Credit for sculpting Earth's surface typically goes to violent collisions between tectonic plates, the mobile fragments of the planets rocky outer shell. The mighty Himalayas shot up when India rammed into Asia, for instance, and the Andes grew as the Pacific Ocean floor plunged beneath South America. But even the awesome power of plate tectonics cannot fully explain some of the planet's most massive surface features.
Take southern Africa, which boasts one of the world's most expansive plateaus, more than 1,500 kilometers across and almost two kilometers high. Geologic evidence shows that southern Africa, and the surrounding ocean floor, has been rising slowly for the past 100 million years, even though it has not experienced a tectonic collision for nearly 400 million years.
The African superswell, as this uplifted landmass is known, is just one example of dramatic vertical movement by a broad chunk of Earths surface. In other cases from the distant past, vast stretches of Australia and North America bowed down hundreds of meters--and then popped up again.
Scientists who specialize in studying Earth's interior have long suspected that activity deep inside Earth was behind such vertical changes at the surface. These geophysicists began searching for clues in the mantle--the middle layer of the planet. This region of scalding-hot rock lies just below the jigsaw configuration of tectonic plates and extends down more than 2,900 kilometers to the outer edge of the globe's iron core. Researchers learned that variations in the mantles intense heat and pressure enable the solid rock to creep molasseslike over thousands of years. But they could not initially decipher how it could give rise to large vertical motions. Now, however, powerful computer models that combine snapshots of the mantle today with clues about how it might have behaved in the past are beginning to explain why parts of Earth's surface have undergone these astonishing ups and downs.
The mystery of the African superswell was among the easiest to decipher. Since the early half of the 20th century, geophysicists have understood that over the immense expanse of geologic time, the mantle not only creeps, it churns and roils like a pot of thick soup about to boil. The relatively low density of the hottest rock makes that material buoyant, so it slowly ascends; in contrast, colder, denser rock sinks until heat escaping the molten core warms it enough to make it rise again. These three-dimensional motions, called convection, are known to enable the horizontal movement of tectonic plates, but it seemed unlikely that the forces they created could lift and lower the planets surface. That skepticism about the might of the mantle began to fade away when researchers created the first blurry images of Earth's interior.
About 20 years ago scientists came up with a way to make three-dimensional snapshots of the mantle by measuring vibrations that are set in motion by earthquakes originating in the planet's outer shell. The velocities of these vibrations, or seismic waves, are determined by the chemical composition, temperature and pressure of the rocks they travel through. Waves become sluggish in hot, low-density rock, and they speed up in colder, denser regions. By recording the time it takes for seismic waves to travel from an earthquakes epicenter to a particular recording station at the surface, scientists can infer the temperatures and densities in a given segment of the interior. And by compiling a map of seismic velocities from thousands of earthquakes around the globe, they can begin to map temperatures and densities throughout the mantle.
These seismic snapshots, which become increasingly more detailed as researchers find more accurate ways to compile their measurements, have recently revealed some unexpectedly immense formations in the deepest parts of the mantle. The largest single structure turns out to lie directly below Africa's southern tip. About five years ago seismologists Jeroen Ritsema, now at the Paris Geophysical Institute (IPGP), and Hendrik-Jan van Heijst, now at Shell Research, calculated that this mushroom-shaped mass stretches some 1,400 kilometers upward from the core and spreads across several thousand kilometers [see illustration below].