If things had been a little different, West Africa would currently be on the other side of the Atlantic.
That’s the startling conclusion of a recently completed study of the West Africa Rift Zone, a subsurface rift structure that passes through Algeria, Niger, and Nigeria on a north-south axis. The study, run by researchers working out of the University of Sydney and the GFZ German Research Centre for Geosciences, used computer models to develop a better understanding of why some rifts succeed and others fail (check out the full scientific article here.).
There are many failed rift zones: the Saint Lawrence Valley in Eastern Canada, the Bass Basin in Australia, and the Basin and Range region of the southwestern US are all examples.
200 million years ago, when the supercontinent Pangea began to break apart, what is now the west coast of Africa was smashed up against what are now the eastern coast of North America and the northern coast of South America. As Pangea broke up, a number of rifts began to form. These included the South Atlantic rift – the one that is responsible for the existence of an ocean between South America and Africa. But there were several other rift zones that failed to spread more than a few hundred kilometers. On the west side of the Atlantic, one of these failed rifts can be found in New England – the Connecticut River Valley. The West African rift zone is this valley’s African counterpart. (to see a video reconstruction of the breakup of Pangea, click here)
Why didn’t these rifts succeed? To find out, the team developed a computer model of the breakup of Pangea. They focused on Gondwana, the portion of Pangea containing the Americas and Africa.
When the continents began to break apart, a triple junction rift zone formed. The team knew that triple junctions only end up spreading along two of the three legs. But why were the Equatorial Rift System and the South Atlantic Rift system successful where the West African rift system was not?
The computer model provided an answer: the bigger the angle between the direction of plate extension and the orientation of a rift, the more energy it takes for the rift to open. Like most things in nature, continental plates break up according to the path of least resistance. In this case, that path was along the Equatorial and South Atlantic rift systems.