The study offers new, sharper evidence of early plate tectonics, the turning of the geomagnetic poles

The study offers new, sharper evidence of early plate tectonics, the turning of the geomagnetic poles

Laying the geological foundations for life on Earth

Interior cross-section of the early Earth highlighting its major geodynamic processes. Magnetic field lines are drawn in blue and red as they emanate from the liquid core that created them, while plate tectonic forces reshape the surface and play a role in the circulation of the rocky mantle below. Credit: Alec Brenner

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

The two results provide clues as to how such geological changes could have resulted in an environment more suitable for the development of life on the planet.

The work, 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 parts of the Earth’s crust. Using new techniques and equipment, researchers show that some of Earth’s earliest surfaces moved at a rate of 6.1 centimeters per year and 0.55 degrees every million years.

That speed more than doubles the speed at which the ancient crust moved previous study by the same researchers. Both the speed and the direction of this geographic deviation leave plate tectonics as the most logical and strongest explanation for it.

“There seems to be a lot of work that suggests that early in Earth’s history, plate tectonics wasn’t actually the dominant way the planet’s internal heat was released as it is today through plate movement,” said Brenner, Ph.D. . candidate in the College of Arts and Sciences and a member of the Harvard Paleomagnetic Laboratory. “This evidence allows us to much more confidently rule out explanations that do not involve plate tectonics.”

For example, researchers can now argue against a phenomenon called “a real polar wander” and “stagnant lid tectonics,” which can cause the Earth’s surface to move, but are not part of modern-style plate tectonics. The results lean more toward plate tectonics because the newly discovered higher speed 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 the Earth reversed its geomagnetic fields, meaning the magnetic north and south poles switched locations. This type of flip-flop is common on Earth geological history with the pole rotating 183 times in the last 83 million years and perhaps a few hundred times in the past 160 million years, according to NASA.

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

“It paints this picture of an early Earth which was already really geodynamically mature,” Brenner said. “It had many of the same kinds of dynamic processes that resulted in an Earth that had fundamentally more stable life and surface conditions, which made it more feasible for life to develop and evolve.”

Today, Earth’s outer mantle consists of about 15 moving blocks of crust, or plates, that hold the planet’s continents and oceans. Over the eons, the plates melted into each other and drifted apart, forming new continents and mountains and exposing new rocks to the atmosphere, leading to chemical reactions that stabilized Earth’s surface temperature over billions of years.

Evidence of when plate tectonics began is hard to come by because the oldest pieces of crust are embedded in the inner mantle, never coming to the surface. Only 5 percent of all rocks on Earth are older than 2.5 billion years, and no rocks are older than about 4 billion years.

Overall, the study adds to the growing body of 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 for about 300 miles. They drilled through the primordial and thick slab of crust to collect samples which, in Cambridge, were analyzed for their magnetic history.

Using magnetometers, demagnetization equipment, and a quantum diamond microscope—which displays a sample’s magnetic fields and precisely identifies the nature of the magnetized particles—the researchers created a set of new techniques to determine the age and how the samples became magnetized. This allows researchers to determine how, when and in what direction the crust moved, as well as the magnetic influence coming from Earth’s geomagnetic poles.

The quantum diamond microscope was developed in collaboration between Harvard researchers in the Department of Earth and Planetary Sciences (EPS) and Physics.

For future research, Fu and Brenner plan to keep their focus on the Pilbara craton, while looking beyond it to other ancient crusts around the world. They hope to find older evidence of modern-like plate motion and when Earth’s magnetic poles flipped.

“Finally being able to reliably read these very ancient rocks opens up so many opportunities to look at a time period that is often known more through theory than hard data,” said Fu, an EPS professor in the College of Arts and Sciences. “Ultimately, we have a good opportunity to reconstruct not only when the tectonic plates began to move, but also how their movements—and thus the deep-seated internal processes of the Earth that drive them—changed over time.

Tectonic plates started moving earlier than thought

More information:
Brenner, Alec R., Plate Motion and the Dipolar Geomagnetic Field at 3.25 Ga, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2210258119.

Citation: Study offers new, sharper evidence of early plate tectonics, geomagnetic pole flips (2022, October 24) Retrieved October 25, 2022 from tectonics .html

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