How does a continent crack open into a brand-new ocean?

In September 2005, the ground in Ethiopia's Afar region tore open along a crack 60 kilometers long and up to eight meters wide. It was a continent showing that it is splitting apart. Seafloor spreading, the process that eventually builds entire new oceans, starts exactly like this: heat rises under a continental land mass, the crust domes upward, and cracks grow into a three-armed rift system, with the arms meeting at roughly 120-degree angles in what geologists call a triple junction. The Red Sea-East Africa rift system is doing this right now.

Usually, only two of the three rift arms keep opening; the third slows down and stalls, becoming what's called a failed rift. One famous failed rift sits in West Africa's Niger Delta, where South America used to fit before the Atlantic pried the two continents apart — the Niger River now flows down that abandoned arm. Meanwhile, the active arms keep widening: the Red Sea itself is a young example of exactly this kind of flooded, still-opening rift. If a rift widens long enough to completely sever a continent, the result is a brand-new ocean basin.

Once an ocean-sized gap opens, the mechanism driving it shifts. The idea that the seafloor itself actively spreads, carrying continents along with it, was proposed in the 1960s by geologists Harold Hess and Robert Dietz. At the spreading center, basaltic magma wells up through fractures and cools into new seabed, with hydrothermal vents dotting the ridge; but the main force pulling plates apart isn't that rising magma — it's the weight of old, cold oceanic plate sinking back into the mantle elsewhere, dragging the rest of the plate along behind it.

Not every ridge spreads at the same pace. Slow ridges like the Mid-Atlantic Ridge open at under 40 millimeters a year; fast ones like the East Pacific Rise currently run up to 145 millimeters a year, and the fastest rate ever recorded, over 200 millimeters a year, happened on that same ridge millions of years ago. This picture took a long time to emerge: when Alfred Wegener proposed continents drifting in 1912, he imagined them plowing through the ocean crust, and scientists rightly objected that this was impossible, since oceanic crust is denser and more rigid. It took until the 1960s for the idea that the ocean floor itself moves to be proposed.