Science

Earth’s core appears to be surrounded by an enigmatic layer, geologists say

For decades, scientists have debated the nature and origin of what some experts consider the most anomalous and enigmatic layer of the Earth’s deep interior: the boundary between its scorching liquid outer core and the solid mantle that surrounds it.

New measurements from 15 seismic stations buried in snow across Antarctica have revealed that this strange boundary layer, nearly 2,000 miles deep, may have once been part of the surface. Evidence suggests that in subduction zones, where one tectonic plate plunges beneath another, an ancient ocean floor is pushed down and falls to the bottom of the mantle for hundreds of millions of years.

The findings, published in the journal Science Advances, provide insight into the geological cycle powered by tectonic plates. They also offer new clues about how heat moves through the Earth’s many layers.

Understanding this planetary rolling is an existential quest: the heat emitted by the planet’s solid metallic core drives the movement of fluid in the outer core, which is believed to generate the protective magnetic field that makes life possible on the surface.

Scientists have discovered a new nucleus in the center of the Earth

But the results also show the difficulty of providing firm descriptions of Earth’s inaccessible innards. Scientists are quick to view their findings as a likely possibility, not a definitive conclusion. And fundamental questions remain: the border could be uneven, with pieces of ocean floor, or it could be a continuous layer variable thickness enveloping the nucleus.

“For a long time there has been a debate in the scientific community about whether subduction slabs could descend to the core-mantle boundary,” said Elizabeth Day, a seismologist at University College London who n did not participate in the study. though she noted that most experts now believe that at least some slabs can fall that far.

“Given the complexity of our planet’s surface,” she added, “it seems very reasonable that there is a lot of complexity at the core-mantle boundary.”

Most people don’t think much of the Earth’s deep interior, thinking of our home planet as mere interlocking layers, like jaw-breaking hard candy. But Samantha Hansen, a seismologist at the University of Alabama who led the work, described the abrupt transition between the liquid outer core and the mantle as more pronounced than the difference between solid rock and air.

She proposes to think of this underground boundary zone as a geological asp.

“It’s kind of a hard thing to visualize,” Hansen said. “You have this very dense, very anomalous structure with these other gap-filling elements around it, if you will. Think of it like stuff stuck in Jell-O, maybe.

Because the depths of the planet are off-limits to direct exploration, scientists measure how seismic waves generated by earthquakes propagate inside. When these waves encounter this boundary layer – officially known as the ultra-low-velocity zones – they bounce and are deflected in a way that allows geoscientists to model the structure.

Hansen and his colleagues made several trips to Antarctica between 2012 and 2015 to dig holes in the snow and set up seismic monitoring stations, then returned to collect data. Their main objective was to study a poorly known mountain range that crosses the continent, the Transantarctic Mountains. But they realized that their instruments also allowed them to scan parts of the deep interior below the southern hemisphere.

What they found was a thin but dense layer between the core and the mantle that varied in thickness from three to 25 miles.

“It’s five times the size of Mount Everest,” Hansen said. Earth’s tallest peak was recently measured at 29,031 feet, or nearly 5.5 miles. “You have this dramatic topography, some being lower and some very high.” This jagged structure, coupled with geological models, suggests that they consist of subducted seabed, conclude the authors of the article.

John Vidale, a University of Southern California seismologist who was not involved in the work, said he was not entirely convinced by the results, stressing that there are still many possible explanations for what happened. passes on the border between mantle and heart.

“The core-mantle boundary is one of the most dynamic parts of the planet. … We just don’t know what’s going on,” Vidale said.

The hope is that understanding the structure of this boundary zone will help unravel other mysteries: how does the core radiate heat to the mantle? Are the old ocean floors being recycled again, carried away in mantle plumes like the one under Hawaii and become surface material by volcanic eruptions?

Even though Earth has been around for about 4.5 billion years and geology isn’t a new field, our understanding of the planet’s innards is still immature, Hansen said. Fundamental questions are still there, and scientists are eager to find answers.

“I think there’s still a lot we don’t know and a lot to learn,” Hansen said. “It’s good that we don’t know much about the planet we live on.”

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