Spreading Continents Kick-Started Plate Tectonics Billions of Years Ago

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First described in the 1960s, plate tectonics explains how volcanoes build mountains and shape oceans. But have these large-scale motions always been this way? A new computer model suggests that plate tectonics was kick-started by gravity and the spreading of continents during the early days of Earth. The work was published in Nature and states:

Spreading continents kick-started plate tectonics

Stresses acting on cold, thick and negatively buoyant oceanic lithosphere are thought to be crucial to the initiation of subduction and the operation of plate tectonics1, 2, which characterizes the present-day geodynamics of the Earth. Because the Earth’s interior was hotter in the Archaean eon, the oceanic crust may have been thicker, thereby making the oceanic lithosphere more buoyant than at present3, and whether subduction and plate tectonics occurred during this time is ambiguous, both in the geological record and in geodynamic models4. Here we show that because the oceanic crust was thick and buoyant5, early continents may have produced intra-lithospheric gravitational stresses large enough to drive their gravitational spreading, to initiate subduction at their margins and to trigger episodes of subduction. Our model predicts the co-occurrence of deep to progressively shallower mafic volcanics and arc magmatism within continents in a self-consistent geodynamic framework, explaining the enigmatic multimodal volcanism and tectonic record of Archaean cratons6. Moreover, our model predicts a petrological stratification and tectonic structure of the sub-continental lithospheric mantle, two predictions that are consistent with xenolith5 and seismic studies, respectively, and consistent with the existence of a mid-lithospheric seismic discontinuity7. The slow gravitational collapse of early continents could have kick-started transient episodes of plate tectonics until, as the Earth’s interior cooled and oceanic lithosphere became heavier, plate tectonics became self-sustaining.

Today, plate tectonics and the resulting movements of the crust and upper mantle (or lithosphere) are primarily driven by the “negative buoyancy” of cold plates—the top is cooler and denser than what’s below—which causes one plate to slip under another in a process called subduction. This happens at mid-oceanic ridges: Hot rocks with low density float and move away from erupting ridges until they cool down, get denser than the underlying hot mantle, and eventually sink below to become recycled. Eight major tectonic plates move above the Earth’s mantle at up to 150 millimeters a year.

During the Archaen eon around 2.5 billion to 4 billion years ago, however, Earth’s interior was much hotter, volcanic activity was more prominent, and the ocean crust was presumably thicker. That made the oceanic lithosphere more buoyant and less cold and dense than nowadays. And it remains unclear if and when subduction and tectonic processes occurred. “Over the past decade, we have kept with the idea that plate tectonics was not something that was born with our planet,” Patrice Rey from the University of Sydney tells Popular Mechanics. “The Earth started with a stagnant plate. The question was, how do we move from there to a situation where we have plate tectonics?”

How Plate Tectonics Got Kick-Started

Scientists propose a new model to explain how plate tectonics, the geological phenomenon behind earthquakes and volcanoes, got started.

The outermost shell of the Earth is constantly in motion. Over eons, our planet’s tectonic plates have dragged the continents apart and slammed them into one another, forming ocean trenches, mountains, and volcanoes.

Plate tectonics explains continental drift, seafloor spreading, and why California has so many earthquakes. The Earth’s shifting plates also created an ideal environment for life itself to evolve and thrive. But exactly how our planet’s seven or eight major plates started shifting remains a mystery. Today, in a study published in Nature, researchers model the conditions that may have kick-started plate tectonics billions of years ago. “Over the past decade, we have kept with the idea that plate tectonics was not something that was born with our planet,” says Patrice Rey, a geoscientist at the University of Sydney in Australia and lead author on the study. “The Earth started with a stagnant plate. The question was, how do we move from there to a situation where we have plate tectonics?”

A Hotter, Heavier Earth

Modern geological plates move about in a predictable fashion. The top portion of a plate is relatively cooler and denser than the Earth below it, and this state of “negative buoyancy” causes one plate to slip, or subduct, under another. But scientists have long suspected that during the Archaean Eon, 2.5 billion years ago, geological plates were more static.

“We look at these old Archaean crusts, for instance, and we don’t see the features that characterize subduction,” Rey says.

This implies that some geological event must have initially set the plates in motion. Rey and his colleagues suspected that early Earth’s unique geology might have had something to do with it. Hundreds of millions of years ago, Earth’s interior was hotter and its crust was thicker. Rey wondered whether heavier continents might have pushed against nearby plates, initiating subduction.

“Continents have a natural tendency to spread horizontally and push other plates on their sides,” Rey says. “When we put this physics into a computer model, we realized that it had the power to force the plate into subduction.”