Sep. 17, 1998 Dynamic Topography And The African Superswell
Scientists at the Carnegie Institution report in this week's Nature magazine that forces deep within the Earth are responsible for the phenomenon known as the African Superswell. Carolina Lithgow-Bertelloni* and Paul Silver of Carnegie's Department of Terrestrial Magnetism write that a large, hot upwelling orginating from the core-mantle boundary causes the mantle above it to flow. This flow, they maintain, extends all the way to the base of the African plate, where it elevates the southern part of the continent. "Imagine a bubble in a vat of maple syrup," says Silver. "It causes the syrup to flow as it rises, and also raises the surface." This change in height is referred to as dynamic topography.
It has been known for many years that the southern African plateau is higher than traditional data would suggest. Southern Africa happens to consist of a craton, which is the oldest part of the African continent. Most other continents have one or more cratons, but they are no higher than 400 or 500 meters above sea level. Southern Africa, however, lies more than 1,000 meters (or 1 km) above average sea level. Most previous explanations for the anomalous elevation have focused on near-surface phenomena such as episodes of volcanic heating and/or lithospheric thinning. None of these processes, however, have been able to account for the elevation.
Lithgow-Bertelloni and Silver believe that the excess elevation is due to an active upwelling of hot mantle material in the lower mantle originating from the core-mantle boundary. They see evidence for this active upwelling in images from seismic tomography. Seismic tomography has persistently indicated the existence of a large, low-velocity seismic anomaly in the lower mantle directly below the African Plate. Low seismic velocities generally reflect hotter, and consequently lighter than normal material. Lithgow-Bertelloni and Silver suspected that the high elevation of Africa is the direct result of this lower-mantle feature. To test this hypothesis, they performed an "instantaneous flow" calculation, which predicts both the mantle flow that would develop from the buoyant, lower-mantle feature, and the resulting dynamic topography of the Earths surface. They found a pattern of surface topography that looked remarkably like southern Africa's actual topography, strongly suggesting a causal link.
Not only does this upwelling raise southern Africa, but the calculations by Lithgow-Bertelloni and Silver show that it is also capable of driving tectonic plates. For many years earth scientists have been debating the forces that drive the plates. It has been conventionally assumed that the driving forces arise directly or indirectly from the plates themselves, as they sink back into the mantle. This study, however, shows that this upwelling originating deep in the Earth's interior constitutes a significant driving force for several tectonic plates.
"This is an exciting result," says Lithgow-Bertelloni. "It clearly shows the link between the dynamics of the deep mantle and features on the surface of the Earth, and that one cannot be understood without the other."
The Department of Terrestrial Magnetism (DTM) is one of five research centers of the Carnegie Institution of Washington, a nonprofit organization devoted to basic research and education in the physical and biological sciences. DTM scientists, including seismologists, geochemists, planetary scientists, and astronomers, are led by Sean C. Solomon, the department's director. The president of the institution is the biologist Maxine F. Singer.
* Lithgow-Bertelloni is currently a faculty member at the University of Michigan, Ann Arbor. Previously, she was a postdoctoral fellow at DTM.
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