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Africa and South America Split 20 Million Years Ago

Mid Atlantic RidgeIn the late 18th century Benjamin Franklin hypothesized that Africa [right] and South America [left] were once joined. We now know that he was right. Approximately 20 million years ago these two continents were one. Then a giant rift, or separation, developed between them. As the two land masses separated, the surrounding water rushed in to fill the gap between them, and the Atlantic Ocean was born. They've been drifting apart about 5 centimeters per year - for the past 20 million years!-as the Atlantic continues to widen.   
What began as a small crack in the Earth's crust is now the Mid-Atlantic Ridge, a giant undersea mountain range. As magma rises to escape along this ridge new crust is formed, sea-floors spread, and continents drift around the globe.
Thanks to Bruce Heezen and Marie Tharpe.

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