Watching a continent splinter.

Ancient human ancestors inhabited some of the rawest land on Earth, amid the East African Rift Valley. There, two sections of the planet's skin are pulling apart like a fresh wound. For years, geoscientists have wondered why this gash disappears south of Mozambique. Two researchers now have an answer.

The East African rift marks the boundary between two tectonic plates: the Nubian plate to the west and the Somalian plate to the east. A general rule of plate tectonics is that such rifts must hook up to other plate boundaries, but researchers have had trouble tracing the connection of the East African rift. South of Mozambique, there are no earthquakes or other signs of disturbance like those seen along the rift to the north.

Dezhi Chu of Exxon Production Research Co. and Richard G. Gordon of Rice University, both in Houston, tracked the continuation of the East African rift by studying ocean rocks. Between Africa and Antarctica, there is a rift in the seafloor where the crust spreads apart and new rock is born. In the process, the two African plates inch away from Antarctica.

Using Antarctica as a signpost, Chu and Gordon discerned the motion between the Nubian and Somalian plates. Instead of pulling straight apart, the two plates rotate like scissors blades around a pivot point east of South Africa, the two report in the March 4 Nature. In the region of the East African rift, far north of the pivot point, the two plates are separating by 6 millimeters a year. Just south of the pivot point, the two plates are crushing together at a rate of only 2 mm per year, so slowly that they don't often produce large earthquakes.

By understanding the motion, or kinematics, of these two plates, geoscientists will have more success in resolving how all Earth's plates behave. "It solves, in a sense, this 30-year mystery of how the East African rift fits in kinematically in the plate-tectonic scheme," says Gordon.