The Atlantic Ocean expands and pushes the Americas to one side and Europe and Africa to the other. But it is not known exactly how.
A new study suggests that deep beneath the Earth’s crust, in a layer called the mantle, hot rocks are rising that push toward the tectonic plates (those rocky pieces that make up the Earth’s crust) that lie beneath the Atlantic.
Previously, scientists thought that continents separated primarily as plates under the ocean moved in opposite directions and crashed into other plates, folding under the force of gravity. But the new study suggests this is not the full picture.
The investigation began in 2016, when a group of researchers set sail on a research vessel to the widest part of the Atlantic Ocean, between South America and Africa; in other words, to the “middle of nowhere,” said lead author Matthew Agius, who at the time was a postdoctoral researcher at the University of Southampton in the United Kingdom but is now at Rome Tre University in Italy.
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The place is not a particularly popular route to travel, said Agius, who noted that sometimes days would pass without seeing a single boat or plane. The interaction is limited to the occasional whales and dolphins swimming and a fleeting signal from the boat’s Wi-Fi. Lightsless nights cover the vast sea in an unobstructed view of the galaxy and the stars, and it’s very, very quiet, Agius said.
But this vast, empty stretch of ocean rests on an incredibly important geological point: the mid-Atlantic ridge, the largest tectonic boundary on the planet that stretches 16,093 kilometers from the Arctic Ocean to the southern tip of Africa. This is where the South American and North American plates move away from the Eurasian and African plates, at a rate of about 1.6 centimeters (4 centimeters) a year, extending the Atlantic Ocean.
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Agius and his team spent five weeks navigating a small portion of the ridge (approximately 1,000 km) dropping seismometers (instruments that detect seismic waves or vibrations such as earthquakes) at the bottom of the sea.
A year later, researchers collected the seismometers.
So far, “we never had good pictures of what’s going on under the ocean,” Agius said. Because seismic waves behave differently depending on the material they move through, researchers could use the data to create images, allowing them to look at various layers of the Earth. In that year of listening, seismometers captured the vibrations of earthquakes that propagated from various parts of the world and through the deep mantle of the Earth, a mostly solid layer of hot rock about 2,900 km thick.
Although the original goal of the team was to find out how the plates were born and aged, and that they really intended to study the lowest depths of the Earth, the researchers found evidence of a phenomenon more deep in play.
They found that in this area of the ridge, the transition zone of the mantle (a higher density region that serves as a control gate between the upper and lower layers of the mantle) was thinner than average, which probably means that it was hotter than normal. Higher temperatures in the transition zone probably facilitated a “rise” of hot rock from the Earth’s lower mantle to the upper mantle that actively pushed the plates apart, Agius said.
Earlier, the researchers thought the plates diverged mainly from each other due to a “stretch” in the subduction zones, places where the plates collide and one sinks beneath the other, recycling material into the mantle, Agius said. . Thus, if you have a plate stretched to one side (and collide with another plate in a subduction zone), and another plate stretched to the other side (collide again with another plate in a subduction zone), ridges would be created in the middle, where the hot material below rises to fill the resulting void.
“This continues to happen, but mountain ranges were thought to be an effect of this process,” he said. But his findings suggest that as subduction zones separate the plates, uplifts under the ridges could actively help separate them. However, it is unclear whether this process is only related to the mid-Atlantic ridge or whether all ridges in the world experience the same thing, Agius said. “The stretch is still there, we would just like to determine now if all the mountain ranges are also experiencing pushes.”
Pushing and stretching
“The findings” add a piece of the puzzle to understanding the flow 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 study.
And while his analysis is “excellent,” the study has limited scope, he said. They observed only a small portion of the Atlantic seabed, so it is unclear whether their findings would remain true along the entire Atlantic ridge or even on other ocean ridges. “It’s hard to infer global rock flow in the Earth’s mantle just 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, the kitchen and the bedrooms upstairs.”
In addition, there could be some other explanations for the warmer-than-normal transition zone.
This is a very “remarkable data set that they collected with a lot of pain,” said Barbara Ramonowicz, a professor at the University of California, Berkeley Earth and Planetary Science Graduate School and professor emeritus at the College de France in Paris. nor was it part of the study. “I have no doubt about his analysis. … I have reservations about his interpretation,” Ramonowicz told LiveScience. There are known feathers nearby that could have been offset and cause this area to heat up, he said.
Vedran Lekic, an associate professor in the Department of Geology at the University of Maryland, who also did not participate in the study, agrees that his explanation is plausible “but not the only one possible to explain the findings.” But if the findings are replicated elsewhere, “it could call into question our dominant view of the ridges,” he added.
These and other similar discoveries could also alter our maps. About 300 million years ago, the seven continents merged into a single supercontinent known as Pangea. Over millions of years, the plates divide the continents, creating ocean boundaries and the modern map. But the spread of the Atlantic Ocean and the shrinking of the Pacific Ocean are slowly aging these maps and making them increasingly inaccurate. “Maps will change a bit [for now] and over millions and millions of years it will change significantly, ”Agius said.
The findings were published in the journal Nature in January. 27.
Originally published in Live Science.