A slice of the interior of early Earth, highlighting its major geodynamic processes. Magnetic field lines are drawn in blue and red, emanating from the liquid core that created them, while plate tectonic forces rearrange the surface and play a role in the churning circulation of the rocky mantle below. Credit: Alec Brenner
Research led by Harvard University provides new, stronger evidence of early plate tectonics and geomagnetic pole flipping.
New evidence points to the role of plate tectonics in the release of internal heat and the exchange of geomagnetic poles in early Earth.
Some of the strongest evidence yet that Earth’s crust was pushing and pulling in ways similar to modern plate tectonics at least 3.25 billion years ago has been revealed by new research analyzing portions of the oldest rocks on the planet. In addition, the study provides the earliest evidence of when the planet’s north and south magnetic poles swapped places. The two findings provide clues as to how such geological changes may have resulted in an environment more conducive to the emergence of life on our planet.
Described in the diary PNAS On October 24, work led by Harvard geologists Alec Brenner and Roger Fu focused on part of the Pilbara craton in Western Australia. This is one of the oldest and most stable parts of the earth’s crust. Using state-of-the-art techniques and equipment, scientists show that some of Earth’s earliest surfaces moved at a rate of 6.1 centimeters (2.4 inches) per year and 0.55 degrees every one million years.
That speed is more than twice as fast as the movement of ancient crust 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 is a lot of work that seems to indicate that early in Earth’s history, plate tectonics was not the dominant way in which the planet’s internal heat is released, as plate shifting does today,” said Brenner, 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.”
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. Credit: Photo by Roger Fu
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. Because the newly discovered higher rate of velocity is inconsistent with aspects of these two processes, the results lean more toward plate tectonic motion.
In the paper, the authors also describe what is believed to be the oldest evidence of when the Earth reversed its geomagnetic fields, meaning that the north and south magnetic poles were reversed. This style of flip flops is common throughout Earth’s geological history. In fact, according to NASA, the poles have flipped 183 times in the last 83 million years and perhaps several hundred times in the last 160 million years.
The reversal says a lot about the planet’s magnetic field 3.2 billion years ago. Among the key 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 geodynamically very mature,” Brenner said. “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 a growing body of research showing 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, members of the project 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 equipment 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 for determining 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.”
Reference: “Plate Motion and a Dipolar Geomagnetic Field at 3.25 Ga” by Alec R. Brenner, Roger R. Fu, Andrew RC Kylander-Clark, George J. Hudak, and Bradford J. Foley, October 24, 2022 Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2210258119
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