On land, most canyons are carved by erosion from rivers over millions of years. In the ocean, things are a bit trickier. The King’s Trough Complex, located more than 600 miles off the coast of Portugal, is a massive canyon that includes one of the deepest points in the Atlantic Ocean—and was once a candidate to become an underwater nuclear waste dumping spot. But how did it get there?
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To find out, geologists from GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany hit the seas in a 300-foot research vessel equipped with high-resolution sonar systems to map the ocean floor and a chain bag dredge to retrieve rock samples. After analyzing the chemical composition of the volcanic rocks, the team was able to determine how and when this deep-sea canyon formed. They published their findings in Geochemistry, Geophysics, Geosystems (G-Cubed).
HIDDEN DEPTHS: This bathymetric map of King’s Trough Complex shows the deep basins at its eastern end, based on new data. Image courtesy of Geomar.
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“Researchers have long suspected that tectonic processes—that is, movements of the Earth’s crust—played a central role in the formation of the King’s Trough,” study author Antje Dürkefälden explained in a statement. “Our results now explain for the first time why this remarkable structure developed precisely at this location.”
Between 37 and 24 million years ago, a tectonic plate boundary shifted to the area, resulting in the crust fracturing and the seafloor between Europe and Africa opening like a zipper in an east-west direction. Prior to the shift, the crust was thickened and heated by an upwelling of molten rock from the mantle, making it particularly fragile.
Read more: “Why Is It So Difficult to Map the Ocean?”
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“This thickened, heated crust may have made the region mechanically weaker, so that the plate boundary preferentially shifted here,” added co-author Jörg Geldmacher. “When the plate boundary later moved farther south toward the modern Azores, the formation of the King’s Trough also came to a halt.”
It’s a remarkable example of how activity deep within our planet’s molten mantle can have a dramatic impact on the surface. 
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Lead image: Igor Kyrlytsya / Shutterstock