In March 2016, a research team dropped 39 seismometers to the bottom of the Atlantic Ocean off the western tip of Africa, to listen for the rumble of earthquakes near and far. A year later, they’ve uncovered a hidden story of how the continents are growing further apart – not pulled from either side by zones of subduction, as previously believed, but perhaps pushed apart by magma bulging in the middle of the ocean.
The probes were placed in parallel lines spanning more than 600 miles, across the seamount ridge that branches off the Atlantic Ocean. The instruments were part of the 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 Earth’s crust and upper mantle, greets the underlying, weaker asthenosphere. To collect the data, a research team from the University of Southampton and Oxford University planted sensors at the bottom of the ocean. The data they collected stretched nearly 400 miles around the planet. The analysis of the data by the team is published today in the journal Nature.
“The transition zone itself was thinner than we expected,” Kate Rychert, a seismologist at the University of Southampton and a chief scientist on the cruises to deposit and restore the deep-sea sensors, said in a video call. “What that suggests is that we have upwelling material from the lower mantle. It’s abnormally hot; usually we don’t think that happens under the ocean ridges. “
Rychert said such lower mantle upwings are typically associated with Hawaii or Iceland – volcanic islands known to spurt over and over. In the Mid-Atlantic Ridge, however, material rises from the lower to the upper mantle, but clearly does not erupt. Just push up firmly, suggesting to the researchers that convection throughout the planet’s mantle may play a substantial role in the tectonics of the plates resting on it.
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“The incredible results shed new light on our understanding of how the Earth’s interior is linked to plate tectonics, with observations not seen before,” explains Matthew Aguis, a seismologist at Università degli studi Roma Tre and lead author of the article, from a university. of Southampton’s press release.
Initially, the plan was to better understand the definition and thickness of the tectonic plates on the Mid-Atlantic Ridge. The ocean floor seismometers and magnetocaloric instruments would image the plate, and the team expected the transition zone to be “very boring,” Rychert said.
The conventional wisdom was that places like the Mid-Atlantic Ridge are relatively quiet when it comes to plate tectonics, and the real geopolitical theater for records was the subduction zones, where two plates come together and one is pushed back into the mantle by the other. Those shifts are responsible for the imperceptible creeping apart of the continents. Below the Pacific, tectonic plates are moving faster, hence the dramatic “Ring of Fire” that causes seismic and volcanic activity at the ocean’s edge. The same cannot be said for the gradual march of the Atlantic plates, which move apart about 1.6 inches per year.
“Why this work is interesting for understanding plate tectonics is that as material wells up through the transition zone, it means that there is an upwelling of convection cell pushing the plates up and pushing them out,” said study co-author Nick Harmon, also a seismologist at the University of Southampton, in a video call.
If you’re considering pizza dough, the difference between making the pie is by pulling on the edges instead of pushing in the center. The difference, of course, is that you’re going up on a planetary scale instead of a culinary scale.
Until better seismic technology is developed, it can be difficult to better understand what is going on so deep in the mantle. Today, even the best data is read as a “blurry CAT scan,” Harmon said. But along the line – and under the sea – they hope to learn something about the dynamics elsewhere along the ridge, as well as the situation in the tectonic boundaries under the Pacific.