When was it, exactly, that plate tectonics started on Earth?
Geodynamicist Fabio Crameri recalled a 2016 conference in which attendees argued over this question. Some thought that plate tectonics started shortly after the planet formed over 4 billion years ago, whereas others thought that tectonics did not truly begin until about 800 million years ago.
In 2018, Crameri was at a different tectonics conference and heard the same argument spark over the same question. It was a rift in opinion, and who was right and who was wrong depended on how you defined plate tectonics, explained Crameri, who works at the University of Oslo in Norway.
“It became obvious to me that semantics was the main problem when discussing when plate tectonics began on Earth,” Crameri said.
In response, Crameri assembled a team of researchers to analyze the issue, and they recently published a paper in Tectonophysics. In it, the scientists outline a concept for what they think plate tectonics has come to mean in the decades since the idea assembled, and they call this concept “ocean plate tectonics.” Crameri and his team’s aim is that the term will bring an end to the semantic impasses that keep upwelling in the discipline.
Traditionally, plate tectonics describes the motion of continental and oceanic plates as they move across the planet’s surface. But that definition does not include much about the engine (Earth’s convection system) that drives the motion.
“It comes from nowhere,” said Louis Moresi, a computational geophysicist at Australian National University in Canberra. “There’s a huge step between understanding what those simple behaviors are and how they work.”
Right now, researchers talk about plate tectonics the way a car enthusiast might talk about their car—the way it handles—without ever talking about the engine or the car’s design.
Part of the reason is that computers were not able to simulate the engine that drives tectonics until the past few decades. That engine is mantle convection, which revs when hot mantle material rises and creates new oceanic crust at mid-ocean ridges. Such a system, because of its planetary scale, is computationally hard to simulate, according to Anne Davaille, a geodynamicist with the French National Centre for Scientific Research. Davaille, who was not involved in the ocean plate tectonics work, thinks that geoscientists who study things like fluid dynamics think about mantle convection and tectonic plates as one and the same and those who study tectonics in terms of what happens on the planet’s surface tend to separate the two.
At the conference in Switzerland that Crameri attended, Davaille gave a presentation about the death of tectonic plates at subduction zones, where plates plunge into the mantle at the end of their long journey after being born at mid-ocean ridges. Subduction, she argued, seems to happen if there is a plume of hot mantle material beneath the plate—something that may have been essential for plate tectonics to get going early on in Earth’s history. In her eyes, subduction zones are where ocean plates and the mantle start to become indistinguishable.
“We can no longer look at plates as separate from mantle convection,” said geodynamicist Carolina Lithgow-Bertelloni, one of the authors behind the new paper.
In addition to sites of plate tectonics and mantle convection, researchers also examined the source of the upwelling magma that drives seafloor spreading in the first place.
Traditionally, some researchers thought new oceanic crust was fed by plumes of magma that rise from the boundary between Earth’s lower mantle and core. But, according to Crameri and his team, this is not the whole story.
“This misconception is mainly due to the many cartoons or sketches that put the deep upwelling flow always right underneath the spreading ridge,” Crameri said.
Those cartoons and sketches create the impression that ocean plates are more distinct from the mantle than they really are, he added. Rather, the smaller-scale updraft made by spreading ridges pulls mantle magma into place, and this magma need not come only from a deep plume—it can come from anywhere in the mantle.
“Our planet, it’s cooling down, and the convection of the mantle is what’s producing the motion of the plate,” said Claudio Faccenna, a structural geologist and geodynamicist at the University of Texas at Austin and Roma Tre University in Rome who was not involved in the work. “What they’re doing is to try to frame the original evolution of the oceanic lithosphere in a fully consistent way.”
Back to the Start
Ocean plate tectonics is not exactly a new concept, explained Laurent Jolivet, a tectonicist at Sorbonne University in Paris who was not involved in the review work. “This so-called new concept is just what we’ve been talking about for 10 years or so,” he said.
Moresi, who was also not involved with the work, agrees. “It’s not a eureka moment. It’s a hard-work-over-two-decades moment. Only on geologic time does this look like a eureka moment,” he said.
Nevertheless, ocean plate tectonics stands to rescript the way geologists speak to one another about the history of plate tectonics on Earth.
If plate tectonics describes just the motion of plates on Earth’s surface relative to one another, for example, the process began shortly after Earth’s formation billions of years ago, “right after the magma ocean or so,” Crameri said.
But if plate tectonics is defined in terms of ocean plate tectonics, that means the process started much later, around 800 million years ago. The reason is that it took Earth millions of years to cool off enough for plates to remain solid for long periods of time.
Older ideas surrounding plate tectonics, once so new, subduct and destruct, making way for something new back at the ridge.
—Lucas Joel, Freelance Journalist
Joel, L. (2019), Tinkering with tectonics, Eos, 100, https://doi.org/10.1029/2019EO131815. Published on 06 September 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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