In the 1960s, J. Tuzo Wilson noticed that old mountain belts mark where ancient oceans once closed, and that oceans open where continents split. Put rifts and collisions on a time line and oceans have a life story: the Wilson cycle.
| Stage | What happens | Example today |
|---|---|---|
| 1. Rifting | a continent stretches and splits | East African Rift |
| 2. Young ocean | a narrow sea floods the rift; a new ridge forms | Red Sea |
| 3. Wide ocean | the ocean grows at a ridge; its edges are quiet | Atlantic Ocean |
| 4. Subduction begins | old, dense seafloor starts sinking at its edges | Pacific Ocean |
| 5. Closing ocean | the ocean shrinks as continents approach | Mediterranean Sea |
| 6. Collision | continents meet and build mountains along a suture | Himalayas |
Why it works
The cycle runs on density. Rifts open where a continent is stretched over hot mantle; the ocean widens as long as its ridge spreads. As the seafloor ages it cools, grows dense, and eventually begins to sink at subduction zones, so the ocean starts to close. When the continents meet, their buoyant crust stops subduction and piles up into mountains, welding the pieces into a larger continent, sometimes a supercontinent, which may later rift again. One full cycle takes very roughly 300 to 500 million years.
Key idea
Oceans open and close in a cycle: rift, young ocean, wide ocean, subduction, closing, collision. Supercontinents form when the continents gather, and Pangaea was only the most recent of several.
Worked examples
Worked example: Opening an ocean
How long does it take to open an ocean 4,500 km wide at a full spreading rate of 25 mm/yr?
- 25 mm/yr is 25 km per million years.
- $4,500 \div 25 = 180$ million years, about the age of the Atlantic.
Answer: 180 million years.
Worked example: Name the stage
What stage is the Red Sea in?
- Arabia split from Africa, and new ocean floor now forms along the middle of the sea.
- A narrow sea with a young ridge: stage 2, a young ocean.
Answer: A young ocean.
Common mistake
It is common to think Pangaea was Earth's first and only supercontinent. Ancient mountain belts record earlier ones: Rodinia came together about a billion years ago and began breaking up around 750 million years ago, and an older one, Columbia (or Nuna), may date from about 1.8 billion years ago. Check the time: Pangaea formed only about 300 million years ago, in the last 7% of Earth's history.