Over the next few hundred million years, the Arctic Ocean and the Caribbean Sea will disappear, and Asia will crash into the Americas forming a supercontinent that will stretch across much of the Northern Hemisphere. That's the conclusion of a new analysis of the movements of these giant landmasses.
Unlike in today's world, where a variety of tectonic plates move across Earth's surface carrying the bits of crust that we recognize as continents, ancient Earth was home to supercontinents, which combined most if not all major landmasses into one. Previous studies suggest that supercontinents last about 100 million years or so before they break apart, setting the pieces adrift to start another cycle.
The geological record reveals that in the past 2 billion years or so, there have been three supercontinents, says Ross Mitchell, a geophysicist at Yale University. The oldest known supercontinent, Nuna, came together about 1.8 billion years ago. The next, Rodinia, existed about 1 billion years ago, and the most recent, Pangaea, came together about 300 million years ago. In the lengthy intervals between supercontinents, continent-sized-and-smaller landmasses drifted individually via plate tectonics, as they do today.
Scientists can track the comings and goings of those landmasses by analyzing the iron-bearing magnetic minerals in various types of rock deposits. That's because the iron atoms, and sometimes even tiny magnetized bits of iron-bearing rock, line up with Earth's magnetic field when they're free to rotate, as they are when the material that contains them is molten. Once the rocks have solidified—and if they aren't heated above the temperature at which their magnetic information is wiped clean—careful analyses can reveal where on Earth those rocks were when they first cooled, even if the rocks are hundreds of millions of years old. In particular, the rocks retain a record of their paleolatitude, or how far they were from Earth's magnetic pole.
Although supercontinents before Nuna may have existed, rocks more than 2 billion years old that still preserve evidence of ancient magnetic fields are scarce, Mitchell says. And although scientists have generally agreed that Nuna, Rodinia, and Pangaea existed, exactly where on Earth each came together has been a matter of strong debate. Some geophysical models have suggested that drifting landmasses have come together in the same spot on Earth's surface each cycle. Other teams have proposed that the wandering pieces reassembled on the opposite side of the planet, 180° away from where the previous supercontinent broke apart.
Now, Mitchell and his colleagues suggest an intermediate answer—that each supercontinent has come together about 90° away from its predecessor. The team's analyses, reported online today in Nature, use techniques that determine the paleolatitude of ancient landmasses but also, for the first time, estimate their paleolongitude by taking into account how the locations of Earth's magnetic poles changed through time. Together, these data suggest that the geographic center of Rodinia was located about 88° away from the center of Nuna, and the center of Pangaea—which was located near present-day Africa—sat about 87° from Rodinia's center.
These angles are no accident, the researchers suggest: The drifting pieces of crust eventually come together along the former edge of the fractured supercontinent—an area approximately 90° away from the former supercontinent's center. That's where relatively dense ocean crust was being shoved beneath the lighter continental crust, causing a downward flow in the underlying mantle that in turn attracted the drifting bits like water running down a drain.
According to this model, the next supercontinent—a sprawling landmass dubbed Amasia, which in its earliest stages will merge Asia with the Americas—will stretch across much of the Northern Hemisphere, the researchers suggest. Over the next few hundred million years, Mitchell says, the motions of tectonic plates will cause the Arctic Ocean and the Caribbean Sea to disappear, the western edge of South America to crowd up against the eastern seaboard, and Australia to slam into southeastern Asia. It's unclear whether Antarctica will join the party or be stranded at the South Pole.
"This is a beautiful piece of work," says Joseph Kirschvink, a geophysicist at California Institute of Technology in Pasadena. Most of the high-quality paleomagnetic data available today has been collected in the past 20 years or so, he notes. "And the more data we have, the more we can recognize the patterns of where chunks of Earth's crust must have been."
The team's ideas about how and where supercontinents form are "reasonable but far from proven," says Bernhard Steinberger, a geodynamicist at the German Research Centre for Geosciences in Potsdam. Although Mitchell and his colleagues have identified statistical trends in their paleomagnetic analyses, he notes, "the data still just look like clouds of points to me."
The team's results "are very impressive," says Brendan Murphy, a geologist at St. Francis Xavier University in Antigonish, Canada. Because the breakup and assembly of supercontinents is arguably one of the most important cycles in Earth's biological and geological evolution, the findings will undoubtedly stimulate further research and analyses, he notes. "And even if the new model is wrong," he adds, "we'll learn a lot by testing it."