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The Atlantic Ocean is widening, pushing the Americas to one side and Europe and Africa to the other. But we don’t know exactly how.
A new study suggests that deep beneath the earth’s crust, in a layer called the mantle, burning rocks rise and grow on tectonic plates – those rocky puzzle pieces that form the earth’s crust – that meet below the Atlantic.
Previously, scientists believed that the continents were mostly separated when the plates under the ocean moved in opposite directions and crashed into other plates, folding back under the force of gravity. But the new study suggests that’s not the big picture.
The research began in 2016, when a group of researchers set sail on a research vessel in the widest part of the Atlantic Ocean between South America and Africa; in other words, “in the middle of nowhere,” said lead author Matthew Agius, who was a postdoctoral researcher at the University of Southampton in the UK at the time, but is now at the University. Roma Tre in Italy.
Related: Under the sea: 50 breathtaking images of our oceans
The place is not a particularly popular route for traveling, Agius said, noting that sometimes days go by without seeing a single other ship or plane. Interaction is limited to the occasional whales and dolphins swimming and a fleeting signal from the ship’s Wi-Fi. Lightless nights blanket the vast sea in breathtaking views of the galaxy and the stars – and it’s very, very calm, Agius said.
But this vast expanse of empty ocean rests on an incredibly important geological point: the Mid-Atlantic Ridge, the largest tectonic frontier on the planet stretching 10,000 miles (16,093 kilometers) from the Arctic Ocean to the southern tip of Africa. This is where the South American and North American plates separate from the Eurasian and African plates, at a rate of about 4 centimeters per year, extending the Atlantic Ocean.
Listen to rumblings
Agius and his team spent five weeks navigating a small portion of the ridge – around 1,000 km (621 miles) – dropping seismometers (instruments that detect seismic waves or vibrations such as those from earthquakes) on the bottom marine.
A year later, the researchers collected the seismometers.
So far, “we’ve never had good footage of what’s going on under the ocean,” Agius said. Since seismic waves behave differently depending on the material they pass through, researchers could use the data to create images, allowing them to scan various layers of the Earth. During this year of listening, the seismometers picked up the vibrations of earthquakes that spread from various parts of the world and through the Earth’s deep mantle – a layer of mostly solid, warm rock from approximately 1800 miles (2900 km) thick.
While the team’s initial goal was to learn how the plates originated and how they aged, and they really intended to study shallower depths of the Earth, the researchers found evidence of a deeper phenomenon at play.
They found that in this area of the ridge, the mantle transition zone – a higher density region that serves as a gatekeeper between the upper and lower mantle layers – was thinner than average, which probably means that she was hotter than normal. The warmer temperatures in the transition zone likely facilitated a “rise” of hot rocks from the Earth’s lower mantle to its upper mantle which actively pushed the plates apart, Agius said.
Researchers previously believed that the plates mainly diverged from each other due to “pulling” at the subduction zones, places where the plates collide and one sinks under the other, recycling the material in the mantle, Agius said. So if you have a plate shot from one side (and crashing with another plate in a subduction zone), and another plate shot from the other side (again crashing with another plate in a subduction zone), this would create ridges in the middle, where hot material from below rises to fill the resulting void.
“It still happens, but the ridges were thought to be an effect of this process,” he said. But their results suggest that when subduction zones separate the plates, upwelling under the ridges could actively help separate them. However, it is not known whether this process is simply related to the Mid-Atlantic Ridge or if all the world’s ridges are experiencing the same thing, Agius said. “The pull is still there, we would just like to determine now if all of the ridges are also pushing.”
Push and pull
“The results” add a piece of the puzzle to understanding flux in the Earth’s mantle, “said Jeroen Ritsema, a professor in the Department of Earth and Environmental Sciences at the University of Michigan, who was not part of the of the study.
And while their analysis is “excellent,” the study is of limited scope, he said. They only looked at a small portion of the Atlantic seabed, so it is not clear whether their findings would hold true along the entire Mid-Atlantic Ridge or even in other Mid-Ocean ridges. “It is difficult to infer a flow of rocks on a global scale in the Earth’s mantle from a single point of view,” Ritsema told Live Sceince. “It’s like looking through a keyhole and trying to figure out what furniture is in the living room, kitchen and bedrooms upstairs.”
Additionally, there might be other explanations for the warmer than normal transition zone.
This is a “remarkable set of data that they collected with great pain,” said Barbara Ramonowicz, professor at the University of California, Berkeley’s Earth and Planetary Science Graduate School and professor emeritus at the Collège de France in Paris, which was also not a part of the study. “I have no doubts about their analysis. … I have reservations about their interpretation,” Ramonowicz told LiveScience. There are well-known plumes nearby that could have been offset and caused this area to warm, she said.
Vedran Lekic, an associate professor in the geology department at the University of Maryland who was also not involved in the study, agrees their explanation is plausible “but not the only one possible to explain the results.” But if the results are replicated elsewhere, it “could call into question our dominant view of ridges,” he added.
These and other similar findings could also change our maps. Around 300 million years ago, the seven continents were united into a single supercontinent known as the Pangea. For millions of years, plates have divided continents, creating ocean boundaries and a modern map. But the spread of the Atlantic Ocean and the shrinking of the Pacific Ocean are slowly, inconspicuously aging these maps and making them increasingly inaccurate. “The cards will change a bit [for now] and over millions and millions of years that will change dramatically, ”said Agius.
The results were published in the journal Nature on Jan 27.
Originally posted on Live Science.
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