signals, researchers can determine both the direction and latitude when a rock was formed, effectively turning it into a kind of ancient GPS.
“Almost everything that is unique about Earth is related to plate tectonics at some level,” Hu said. “At some point, Earth went from being just another planet in the solar system with similar materials, which wasn’t very special, to something very special. There’s a very strong suspicion that plate tectonics started putting Earth into this divergent orbit.”
Analysis of giant rocks reveals plate drift
To investigate, the team studied more than 900 rock samples from more than 100 locations in the area known as the Arctic Dome.
They used special equipment to excavate cylindrical “cores” from the rock, and carefully recorded the position of each sample using tools such as compasses and goniometers (devices that measure angles).
Back at the lab, the core was sliced ​​into thin sections and analyzed with a highly sensitive magnetometer that can detect signals much weaker than a compass needle. The samples were gradually heated to 590 degrees Celsius to isolate magnetic signals from different periods in history. The complete analysis took approximately two years.
“We took a very big gamble,” said Brenner, now a postdoctoral fellow at Yale University. “Degaussing thousands of cores takes years. And did it pay off! These results were beyond our wildest imaginations.”
Evidence of movement 3.5 billion years ago
In magnetic minerals, the arrangement of electrons acts like a small compass pointing to the Earth’s magnetic poles. This placement also reveals where on Earth the rock was located when it was formed, including its latitude.
Examining rocks spanning about 30 million years, just after 3.5 billion years ago, researchers found that parts of the East Pilbara region shifted from 53 degrees to 77 degrees latitude, shifting tens of centimeters a year and rotating more than 90 degrees clockwise over millions of years. (The magnetic poles occasionally reverse, so it remains unclear whether this movement occurred in the northern or southern hemisphere.) After about 10 million years, the movement slowed and eventually stabilized.
For comparison, the researchers looked at rocks from South Africa’s Barberton Greenstone Belt. Previous studies had shown that this region remained near the equator and remained mostly stationary for the same period of time. This suggests that different parts of the Earth’s crust were moving in different ways.
Even today, the plates are still moving, albeit slowly. For example, the North American and Eurasian plates are moving apart by about 2.5 centimeters, or 1 inch, per year.
Rethinking how plate tectonics began
Scientists are still trying to figure out exactly when and how Earth developed the modern plate tectonics system known as the “active lid.” Some theories propose that the early Earth had a “stagnant lid” (a single unbroken Earth plate), a “sluggish lid” (slowly moving plates), or an “episodic lid” (a plate that moved sporadically).
This study rules out the idea of ​​a stagnant lid and shows that the Earth’s surface is already divided into moving parts. However, it has not yet been distinguished which type of initial plate behavior was predominant. Further research is underway to address this question.
“We are observing the movement of tectonic plates, but this requires that boundaries exist between the plates and that the lithosphere is not a large, unbroken shell extending across the globe, as many have argued,” Brenner said. “Instead, it was divided into different parts that could be moved around each other.”
Oldest magnetic flip ever detected
The researchers also identified the oldest known geomagnetic reversal, a process in which the Earth’s magnetic field flips so that the compass points south instead of north.
This reversal is thought to be caused by the “dynamo effect” of molten iron circulating within the Earth’s core, generating electrical currents and magnetic fields. The most recent reversal occurred about 780,000 years ago.
Hu said the new findings suggest that such reversals were less frequent 3.5 billion years ago than they are now. “It’s not conclusive in itself, but it suggests that perhaps the generators were in a slightly different regime than they are now,” he said.
