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.
"Understanding these processes will certainly provide new insights on how subduction zones may have initiated in the past and how oceans start to close," Duarte said in a statement.
Geologic puzzle: Why do the coastlines of South America and Africa on opposite sides of the Atlantic Ocean seem to fit together? In 1915, German scientist Alfred Wegener proposed that continents could move around on the earth’s surface.
• Wegener suggested that the continents were all together in the geologic past, forming the supercontinent Pangaea.
(Diagrams courtesy of the United States Geological Survey)
Evidence suggested that Pangaea began to break up about 200 million years ago. By 160 million years ago, continents had begun to drift to their present locations. Today’s coastlines of South America and Africa are a match because these two continents were once joined together.
• Evidence supported the hypothesis. Similar fossils were found on both sides of the Atlantic. Glacial features on widely distant continents matched.
Glossopteris fossil (circled) (Photograph by Parvinder Sethi)
This seed fern fossil, called Glossopteris, is one of the many fossils that were found on both sides of the Atlantic Ocean. Scientists could not understand how the seeds could have migrated across the oceans unless the continents were connected by mysterious land bridges or perhaps the oceans were much smaller.
• Continental drift was rejected because no known force could move continents through the seafloors between them.
Continental drift describes one of the earliest ways geologists thought continents moved over time. Today, the theory of continental drift has been replaced by the science of plate tectonics. The theory of continental drift is most associated with the scientist Alfred Wegener. In the early 20th century, Wegener published a paper explaining his theory that the continental landmasses were “drifting” across the Earth, sometimes plowing through oceans and into each other. He called this movement continental drift. Pangaea Wegener was convinced that all of Earth’s continents were once part of an enormous, single landmass called Pangaea. Wegener, trained as an astronomer, used biology, botany, and geology describe Pangaea and continental drift. For example, fossils of the ancient reptile mesosaurus are only found in southern Africa and South America. Mesosaurus, a freshwater reptile only one meter (3.3 feet) long, could not have swum the Atlantic Ocean. The presence of mesosaurus suggests a single habitat with many lakes and rivers. Wegener also studied plant fossils from the frigid Arctic archipelago of Svalbard, Norway. These plants were not the hardy specimens adapted to survive in the Arctic climate. These fossils were of tropical plants, which are adapted to a much warmer, more humid environment. The presence of these fossils suggests Svalbard once had a tropical climate. Finally, Wegener studied the stratigraphy of different rocks and mountain ranges. The east coast of South America and the west coast of Africa seem to fit together like pieces of a jigsaw puzzle, and Wegener discovered their rock layers “fit” just as clearly. South America and Africa were not the only continents with similar geology. Wegener discovered that the Appalachian Mountains of the eastern United States, for instance, were geologically related to the Caledonian Mountains of Scotland. Pangaea existed about 240 million years ago. By about 200 million years ago, this supercontinent began breaking up. Over millions of years, Pangaea separated into pieces that moved away from one another. These pieces slowly assumed their positions as the continent we recognize today. Today, scientists think that several supercontinents like Pangaea have formed and broken up over the course of the Earth’s lifespan. These include Pannotia, which formed about 600 million years ago, and Rodinia, which existed more than a billion years ago. Tectonic Activity Scientists did not accept Wegener’s theory of continental drift. One of the elements lacking in the theory was the mechanism for how it works—why did the continents drift and what patterns did they follow? Wegener suggested that perhaps the rotation of the Earth caused the continents to shift towards and apart from each other. (It doesn't.) Today, we know that the continents rest on massive slabs of rock called tectonic plates. The plates are always moving and interacting in a process called plate tectonics. The continents are still moving today. Some of the most dynamic sites of tectonic activity are seafloor spreading zones and giant rift valleys. In the process of seafloor spreading, molten rock rises from within the Earth and adds new seafloor (oceanic crust) to the edges of the old. Seafloor spreading is most dynamic along giant underwater mountain ranges known as mid-ocean ridges. As the seafloor grows wider, the continents on opposite sides of the ridge move away from each other. The North American and Eurasian tectonic plates, for example, are separated by the Mid-Atlantic Ridge. The two continents are moving away from each other at the rate of about 2.5 centimeters (1 inch) per year. Rift valleys are sites where a continental landmass is ripping itself apart. Africa, for example, will eventually split along the Great Rift Valley system. What is now a single continent will emerge as two—one on the African plate and the other on the smaller Somali plate. The new Somali continent will be mostly oceanic, with the Horn of Africa and Madagascar its largest landmasses. The processes of seafloor spreading, rift valley formation, and subduction (where heavier tectonic plates sink beneath lighter ones) were not well-established until the 1960s. These processes were the main geologic forces behind what Wegener recognized as continental drift.
Fast Fact
Colliding Skyward
The collision of the Indian subcontinent and Asian continent created the Himalayan mountain range, home to the world's highest mountain peaks, including 30 that exceed 7300 meters (24,000 feet). Because continental drift is still pushing India into Asia, the Himalayas are still growing.
Fast Fact
UrkontinentAlfred Wegener’s original name for his proposed, ancient continent was “Urkontinent”—ur meaning “first or original,” and kontinent meaning “continent” in Wegener’s native language, German. A more popular name for this huge ancient landmass is Pangaea, which means “all lands” in Greek.