Whatever you learned about plate tectonics and the continental drift in school, it's still hard to really imagine imagine that giant landmasses built of diverse geology and populated by massive systems of ecology are actually floating, transient things. That's the story though—our solid-as-a-rock homelands are just the solid mineral scum on the soup of superheated semisolid rock that makes up the planet's 1,800 mile deep mantle.
This drifting around of the Earth's crust means, for one thing, that the planet used to look a lot different as far as landmasses are concerned. The continents/subcontinents of South America, Africa, Antarctica, Australia, and India all used to be part of the supercontinent Gondwana. And Gondwana would have been the southern portion of the super-supercontinent Pangea (the northern part was Laurasia), which began breaking up 200 million years ago. It's thought that Gondwana itself broke up around 70 million years after that, eventually leaving us with one of Earth's most glaring statements of continental drift: the nearly perfect puzzle piece match between the coastlines of western Africa and eastern South America.
It was this neat relationship between two coastlines that inspired German scientist Alfred Wegener to first suggest the idea of drifting continents in the early 20th century. He didn't how or why this phenomenon was occurring, but Earth was less solid than it seemed. If Wegener were around today, he might be a bit taken aback to learn that the neat coherence of the two continental coastlines is only due to a late-in-the-game geologic plot twist. This is according to a new paper in Geology explaining why the Sahara Desert, the bulk of the big northwestern lobe of Africa, stayed with the continent instead of being pulled along with the new South American continent, a fate that would have left the two landmasses with coastlines that look like this:
A hypothetical model of the circum-Atlantic region at present-day, if Africa had split into two parts along the West African Rift system. Here, the north-west part of present day Africa would have moved with the South American continent, forming a "Saharan Atlantic ocean"/Sascha Brune/Christian Heine
The reason, put simply, is the relationship between the direction of drift—at what angle was South American pulling at Africa—and the relative “rift trend.” In the cast of Africa, the West African rift runs more or less north-south, while you can see well enough what direction the drift was pulling in. The result was a large angle, too large, according to the researchers, for Saharan Africa to be pulled off into the new abyss of the southern Atlantic Ocean. And so the West African rift is a "failed rift," though that depends on your perspective.
What happened instead of the Saharan Sea is, basically, that the Sahara stayed stuck to Africa as the continents drifted. The angle of South American drift relative to the West African Rift System—which extends south-north from Nigeria to Libya—was too big and so, instead of the Saharan Sea (the “Saharan Atlantic Ocean” more officially) we have our puzzle pieces.
“Extension along the so-called South Atlantic and West African rift systems was about to split the African-South American part of Gondwana North-South into nearly equal halves, generating a South Atlantic and a Saharan Atlantic Ocean," geoscientist and study co-author Sascha Brune summarizes. "In a dramatic plate tectonic twist, however, a competing rift along the present-day Equatorial Atlantic margins, won over the West African rift, causing it to become extinct, avoiding the break-up of the African continent and the formation of a Saharan Atlantic ocean."