The Atlantic Ocean grows 1. 5 inches wider every year.
The plates that support the Americas and Europe are moving apart. But precisely how and why that is happening was a mystery to scientists, since the Atlantic doesn’t have the same dense, subducting plates that the Pacific does.
A 2021 study published in the journal Nature suggested the key to the Atlantic’s expansion lies beneath a large underwater mountain range in the middle of the ocean.
The Mid-Atlantic Ridge, a set of mostly underwater peaks that separates the North American Plate from the Eurasian Plate, as well as the South American Plate from the African, is known.
The researchers behind the study found that material from deep within the Earth is rising to the surface under the MAR, pushing the plates on either side of the divide apart.
The Atlantic seafloor is spreading
A 1,800-mile-thick, mainly solid mantle surrounds the Earth’s core, which is as hot as the surface of the sun.
Fragmented into tectonic plates, the Earth’s crust fits together like a puzzle. These plates interact in various ways, moving together or apart or sliding under one another.
Volcanoes are one way magma can bubble up from inside the Earth. Seafloor spreading, which occurs at divergent tectonic plates that are pulling apart like the MAR, is another. Another way is below the crust where, hot, softened rock rises from the mantle, and convection currents pull it toward the surface.
Generally, any material oozing upward under tectonic boundaries like the Mid-Atlantic Ridge usually starts from the mantle very close to the Earth’s surface, about 3 miles below the crust. The material from the mantle’s lower part, closest to the center, doesn’t usually appear there.
But the 2021 study found that the Mid-Atlantic Ridge is a convection hotspot.
The researchers measured geologic activity across a 621-mile span. They dropped 39 seismometers under the waves in 2016, then left them for a year to collect data from earthquakes around the world.
Seismic waves reverberating through material in Earth’s core offered scientists a peek into what was happening in the mantle under the Mid-Atlantic Ridge.
The group found that magma and rock from 410 miles under the crust can push their way to the surface there. That upwelling of material is what is spreading the tectonic plates — and the continents on top — apart at a rate of 1. 5 inches (4 centimeters) per year.
“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,” Matthew Aguis, a seismologist at Roma Tre University and a co-author of the study, told Insider in 2021. “This is what makes this result exciting because it was completely unexpected.”
Often, material trying to make its way from the lower to upper mantle is hindered by a band of dense rock known as the mantle transition zone, between 255 miles and 410 miles under our feet.
But Agius and his colleagues estimated that beneath the MAR, temperatures in the deepest part of that transition zone were higher than expected, making the zone thinner in the area. This is why the material that rises to the sea floor in this area can be more readily deposited than anywhere else on Earth.
Solving a geological mystery
The discovery helped solve a long-standing geological puzzle.
Typically, plates move under the force of gravity as it pulls denser parts of plates into the Earth. Scientists wonder why the Atlantic Ocean plates are moving, as they’re not nearly so dense.
The research suggests the upwelling of material from deep within the mantle could be the engine of that Atlantic expansion.
Catherine Rychert, a geophysicist from the University of Southampton and a co-author of the study, said this process started 200 million years ago. One day, however, this rate could increase.
“Most likely, the rate of expansion will not change during our lifetime. Rychert said that the rate is most likely to change in millions of years, as it has done before.
Correction August 22, 2023: An earlier version of this article misstated how subduction zones affect plate tectonics and oceanic expansion. This article was first published in 2021 and has been updated.
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