From time to time, all of the Earth's continents get together and have a jamboree. Then their personalities begin to move in different directions and the band splits up.
One of the more startling ideas in Alfred Wegener's theory of Continental Drift was the idea that all of the continents had massed together as "Pangaea", between 300 and 200 million years ago. Decades after the laughter died down, Wegener was shown to be correct, and the theory of plate tectonics was born.
During the 1960's, Canadian geophysicist J. Tuzo Wilson noticed how oceans east of the Appalachian Mountains were repeatedly closed up by a continent colliding with North America, and then opened again. He was able to show how sea-floor spreading and subduction were linked: In a process called retrenchment, old oceanic crust subsides as it moves away from a spreading center. The oceanic crust eventually detatches from the continent it had been pushing, and begins to subduct beneath that continent. reversing the spreading of the ocean. Eventually the fragments of the earlier supercontinent join with each other to form a new supercontinent. The Coriolis Force makes different parts of the supercontinent move in different directions, and it breaks up again.
Things are somewhat more complicated than this, but retrenchment, rifting, and other geological evidence seemed to suggest that the continents have repeatedly smashed together and broken up again. To date, three supercontinents have been identified, three turnings of the Wilson Cycle:
- Pangaea (occasionally called "Pangaea III") formed 300 million years ago with the Alleghenian Orogeny at the beginning of the Permian period. It broke up 200 million years ago during the Triassic Period.
- Pannotia (or "Pangaea II" if you like) formed 650 million years ago, causing the Pan-African Orogeny. It existed in the Southern Hemisphere throughout the Cambrian Period, then broke up 550 million years ago during the Ordovician Period. Pannotia was named in 1997 by Ian W. D. Dalziel of the University of Texas.
- Rodinia (or "Pangaea I") formed 1100 million years ago, causing the Grenville Orogeny. Its entire existence was during the late Proterozoic Era, breaking up 750 million years ago. Rodinia was named by Dalziel in 1991.
Professor John J.W. Rogers of the University of North Carolina, Chapel Hill, has found evidence for a supercontinent before Rodinia, proposing the name "Columbia" for it.
Presumably, the Wilson Cycle operated well into the Precambrian before this, and there probably were supercontinents before, but no-one has figured out when they existed, nor named them yet.
What is certain is that the Wilson Cycle will continue to operate into the future. Geologists predict that "Pangaea Ultima" will form about 250 million years from now.
Lynn S. Fichter, James Madison University
Introduction to Wilson Cycles: Some Philosophy of Science