Continents in Collision

"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.

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!