The Atlantic Ocean widens each year, pushing the Americas away from Europe and Africa. We can finally find out why.

• The tectonic plates under the Americas, Europe, and Africa are spreading apart as the Atlantic Ocean widens year after year.

• New research reveals what separates the plates: Material from deep within the Earth is erupting upward on an undersea ridge in the mid-Atlantic.

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The Atlantic Ocean widens 1.5 inches each year.

This is because the tectonic plates that underlie the Americas are separating from those under Europe and Africa. But precisely how and why this happens is a mystery to scientists, since the geological forces that typically separate the plates are not widespread in the Atlantic.

A study published Wednesday finally answers this question. The research, published in the journal Nature, suggests that the key to the expansion of the Atlantic lies under a large underwater mountain range in the middle of the ocean.

This set of submarine peaks is known as the Mid-Atlantic Ridge and separates the North American Plate from the Eurasian Plate and the South American Plate from the African Plate. The researchers behind the study found that material from deep within the Earth was rising to the surface below the Mid-Atlantic Ridge, pushing the plates on either side of the fracture.

The Atlantic seabed is spreading

Imagine the Earth as a chocolate truffle – a slimy center embedded in a hardened shell.

The center consists of an 1,800 mile thick semi-solid mantle that surrounds a super hot core. The top layer of the truffle – only about 21 miles thick – is the earth’s crust, which is fragmented into tectonic plates that fit together like a puzzle. These plates surf the mantle, moving as the warmer, less dense material from the depths of the Earth rises toward the crust and the cooler, denser material sinks toward the core. This process, known as convection, usually occurs when two plates collide and one sub-duct, or sink, under another.

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In general, any upward seepage of matter below tectonic boundaries like the Mid-Atlantic Ridge typically begins from a portion of the mantle very close to the Earth’s surface, about 2 miles below the crust. The material of the lower mantle, the part closest to the core, is not usually found bursting towards the crust.

But the new study found that the Mid-Atlantic Ridge is a convection hotspot.

The researchers measured geological activity over a range of 621 miles. They dropped 39 seismometers under the waves in 2016, then left them for a year to collect data on earthquakes around the world. The seismic waves reverberating through the material of the Earth’s core gave scientists a glimpse of what was happening in the mantle under the Mid-Atlantic Ridge.

The group found that magma and rock located 410 miles below the crust can make their way to the surface. This upwelling is what spreads the tectonic plates – and the continents above – at a rate of 4 centimeters per year.

“Upwelling from the lower mantle to the upper mantle and down to the surface is generally associated with localized places on Earth, such as Iceland, Hawaii and Yellowstone, and not with mid-ocean ridges”, Matthew Aguis, seismologist at Roma Tre University and co-author of the new study, told Insider. “This is what makes this result so exciting because it was completely unexpected.”

Often times, materials trying to make their way from the lower mantle to the upper mantle are hampered by a dense band of rock known as the mantle transition zone, between 255 miles and 410 miles below our feet.

But Agius and his colleagues estimated that below the Mid-Atlantic Ridge, temperatures in the deepest part of this transition zone were warmer than expected, making the area thinner in the region. This is why materials can reach the ocean floor more easily than in other parts of the Earth.

Solve a geological mystery

The discovery helps solve a long-standing geological puzzle.

Researchers knew that the oceans were expanding and contracting at different rates. They also knew that the plates separated most sharply in subduction zones, which typically occur at active continental margins – where the boundary between a continent and the ocean is also a tectonic plate boundary. This is why the Pacific Ocean is developing faster than the Atlantic: most of the Pacific sits atop a tectonic plate, and its boundaries align almost perfectly with the continental borders on the east and west sides, the North American and Eurasian Plates. Subduction at these limits causes the earthquakes and volcanic eruptions that characterize the region’s aptly named “Ring of Fire”.

The Atlantic Ocean, however, sits atop four major plates with boundaries that do not correspond to continental boundaries – the boundaries are in the middle of the ocean. Scientists were therefore perplexed as to the expansion of its seabed.

But new research suggests that the upwelling of material from the depths of the mantle could be the engine of this Atlantic expansion.

Catherine Rychert, a geophysicist at the University of Southampton and co-author of the new study, said this process began 200 million years ago. But someday the rate of expansion could accelerate.

“Most likely, the rate will stay the same over our lifetime. However, it is likely that the rate will change over millions of years because it has varied in the past,” Rychert told Insider.

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