Hundreds of millions of years ago, a single mega-continent on the Earth broke up and began to drift apart. The pieces spread out across the surface of the globe, eventually settling in their current positions as continents and creating oceans in between. Now scientists have observed the first indication that this movement may be shifting into reverse, setting the United States and Europe on an eventual collision course.
Such an expansion and contraction of the Earth’s crust is directed by a recurring process called the Wilson cycle. Step one: tectonic plates move apart, breaking up supercontinents and filling oceans. (Picture Pangea.) Step two: tectonic plates fracture, creating new subduction zones that force one plate under the other, into the Earth’s molten mantle. Step three: subduction brings the continents back together to create a new supercontinent.