Deep rock gives lift to Africa.

In the world of geophysics, southern Africa stands out as a puzzle. All continents have a geologically stable heartland, called a craton, which usually forms a flat, low-lying plain. The craton of southern Africa, however, hovers as a vast plateau about a kilometer above sea level. Two researchers now propose that the reason for this unusual height lies deep inside the planet.

In the past, geoscientists studying Africa's elevation have searched for answers in the lithosphere, the planet's outermost shell, Which is broken up into large plates. But these efforts have come up dry. "This has been a long-standing mystery in plate tectonics," says Carolina Lithgow-Bertelloni of the University of Michigan in Ann Arbor.

She and Paul G. Silver of the Carnegie Institution of Washington (D.C.) decided to take a deeper look. They studied images of Earth's mantle that seismologists produce from recordings of earthquake waves. For many years, these images have shown that the mantle--the thick rocky layer beneath Earth's crust--is warmer and less dense than normal under Africa.

Although rock in the mantle is solid, it can flow slowly because of the extreme temperatures. Lithgow-Bertelloni and Silver used the seismic images to calculate how the mantle should flow under Africa and how that movement would affect the surface elevation. According to their analysis, the buoyant rock deep in the mantle rises at a rate of a few centimeters per year, forcing the surface upward by about a kilometer, the researchers report in the Sept. 17 NATURE.

Their conclusions have swayed some geoscientists who have studied the elevated African terrain. "It makes a lot of sense, given what we are learning about the structure of the lower mantle," says Andrew A. Nyblade of Pennsylvania State University in State College. In 1994, Nyblade noted that the seafloor around southern Africa was also unusually high. He called the entire region the African superswell.