A Tectonic Plate May Have Peeled Apart…
and that could shrink the Atlantic Ocean
Something strange is happening off the coast of Portugal, and scientists have now proposed a groundbreaking explanation.
BY MAYA WEI-HAAS | National Geographic
PUBLISHED MAY 6, 2019
For years, João Duarte has puzzled over a seemingly boring underwater expanse off the coast of Portugal. In 1969, this site spawned a massive earthquake that rattled the shore and sparked a tsunami. But you would never know why just from looking at the broad, featureless surface of the seabed. Duarte, a marine geologist from the Instituto Dom Luiz at the University of Lisbon, wanted to find out what was going on.
Now, 50 years after the event, he may finally have an answer: The bottom of the tectonic plate off Portugal’s coast seems to be peeling away from its top. This action may be providing the necessary spark for one plate to start grinding beneath another in what’s known as a subduction zone, according to computer simulations Duarte presented in April at the European Geosciences Union meeting.
If confirmed, the new work would be the first time an oceanic plate has been caught in the act of peeling—and it may mark one of the earliest stages of the Atlantic Ocean shrinking, sending Europe inching toward Canada as predicted by some models of tectonic activity. (Find out what scientists think will happen when Earth’s tectonic plates grind to a halt.)
“It’s certainly an interesting story,” says the University of Oslo’s Fabio Crameri, who was not part of the research team but who attended the EGU lecture. Duarte presented some strong arguments, he says, but he cautions that the model needs further testing—not an easy feat when your data comes from a natural process that works at the speed at which fingernails grow.
“It’s a big statement,” Duarte says of the conclusions, acknowledging that he and his team still have work to do. “Maybe this is not the solution to all the problems. But I think we have something new here.”
The tectonic parade
Earth’s tectonic plates are constantly in a slow-motion march, with some edges pulling apart and others colliding. At least three times in our planet’s 4.54-billion-year history, the ever-shifting landmasses glommed into mighty supercontinents, only to eventually reverse course and break apart. Subduction zones are major driving forces behind this tectonic conveyor belt, as they pull oceanic crust and upper mantle down to depth, recycling the rocks and dragging continents around in the process. (Learn more about the possible future supercontinent, Pangaea Proxima.)
So how do subduction zones start? “It’s one of the biggest unsolved problems in plate tectonics,” Duarte says.
One way to locate subduction zones—and perhaps also baby subduction zones—is to follow the earthquakes. Around 90 percent of the world’s quakes pop off in the disjointed string of subduction zones that trace the so-called ring of fire, which stretches in an arc around the Pacific Ocean from the southern tip of South America to New Zealand, by way of the Bering Sea. (Read about how the powerful 2017 earthquake in Mexico seems to have snapped a tectonic plate in two.)
But the Iberian Peninsula, which includes Spain and Portugal, is on the other side of the world, touching the Atlantic Ocean. Here, plates instead pull apart down the ocean’s middle and form new crust, and the edges of most surrounding landmasses transition from continent to ocean on a single plate.
It’s like the plains of Kansas under 4.8 kilometers [three miles] of water.
MARC-ANDRÉ GUTSCHER, UNIVERSITY OF BREST
“It sounds wild, it sounds crazy, but it was not my idea,” Duarte quips. “He drew in 1975 the result I have in my numerical model—it’s mind-blowing.”
The work has yet to appear in a peer-reviewed journal, and for now, other geologists are approaching the results with a mix of cautious excitement and healthy skepticism.
“Most of what we know so far is that new subduction tends to stay in the places where we already have ongoing subduction,” Crameri says. “But that doesn’t mean it won’t happen.”
Importantly, the model does seem to explain the unusual featureless expanse that lies above the earthquake’s point of origin, Gutscher notes. The thorough work also includes many of the forces that would be at play due to the spidery fractures that surround the area of interest, adds Valentina Magni of the University of Oslo, who was an organizer of the EGU session. But she remains dubious that the model actually matches reality.
“I think it’s very hard to start subduction just like that where nothing around is happening,” she says.
Duarte and his coauthors are currently working on writing up their research to submit for publication, so their data can be more widely reviewed and debated. If accepted, he says, he’s sending the first copy to Purdy.