It was one heck of a merger. Around 300 million years ago, nearly all the world's landmasses clumped together, forming a supercontinent called Pangea. On it, the first dinosaurs evolved, along with the earliest mammals. Then, roughly 100 million years later, the band started to break up. The smaller continents that once made up Pangea were driven apart by the same phenomenon that had united them in the first place: plate tectonics. So was this a one-time thing or could it happen again in the geological future?
First let's talk about Earth's outermost layer. It's a hard, rocky crust that sits on top of its interior layer, the mantle. Together, the crust and upper mantle comprise what's known as the lithosphere, which is fragmented into a set of moving segments called "tectonic plates." Like rafts floating on a suburban pool, the plates slowly glide across the lower mantle. Two varieties exist: continental and oceanic. Remember that little fun fact — we'll come back to it later.
Interesting things happen at the boundaries where these tectonic plates meet. Sometimes, two of them will be pushed apart by a rift that generates brand-new crust. On other occasions, the plates slide past each other horizontally, establishing what scientists refer to as a transform boundary. However, the most dramatic interactions are when two plates collide head-on. When tectonic plates smack into each other, there are two possible outcomes. Push a pair of continental plates together and a mountain range will rise up at the point of convergence. But if there's an oceanic plate (or two) involved with a collision, the result will be a subduction zone. Subduction is the process whereby the denser of two converging plates is slowly pulled beneath the other and down into the Earth's mantle.
Tectonic forces see to it that our continents are always moving — albeit very, very slowly. The landmasses travel at a rate of about 20 millimeters (0.8 inches) per year; that's the approximate growing speed of human fingernails.
So what does this have to do with the possibility of another Pangaea? Well there's only so much room on the face of our home planet, and the ever-shifting continents can't help but crash into each other once in a while. So it's not too shocking to learn that Pangea wasn't the first supercontinent. Not by a long shot.
The Supercontinent Cycle
Fossil distribution, glacier-made scars and other lines of evidence tell us that at least three Pangea-like supercontinents have occurred throughout the course of Earth's lifetime. There was Nuna, which came together about 1.8 billion years ago. After it split apart, the continents recombined into Rodinia roughly 800 million years later. Eventually, this too broke into fragments. Pangea represented the next — and to date, most recent — reunion of Nuna and Rodinia's former components.
Going forward, the established pattern of drifting and merging will only continue. Among geologists, the consensus is that a future Pangea-style supercontinent is going to form at some point within the next 300 million years. What's debatable, though, is the manner in which that'll happen.
Forecasting the Future
At present, scientists are using GPS technology to track the movements of our modern continents. As you're reading this, the Americas are inching closer to Asia while Australia and Antarctica are shifting north.
Dr. J. Brendan Murphy, a geologist at St. Francis Xavier University, explains that if the Americas stick to their present course, "the Pacific Ocean will close and the Atlantic will widen" until we get an all-new supercontinent in 70 million years or so.
But maybe the Pacific isn't doomed after all. A different school of thought holds that it's the Atlantic Ocean whose days are numbered.
The argument goes like this: Oceanic crust gets denser with age. Eventually, the crust becomes so dense and heavy that it starts subducting. At the Atlantic's center is a mid-ocean ridge, which is the reason why the body of water is currently expanding. The ridge is constantly producing new crust on the sea floor that pushes older crust — which had previously been created by the same ridge — further and further outward. Hence, the Atlantic grows ever wider.
How long can this status quo persist? Some geologists think that the very old crust on the Atlantic's edges is bound to go under. Eventually, these scientists say, new subduction zones will emerge along the coasts of Africa and the Americas. As the zones devour old, dense ocean crust, the Atlantic will theoretically shrink, pulling the American continents backward until they slam into Europe and Africa.
Don't hold your breath waiting for that to happen, though. "The Atlantic's been expanding for 200 million years, so if you assume that the rate at which it will subduct is similar, it'd probably take [the same amount of time] to close," Murphy notes.
A third possibility was put forth in 2012. That February, a Ph.D. candidate at Yale University by the name of Ross Mitchell published his thesis, which centered on supercontinent formation. After evaluating prehistoric trends, he predicted an impending closure of both the Caribbean Sea and the Arctic Ocean. This means that the Americas would merge together and meet up with Eurasia somewhere around the North Pole.
Mitchell now works at Curtin University in Perth, Australia, where he is a member of the Earth Dynamics Research Group, which seeks to enhance our understanding of plate tectonics and supercontinents. Mitchell informs us via email that he’s sticking by the viewpoint expressed in his 2012 thesis. "Nothing has changed my mind yet," he says, "but hopefully some additional lines of evidence we have in the pipeline should help convince others."
For his money, Murphy says he thinks that of these three scenarios, the first one seems like the safest bet. But when all's said and done, there'd be no way to collect on that gamble — not unless somebody devises a way to live for 70 to 200 million years. "It's not really a testable hypothesis, that's for sure!" he says.