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 this week.
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?"
The numerical model that Rey and colleagues developed shows how thick, buoyant early continents produced gravitational stresses on surrounding plates—flattening whole continents under their weight, LiveScience explains. These major stresses were strong enough to drive continental spreading and ultimately initiate subduction at continental margins. "The driving engine for plate tectonics didn't exist," Rey explains in a news release. "Instead, thick and buoyant early continents erupted in the middle of immobile plates...Because they were buoyant they spread horizontally, forcing adjacent plates to be pushed under at their edges."
Early continental spreading could have produced intermittent episodes of plate tectonics until Earth’s interior cooled, the ocean lithosphere became heavier, and plate tectonics became self-sustaining.
Watch an 87-million-year-long story in this nine-second video. A continent (depleted lithospheric mantle in green and continental crust in red) slowly spreads toward the adjacent oceanic lithosphere (blue). A short-lived subduction zone develops following 45 million years of slow spreading. This subduction episode allows the continent to surge toward the ocean, leading to its "boudinage" and allowing the detachment of a continental block.
Image/video: Patrice F Rey, Nicolas Coltice and Nicolas Flament