In March 2016, a team of researchers dropped 39 seismometers at the bottom of the Atlantic Ocean off the western tip of Africa to hear the sound of earthquakes near and far. A year later, they found a hidden story of how the continents move away – they are not drawn on both sides by subduction zones, as previously thought, but probably being pushed by magma rising into the center of the ocean.
The probes were placed in parallel lines that stretch for more than 600 miles, over the submarine ridge that forks the Atlantic. The tools were part of PI-LAB and EURO-LAB projects, efforts to better understand a transition zone in the Earth’s mantle, the boundary where the rigid lithosphere, which makes up the planet’s crust and the upper mantle, welcomes the underlying asthenosphere, weaker. To collect the data, a team of researchers from the University of Southampton and Oxford University planted sensors on the ocean floor. The data they collected spanned nearly 400 miles across the planet. Data analysis by the team is published today in the journal Nature.
“The transition area itself has been thinner than we expected,” said Kate Rychert, a seismologist at the University of Southampton and chief scientist on cruises for the deposition and recovery of depth sensors, said in a video call. “What it suggests is that we have ascending material in the lower mantle. It is abnormally hot; we usually believe that this does not happen under the ridges in the middle of the ocean. ”
Rychert said such increases in the lower mantle are usually associated with Hawaii or Iceland – volcanic islands that are known to always spill over. At the mid-Atlantic ridge, however, the material rises from the lower to the upper mantle, but obviously does not erupt. Just by pressing firmly upwards, suggesting to researchers that convection along the planet’s mantle may play a substantial role in the tectonics of the plates above it.
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“Incredible results shed new light on our understanding of how the Earth’s interior is related to plate tectonics, with hitherto unseen observations,” Matthew Aguis, a seismologist at the Università degli studi Roma Tre and lead author, explained at a university. . of the Southampton press release.
Initially, the plan was to better understand the definition and thickness of tectonic plates in the Mid-Atlantic ridge. The seismometers and magnetocaloric instruments on the ocean floor were to imagine the plate, and the team expected the transition area to be “very boring,” Rychert said.
Conventional wisdom held that places like the Mid-Atlantic ridge are relatively quiet when it comes to plate tectonics, and the real geopolitical theater for plates was the subduction zones, where two plates converge and one is pushed back into the other’s mantle. These changes are responsible for the imperceptible flow of continents away from each other. Below the Pacific, tectonic plates move faster, hence the dramatic “ring of fire” that produces seismic and volcanic activity on the perimeter of the ocean. The same cannot be said of the gradual march of the Atlantic plates, which move at about 1.6 inches per year.
“Why this paper is interesting for understanding plate tectonics is that if the material hides through the transition zone, it means there is an ascending convection cell that pushes the plates up and pushes them out,” said co-author Nick Harmon. , seismologist at the University of Southampton, in a video call.
If you are considering a pizza dough, it is the difference between making the pie by pulling the edges, rather than pushing in the center. Of course, the difference is that one is pushing up on a planetary scale, rather than pushing down on a culinary one.
Until better seismic technology develops, it can be difficult to better understand what is going on so deep in the mantle. Today, even the best data reads as a “blurry CAT scan,” Harmon said. But along the line – and under the sea – he hopes to learn about the dynamics elsewhere along the ridge, as well as the situation in the tectonic borders below the Pacific.