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An ascent of rocks under the Atlantic can separate the continents

A rise of hot rock from the bottom of the Atlantic Ocean may be separating the continents on either side.

The Americas move away from Europe and Africa a few inches each year, as the tectonic plates underlying those continents deviate. Researchers typically think that tectonic plates separate as the distant edges of those plates sink into the Earth’s mantle, creating a gap (SN: 1/13/21). The material from the upper mantle then drains through the gap between the plates to fill the seabed.

But new seismic data from the Atlantic Ocean floor shows that hot rock is emerging beneath a seabed fissure called the Mid-Atlantic Ridge from hundreds of miles deep in the Earth's mantle. This suggests that the material rising below the ridge is not just a passive response to the tectonic plates slipping. Conversely, deep rock pushing toward the Earth’s surface may be driving a wedge between the plates that helps separate them, researchers reported online on Jan. 27 in Nature.

A better understanding of plate tectonics – which causes earthquakes and volcanic eruptions – could help people better prepare for these natural disasters (SN: 9/3/17).

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Matthew Agius, a seismologist at Rome Tre University in Rome, and colleagues glimpsed what is happening below the Mid-Atlantic Ridge using 39 seismometers on the seabed near a point along the ridge between South America and Africa. Those sensors monitored rumors of earthquakes around the world for about a year. Because the seismic waves from those earthquakes traveled deep through the Earth’s mantle on their way to the seismometers, the tremors recorded contained clues about the location and movement of the material far below the seabed.

At those signs, the Agius team saw signs of material from the Earth's lower mantle, more than 600 kilometers below the seabed, coming out toward the mid-Atlantic ridge. “This was completely unexpected,” says Agius, and could be a powerful force to push the tectonic plates away on both sides of the cleft.

“It’s definitely an interesting observation,” says Jeroen Ritsema, a seismologist at the University of Michigan in Ann Arbor who didn’t participate in the work. But it is difficult to say how much the deep outcrop of the mantle contributes to the spread of the Atlantic bottom, based on observations of only a group of seismometers near the equator, he said. It's like "you're looking through a keyhole and trying to see what's in the living room, the bedroom, and the kitchen."

Observations elsewhere along the Mid-Atlantic ridge, as well as in other mid-ocean ridges around the world, could help determine whether the deep mantle material that emerges beneath these reefs actually plays an important role in the spread of the seabed.

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