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African landscape shaped by single magma plume
The majestic slopes of Mount Kilimanjaro. The burning sands of the Ethiopian plateau. The rugged cliffs of the African rift valley where Homo sapiens early ancestors thrived millions of years ago. The Comoro Islands where that most ancient of fishes, the coelacanth, is found.
All these and many of the other striking geological features found in north and central Africa are the result of a single giant plume of magma that rose up from Earth's mantle about 45 million years ago and is still present today.
That is the conclusion of a geophysical modeling project conducted by Norman H. Sleep, professor of geophysics at Stanford, and Cindy J. Ebinger, lecturer in geophysics at the University of Leeds in the United Kingdom. Their results were published in the Oct. 22 issue of the journal Nature.
If confirmed by more detailed, three-dimensional modeling efforts now under way, their explanation will strengthen the position of scientists who argue that large magma plumes have played a major role in creating many of the planet's outstanding geological features.
In this model, giant plumes of hot rock rise up from the molten mantle to within 10 to a 100 miles of Earth's surface to form hot spots. Lava melted from this material periodically rises up and punches through the lithosphere, the upper layers of Earth that behave like a solid. When the rigid tectonic plate above such a hot spot moves, the resulting episodes of vulcanism occur at different places.
The strongest evidence for this mechanism is the Hawaiian island chain, which is formed by a series of successively younger volcanoes. According to Sleep, the African plume is about the same size as the Hawaiian plume.
Supporters of the giant plume theory see the fingerprints of hot spots all over the globe. They argue that the Snake River Valley in eastern Idaho is the path of the magma plume that now lies underneath Yellowstone and is the source of its geysers and hydrothermal wonders. Lake Superior lies in a rift valley created by a giant magma plume that was active 1.1 billion years ago and sent lava all the way down to Mexico. A plume that emerged beneath present-day Jacksonville, Fla., 200 million years ago triggered the breakup of North America and Europe.
Not all scientists agree with the giant plume theory. Critics maintain that hotspot volcanoes can be more readily explained as the result of magma forcing its way through existing cracks in the crust. In fact, they frequently point to the complex geology in northeastern Africa to support their argument. Instead of a single large plume, critics argue, number of small plumes adequately explain the confusing African topography.
Sleep and Ebinger were able to explain how a single plume could cause volcanic features in an area the size of the continental United States by modeling the way in which the hot, magmatic material can be channeled at depths of around 100 miles beneath old rift valleys, areas where the lithosphere is thinnest. Instead of forming a giant volcano at the plume site, magmatic material is diverted for distances up to 2,000 kilometers.
"Something like an upside-down drainage system formed. It is similar to what you get when oil is released under a layer of ice," Sleep says.
The African plume is currently centered near the southern border between Ethiopia and Sudan. Its exact size and location are a matter of conjecture because the seismic network needed to measure such a feature does not exist in that part of the world, Sleep says. The crustal plate in this area moves very slowly, so the position of the plume cannot have moved by more than 1,000 kilometers in 45 million years.
Using a two-dimensional computer model, Sleep and Ebinger successfully duplicated the major features of this unusual drainage system. They show plume material moving south as far as the Comoro Islands, north as far as the Red Sea and west as far as Cameroon.
"Actually, we think that the plume material reached even further to the west and north into Niger and Chad than we calculated in the model," Sleep says.
By David F. Salisbury