Research from Harvard University offers new, sharper evidence for early plate tectonics and geomagnetic pole shifts.
New evidence points to the role of plate tectonics in early Earth’s internal heat release and geomagnetic pole swapping.
Some of the clearest evidence to date that the Earth’s crust was pushing and pulling in a manner similar to modern plate tectonics at least 3.25 billion years ago has been revealed by new research that analyzed pieces of the oldest rocks on the planet. Additionally, the study provides the first evidence of when the planet’s north and south magnetic poles swapped places. Both discoveries offer clues as to how such geological changes may have created an environment more conducive to the emergence of life on our planet.
Described in the newspaper PNAS on October 24 and led by Harvard geologists Alec Brenner and Roger Fu, the work focused on part of the Pilbara Craton in Western Australia. It is one of the oldest and most stable pieces of the earth’s crust. Using state-of-the-art techniques and equipment, scientists have shown 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 million ‘years.
This speed is more than double the speed at which the ancient crust was moving in a previous study by the same researchers. The speed and direction of this latitudinal drift make plate tectonics the most logical and robust explanation.
“There’s a lot of work that seems to suggest that early in Earth’s history, plate tectonics was not actually the dominant way in which the planet’s internal heat is released, as it is. cases today, by moving plaques,” said Brenner, a Ph.D. candidate at the Graduate School of Arts and Sciences and member of the Harvard Paleomagnetics Lab. “This evidence allows us to rule out with much more confidence explanations that do not involve plate tectonics.”
For example, investigators can now argue against phenomena called “true polar wander” and “stagnant lid tectonics”, both of which can cause the Earth’s surface to move but are not part of style plate tectonics. modern. Since the recently discovered higher velocity rate is inconsistent with some aspects of these two processes, the results lean more towards 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 the reversed magnetic locations of the North and South poles. This type of seesaw is a common phenomenon in the geological history of the Earth. In fact, according to NASA, the poles have flipped 183 times in the last 83 million years and possibly 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. One of the main implications is that the magnetic field was probably stable and strong enough to keep solar winds from eroding the atmosphere. This idea, combined with results on plate tectonics, offer clues to the conditions under which the first forms of life developed.
“It paints a picture of an early Earth that was already really geodynamically mature,” Brenner said. “There were a lot of the same kinds of dynamic processes that result in an Earth that essentially has more stable environmental and surface conditions, which makes it more possible for life to evolve and develop.”
Today, Earth’s outer shell is made up of about 15 crustal blocks, or plates, which hold the planet’s continents and oceans together. Over eons, the plates moved closer together and apart, forming new continents and mountains and exposing new rocks to the atmosphere, leading to chemical reactions that stabilized Earth’s surface temperature for eons. billions of years.
Finding evidence of the beginning of plate tectonics is difficult because the oldest pieces of crust are sunk into the inner mantle, never to resurface. Only 5% of all rocks on Earth are over 2.5 billion years old and no rock is over 4 billion years old.
Overall, the study adds to growing research that shows that tectonic movement occurred relatively early in Earth’s history of 4.5 billion years and that the first forms of life took place. are produced in a more moderate environment. In 2018, project members revisited the Pilbara Craton, which spans about 300 miles in diameter. They drilled into the primordial, thick slab of crust there to collect samples which, back in Cambridge, were analyzed for their magnetic history.
Using magnetometers, degaussing equipment and the quantum diamond microscope – which images the magnetic fields of a sample and precisely identifies the nature of the magnetized particles – researchers have created a suite of new techniques to determine the age and how the samples were magnetized. This allows researchers to determine how, when, and in what direction the crust moved as well as the magnetic influence from Earth’s geomagnetic poles.
The quantum diamond microscope was developed through a collaboration between Harvard researchers from the departments of Earth and Planetary Sciences (EPS) and Physics.
For future studies, Fu and Brenner plan to stay focused on the Pilbara Craton while looking beyond to other ancient crusts around the world. They hope to find older evidence of modern-type plate motion and the shifting of the Earth’s magnetic poles.
“Finally, being able to reliably read these very ancient rocks opens up many possibilities for observing a time period often better known by theory than by hard data,” said Fu, professor of PE at the Faculty of Arts and Sciences. science. “Ultimately, we have a good chance of reconstructing not only when the tectonic plates started moving, but also how their movements – and therefore the deep inner Earth processes 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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