Study offers new, sharper evidence for early plate tectonics and geomagnetic pole shift

Study offers new, sharper evidence for early plate tectonics and geomagnetic pole shift

An interior cross-section of the early Earth highlighting its major geodynamic processes. Magnetic field lines are drawn in blue and red emanating from the liquid core that generated them, as plate tectonic forces rearrange the surface and play a role in the bubbling circulation of the rocky mantle below. Credit: Alec Brenner

New research analyzing chunks of the planet’s oldest rocks provides some of the clearest evidence yet that Earth’s crust was pushing and pulling in a similar way to modern plate tectonics at least 3.25 years ago. billions of years. The study also provides the first evidence of when the planet’s north and south magnetic poles swapped places.

Both results offer clues as to how such geological changes may have created an environment more conducive to the development of life on the planet.

The book, described in PNAS and led by Harvard geologists Alec Brenner and Roger Fu, focused on part of the Pilbara Craton in Western Australia, one of the oldest and most stable pieces of Earth’s crust. Using innovative techniques and equipment, researchers have shown that some of Earth’s earliest surfaces moved at a rate of 6.1 centimeters per year and 0.55 degrees every million years.

This speed more than doubles 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 strongest explanation.

“There’s a lot of work that seems to suggest that early in Earth’s history, plate tectonics wasn’t actually the dominant way in which the planet’s internal heat is released as it is. today by plate displacement,” 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, researchers can now oppose 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 polar tectonics. modern style plates. The results lean more towards plate tectonic motion because the newly discovered higher velocity rate is inconsistent with aspects of the other two processes.

In the paper, the scientists also describe what is believed to be the oldest evidence of when Earth reversed its geomagnetic fields, meaning the reversed magnetic locations of the North and South poles. This type of flip is a common occurrence in Earth’s geological history, with the pole having reversed 183 times in the last 83 million years and possibly several hundred times in the last 160 million years. years, according to NASA.

The reversal says a lot about the planet’s magnetic field 3.2 billion years ago. Key to these implications is that the magnetic field was likely stable and strong enough to keep solar winds from eroding the atmosphere. This idea, combined with results on plate tectonics, offers clues to the conditions under which the first forms of life developed.

“It paints a picture of a primeval 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 the growing research that tectonic movement occurred relatively early in Earth’s history of 4.5 billion years and that the earliest forms of life occurred. produced in a more moderate environment. Project members revisited the Pilbara Craton in 2018, 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. ”

Tectonic Plates Started Moving Earlier Than We Thought

More information:
Brenner, Alec R., Plate motion and dipole geomagnetic field at 3.25 Ga, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2210258119.

Provided by Harvard University

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