A newly discovered crack in the Earth's crust could pull North America and Europe together and cause the Atlantic Ocean to vanish in about 220 million years, scientists say.

A new map of the seafloor off the coast of Iberia—the region of Europe that includes Portugal and Spain—has revealed what could be the birth of a new subduction zone.

Subduction zones happen when tectonic plates—the large rock slabs that make up the Earth's crust—crash into one another. The edge of the heavier plate slides, or subducts, below the lighter plate. It then melts back into the Earth's mantle—the layer just below the crust.

The discovery of this new subduction zone, published on June 6 in the journal Geology, could signal the start of an extended cycle that fuses continents together into a single landmass—or "supercontinent"—and closes our oceans.

This breakup and reformation of supercontinents has happened at least three times during Earth's approximately four-billion-year history.

In the far future, Earth's continents could "look very much like the Pangea," said study first author João Duarte, referring to a supercontinent that existed about 200 million years ago.

So what's new? The newly discovered subduction zone is located in the Atlantic Ocean about 120 miles (200 kilometers) off the southwest coast of Portugal. It is made up of six distinct segments that together span a distance of about 186 miles (300 kilometers).

The subduction zone is actually a newly formed crack in the Eurasian plate—one of about a dozen tectonic plates that make up the Earth's crust. The Eurasian plate contains all of Europe and most of Asia.

"In this case, the Eurasia plate is breaking in two," said Duarte, a geoscientist at the University of Monash in Australia.

Why is it important? Scientists have long suspected that a new subduction zone was forming near the western margin of the Eurasian plate, off the coast of Portugal.

Part of the reason is that the region has been the site of significant earthquake activity, including an 8.7-magnitude quake in 1755 that devastated Lisbon.

Over the past 20 years, several scientific teams from different countries have launched research cruises to map the seafloor around the region to look for proof that a new subduction zone was forming.

As part of his research project while at the University of Lisbon, Duarte gathered together the data from all of the different mapping projects and combined them to create a new tectonic map of the seafloor off the coast of Portugal.

The updated map provided the first conclusive proof that the ocean floor off the coast of Iberia is indeed beginning to fracture, and that a new subduction zone is starting to form.

"It is not a fully developed subduction, but an embryonic one," Duarte said.

What does this mean? The evidence collected by Duarte's team indicate that the Eurasia plate could eventually split into separate oceanic and continental sections.

If this happens, the oceanic section—which is made of denser rock—will dive beneath the continental section. This will cause the Atlantic Ocean to shrink and pull North America and Iberia closer together.

Other studies have indicated that geologic activity in the region could also pull Africa and Iberia together, causing the Mediterranean Sea to vanish.

"Eventually North America and Iberia will be together again, and the collision will give origin to new mountain chains like the Himalaya," Duarte said.

What's next? Scientists will continue to study the nascent subduction zone because it could help answer a long-lasting mystery: How do oceans—especially ones like the Atlantic that have "passive" margins that are free of fractures—start to close?

"For the first time we are seeing a [passive] Atlantic margin turn into a Pacific one," for which subduction zones are common, Duarte said.

His team plans to continue collecting data about the crust and seafloor in the region to further investigate the subduction zone. They are also developing computer and physical models of the subduction process and plate motions.