Creeping more slowly than a human fingernail grows, Earth's massive continents are nonetheless on the move.

by Patrick L.Barry

The Earth is going to be a very different place 250 million years from now.

Africa is going to smash into Europe as Australia migrates north to merge with Asia. Meanwhile the Atlantic Ocean will probably widen for a spell before it reverses course and later disappears.

Two hundred and fifty million years ago the landmasses of Earth were clustered into one supercontinent dubbed pangaea. As Yogi Berra might say, it looks like "deja vu all over again" as the present-day continents slowly converge during the next 250 million years to form another mega-continent: Pangaea Ultima.

The surface of the Earth is broken into large pieces that are slowly shifting -- a gradual process called "plate tectonics." Using geological clues to puzzle out past migrations of the continents, Dr. Christopher Scotese, a geologist at the University of Texas at Arlington, has made an educated "guesstimate" of how the continents are going to move hundreds of millions of years into the future.

"We don't really know the future, obviously," Scotese said. "All we can do is make predictions of how plate motions will continue, what new things might happen, and where it will all end up." Among those predictions: Africa is likely to continue its northern migration, pinching the Mediterranean closed and driving up a Himalayan-scale mountain range in southern Europe.

What's it like to see two continents collide? Just look at the Mediterranean region today.

Africa has been slowly colliding with Europe for millions of years, Scotese said. "Italy, Greece and almost everything in the Mediterranean is part of (the African plate), and it has been colliding with Europe for the last 40 million years."

That collision has pushed up the Alps and the Pyrenees mountains, and is responsible for earthquakes that occasionally strike Greece and Turkey, Scotese noted.

"The Mediterranean is the remnant of a much larger ocean that has closed over the last 100 million years, and it will continue to close," he said. "More and more of the plate is going to get crumpled and get pushed higher and higher up, like the Himalayas."

Australia is also likely to merge with the Eurasian continent. 

"Australia is moving north, and is already colliding with the southern islands of Southeast Asia," he continued. "If we project that motion, the left shoulder of Australia gets caught, and then Australia rotates and collides against Borneo and south China -- sort of like India collided 50 million years ago -- and gets added to Asia."

Image courtesy of Dr. Christopher Scotese. A map of the world as it might appear 250 million years from now. Notice the clumping of most of the world's landmass into one super-continent, "Pangaea Ultima," with an inland sea -- all that's left of the once-mighty Atlantic Ocean.

Meanwhile, the Americas will be moving further away from Africa and Europe as the Atlantic Ocean steadily grows. The Atlantic sea floor is split from north to south by an underwater mountain ridge where new rock material flows up from Earth's interior. The two halves of the sea floor slowly spread apart as the ridge is filled with the new material, causing the Atlantic to widen.

"It's about as fast as your fingernails grow. Maybe a little bit slower," Scotese said. Still, over millions of years that minute movement will drive the continents apart. 

That part of the prediction is fairly certain, because it is just the continuation of existing motions. Beyond about 50 million years into the future, prediction becomes more difficult.

"The difficult part is the uncertainty in (new behaviours)," Scotese said. 

"It's like if you're travelling on the highway, you can predict where you're going to be in an hour, but if there's an accident or you have to exit, you're going to change direction. And we have to try to understand what causes those changes. That's where we have to make some guesses about the far future -- 150 to 250 million years from now."

In the case of the widening Atlantic, geologists think that a "subduction zone" will eventually form on either the east or west edges of the ocean. At a subduction zone, the ocean floor dives under the edge of a continent and down into the interior of the Earth.

"The subduction zone turns out to be the most important part of the system if you want to understand what causes the plates to move," Scotese said.

Like cold air drifting down from an open attic in winter, the cold, dense seabed at the ocean's edges sometimes starts sinking into the playdough-like layer beneath the crust, called the "mantle."

"As it sinks, it pulls the rest of the plate with it," like a tablecloth sliding off a table. This accounts for most of the force that moves the plates around, Scotese said.

This "slab pull" theory for the mechanism driving the motion of the plates stands in opposition to the older "river raft" theory.

"For a long time, geologists had this model that there were 'conveyer belts' of mantle convection, and the continents were riding passively on these conveyer belts, sort of like a raft on a river," Scotese said. "But that theory's all wrong."

If a subduction zone starts on one side of the Atlantic -- Scotese thinks it will be the west side -- it will start to slowly drag the sea floor into the mantle. If this happens, the ridge where the Atlantic sea floor spreads would eventually be pulled into the Earth. The widening would stop, and the Atlantic would begin to shrink.

A diagram showing the major processes of plate tectonics.

Tens of millions of years later, the Americas would come smashing into the merged Euro-African continent, pushing up a new ridge of Himalayan-like mountains along the boundary. At that point, most of the world's landmass would be joined into a super-continent called "Pangaea Ultima." The collision might also trap an inland ocean, Scotese said.

"It's all pretty much fantasy to start with. But it's a fun exercise to think about what might happen," he said. "And you can only do it if you have a really clear idea of why things happen in the first place."

For now it appears that in 250 million years, the Earth's continents will be merged again into one giant landmass...just as they were 250 million years before now. From Pangaea, to present, to Pangaea Ultima!