Every year, the Americas are moving further away from Europe and Africa . The separation between the Atlantic-facing continents increases by 4 centimeters (1.6 inches) per year, but what was driving this has been a bit of a puzzler. The dividing line where new plates are formed is known as the Mid-Atlantic Ridge. A ridge is usually driven by a balancing act between the lighter and heavier parts of tectonic plates. But this is not the case for the Atlantic ocean, which is not surrounded by dense sinking plates. While the Atlantic ocean is widening, the Pacific ocean is shrinking.
This glacially slow shifting of oceans is due to the ongoing movement of Earth’s tectonic plates, as the plates underneath the Americas pull apart from those underneath Europe and Africa.
The deep, geophysical forces underpinning this epic phenomenon remain far from fully understood, but researchers may have just identified an important contributor to what’s happening.
In a new study, scientists suggest that mid-ocean ridges – mountainous formations that emerge along the seafloor in-between tectonic plates – could be more implicated in the transfer of material between the upper and lower mantle beneath Earth’s crust than we previously realised.
“Sinking slabs and rising plumes are generally accepted as locations of transfer, whereas mid-ocean ridges are not typically assumed to have a role,” a team led by seismologist Matthew Agius from the University of Southampton in the UK explains in a new paper.
“However, tight constraints from in situ measurements at ridges have proved to be challenging.”
“Upwelling from the lower to the upper mantle and all the way up to the surface is typically associated with localized places on Earth, such as Iceland, Hawaii and Yellowstone, and not with mid-ocean ridges,” Roma Tre University seismologist and study coauthor Matthew Aguis told Insider. “This is what makes this result exciting because it was completely unexpected.”
To work out this mystery the team of researchers embarked on two research cruises and placed 39 seismometers several miles deep along the Mid-Atlantic Ridge, the ridge boundary that tectonically separates the Americas from Europe and Africa. VICE reported. They left them there for a full year, from 2016 to 2017. This gifted the scientists with a wealth of data, allowing them to image variations in Earth’s mantle at depths of around 410 to 660 kilometers (approximately 255 to 410 miles), the press release explained. It also provided them with the first high-resolution and large-scale imaging of the mantle beneath the Mid-Atlantic Ridge.
Until now, “we never had good images of what’s happening beneath the ocean,” Agius said. Since seismic waves behave differently depending on the material they move through, the researchers could use the data to create images, allowing them to peer into various layers of the Earth. In that year of listening, the seismometers picked up vibrations from earthquakes that propagated from various parts of the world and through Earth’s deep mantle — a layer of mostly solid, hot rock about 1,800 miles (2,900 km) thick.
“There are similar experiments around the world, but this was a large scale, with so many instruments for such a long time,” Agius told VICE.
“This work is exciting and that it refutes long held assumptions that mid-ocean ridges might play a passive role in plate tectonics,” study coauthor and University of Oxford professor Mike Kendall said in the press release. “It suggests that in places such as the Mid-Atlantic, forces at the ridge play an important role in driving newly-formed plates apart.”
Seismic readings recorded in the experiment monitored the flow of material in the mantle transition zone that lies between the upper mantle and lower mantle, enabling the team to image material transfer at depths as far underground as 660 kilometres (410 miles) below the surface.
The results suggest that upwellings of chemical material are not limited to shallow depths in the Mid-Atlantic Ridge, but can emerge at the deepest reaches of the mantle’s transition zone, suggesting material from the lower mantle rising upwards.
“The observations imply material transfer from the lower to the upper mantle – either continuous or punctuated – that is linked to the Mid-Atlantic Ridge,” the researchers explain.
“Given the length and longevity of the mid-ocean ridge system, this implies that whole-mantle convection may be more prevalent than previously thought.”
While it was already known that mid-ocean ridges contributed to the phenomenon of seafloor spreading, the new findings show that the overall processes involved extend far deeper into the Earth than has previously been measured, and may still occur even in areas of the seafloor not marked by overt regions of plate subduction.
“[The work] refutes long held assumptions that mid-ocean ridges might play a passive role in plate tectonics,” says senior researcher and geophysicist Mike Kendall from the University of Oxford.
“It suggests that in places such as the Mid-Atlantic, forces at the ridge play an important role in driving newly-formed plates apart.”
The findings “add a piece of the puzzle towards understanding flow in Earth’s mantle,” said Jeroen Ritsema, a professor in the department of Earth and Environmental Sciences at the University of Michigan, who was not a part of the study.
It’s a very “remarkable data set that they collected at great pains,” said Barbara Ramonowicz, a professor of the University of California, Berkeley’s Earth and Planetary Science Graduate School and professor emeritus of the College de France in Paris, who was also not a part of the study. “I have no doubt 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 that area to heat up, she said.
Vedran Lekic, an associate professor at the University of Maryland’s Department of Geology who was also not involved with the study, agrees that their explanation is plausible “but not the only possible one to explain the findings.” But if the findings are replicated elsewhere, it “might bring into question our prevailing view of ridges,” he added.
These and other similar findings could also alter our maps. Some 300 million years ago, all seven continents were smooshed together into a single supercontinent known as Pangaea. Over millions of years, plates split the continents, creating ocean boundaries and the modern map. But the spreading of the Atlantic Ocean and the shrinking of the Pacific Ocean is slowly, inconspicuously aging those maps and making them increasingly inaccurate. “The maps will alter a little bit [for now] and over millions and millions of years will alter significantly,” Agius said.
And though their analysis is “excellent,” the study is limited in scope, he said. They looked at only a small portion of the Atlantic seafloor, so it’s not clear if their findings would hold true along the entire mid-Atlantic ridge or even in other mid-ocean ridges. “It is difficult to infer global-scale rock flow in Earth’s mantle from only a single viewpoint,” Ritsema told Live Sceince. “It is like peeking through a keyhole and trying to find out what furniture is in the living room, kitchen and the bedrooms upstairs.”
What’s more, there could be some other explanations for the warmer-than-normal transition zone.
The findings are reported in Nature.
