New research analyzing pieces of the planet’s oldest rocks adds some of the strongest evidence yet that the Earth’s crust was pushing and pulling in a way similar to modern plate tectonics at least 3.25 billion years ago. The study also provides the first evidence of the timing of the interchange of the planet’s north and south magnetic poles.
Both results provide clues to how these geological changes have led to an environment more favorable for the development of life on this planet.
The work described in PNAS Led by Harvard geologists Alec Brenner and Roger Foo, they focused on a portion of the Pilbara Craton in Western Australia, one of the oldest and most stable pieces of Earth’s crust. Using new techniques and equipment, the researchers showed that some of Earth’s oldest surfaces were moving at a rate of 6.1 cm per year and 0.55 degrees every million years.
This speed is more than twice the rate at which the ancient crust moved in a previous study by the same researchers. Both the speed and direction of this latitude drift make plate tectonics the most logical and powerful explanation for it.
“There is a lot of work that seems to indicate that plate tectonics early in Earth’s history wasn’t actually the dominant way in which the planet’s internal heat is released as it is today through plate change,” said Brenner, Ph.D. . . Candidate in the Graduate School of Arts and Sciences and a member of Harvard University’s Paleomagnetics Laboratory. “This evidence allows us to more confidently rule out explanations that do not involve plate tectonics.”
For example, researchers can now argue against a phenomenon called “true polar wandering” and “stagnant cap tectonics,” which can cause Earth’s surface to shift but are not part of modern-style plate tectonics. The results are skewed more towards plate tectonic motion because the newly discovered higher rate of velocity is incompatible with aspects of the other two processes.
In the paper, the scientists also described what is believed to be the oldest evidence that the Earth reversed its geomagnetic fields, meaning that the magnetic north and south poles flipped. This type of slipper is a common occurrence in Earth’s geological history as the pole has reversed 183 times in the past 83 million years and possibly several hundred times in the past 160 million years, according to NASA.
The reversal tells a lot about the planet’s magnetic field 3.2 billion years ago. The key among these effects was that the magnetic field was likely stable and strong enough to prevent the solar wind from eroding the atmosphere. This insight, along with findings on plate tectonics, provides clues to the conditions under which the first life forms evolved.
“It paints this picture of an early Earth that was already already geodynamically mature,” Brenner said. “It had many of the same kinds of dynamic processes that lead to Earth having essentially more stable environmental and surface conditions, which make life more amenable to evolution and evolution.”
Today, Earth’s outer crust is made up of about 15 moving masses of crust, or plates, that hold the planet’s continents and oceans. Over eons, the plates drifted toward and away from each other, forming new continents and mountains and exposing new rocks to the atmosphere, triggering chemical reactions that stabilized Earth’s surface temperature over billions of years.
It’s hard to get evidence of when tectonic plates began because the oldest pieces of the Earth’s crust are pushed into the inner mantle, and never show up. Only 5 percent of all rocks on Earth are over 2.5 billion years old, and no rocks are older than about 4 billion years old.
Overall, the study adds to the growing research that tectonic movement occurred relatively early in Earth’s 4.5 billion year history and that early life forms arose in a more temperate environment. Project members revisited the Pilbara Craton in 2018, which stretches about 300 miles across. They dug into the thick, primitive slab of crust there to collect samples that were analyzed, at Cambridge, for their magnetic history.
Using magnetometers, demagnetization equipment, and a quantum diamond microscope — which visualizes the magnetic fields of a sample and accurately determines the nature of magnetized particles — the researchers devised a range of new techniques to determine the age and way samples became magnetized. This allows researchers to determine how, when, and in which direction the crust shifts, as well as magnetic forcing from Earth’s magnetic poles.
The quantum diamond microscope was developed in collaboration with Harvard researchers in the Departments of Earth and Planetary Sciences (EPS) and Physics.
For future studies, Fu and Brenner plan to maintain their focus on the Pilbara Craton while also searching for other ancient crustaceans around the world. They hope to find ancient evidence of modern-like plate movement and when Earth’s magnetic poles flipped.
“Finally, being able to reliably read these very ancient rocks opens up many possibilities for observing a time period that is often known more through theory than from hard data,” said Fu, an EPS professor in the College of Arts and Sciences. “Ultimately, we have a good chance of rebuilding not only when the tectonic plates began to move, but also how their movements—and thus the internal processes of the deep Earth that push them—changed through time.”
Plate tectonics began shifting earlier than previously thought
Brenner, Alec R., Plate motion and a dipole geomagnetic field at 3.25 Ga, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2210258119. doi.org/10.1073/pnas.2210258119
Presented by Harvard University
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