The history of the Earth is written on the great tablets of tectonic plates.
The motions of plates shaped land masses, formed oceans, and created the varied climates and habitats that set the stage for evolution and the diversity of life.
But this grand drama begins with a deep mystery: just when did the continental and oceanic plates begin to drift? Did the lithosphere begin to move soon after the formation of the Earth 4.5 billion years ago or only in the last billion years?
A new study by Harvard geoscientists shows the oldest-yet direct evidence of plate movement by 3.5 billion years ago. In a study published March 19 in Science, the team found that plate movements—though not necessarily the modern type—shaped the early history of our planet.
“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 surface.”
The new revelations came from some of the oldest well-preserved rocks in the world, the Pilbara Craton in western Australia, which contains formations from the Archean Eon when the Earth was hosting early microbial life and under heavy bombardment by astronomical objects. The Pilbara area contains evidence of some of the earliest known life, stromatolites and microbialite rocks deposited by single-celled organisms such as cyanobacteria.
A team led by Roger Fu, Professor of Earth and Planetary Sciences at Harvard University, has been conducting research in East Pilbara since 2017. Fu specializes in paleomagnetism, a branch of geophysics that examines changes in the Earth’s magnetic fields to reconstruct the early history of the planet. Last year, they published a paper about an ancient meteor impact at the same site.
In addition to revealing the properties of the Earth’s magnetic field, paleomagnetism can also be used to track the motions of plates. By analyzing the magnetic signals of ancient mineral grains, the researchers can infer the orientation and latitude of the rocks at the time of formation—thus using the ancient samples like paleo GPS units.
“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.”
In the new study, the researchers analyzed more than 900 rock samples collected from more than 100 sites scattered across an area called the North Pole Dome.
They extracted cylindrical samples or “cores” using an electric drill with a hollow bit and diamond teeth, kept cool by a hand-pump garden sprayer. Afterwards the position of the sample was precisely recorded with an instrument inserted into the hole containing a compass and goniometer (a device for measuring angles).
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.
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 now are 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.”
The team also discovered the oldest-known case of a geomagnetic reversal—a phenomenon in which the magnetic field of the planet occasionally flipped. 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 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.
Leave a Reply