PASADENA - A superheated plume of molten rock rises deep beneath Africa, buoying the continent like a rubber ducky atop spouting bathwater.
Now Caltech scientists have discovered the African superplume, which drills outward from near Earth's iron core, is more than just hot rock. Earthquake waves reveal the plume has sharp boundaries, implying it has a different chemical composition from the surrounding mantle rock.
The results could change theories about how the heat circulates within the Earth, said Sidao Ni, a Caltech researcher and lead author of the study published in today's issue of the journal Science.
"This gives people new ideas about the interior of the Earth," Ni said.
The Earth's crust is a thin veneer of solid rock that floats on an essentially molten layer known as the mantle. The mantle churns as hotter material moves outward from Earth's core and colder material sinks back down, a process called thermal convection.
The convection cycles drive plate tectonics, causing collisions between mobile pieces of the Earth's crust that create earthquakes, volcanoes and mountains.
While scientists can paint a broad picture of mantle convection, the details are still not well understood.
Geophysicists such as Ni and his Caltech colleagues use earthquake waves to produce pictures of temperature and density changes in the mantle, trying to understand the complexities of convection.
Earthquake waves travel slowly through the hotter regions of the mantle and speed up in colder, denser areas.
Using a global network of seismometers to track the speed of earthquake waves, Ni discovered the African superplume has an unusually sharp boundary that extends nearly 900 miles above the core.
"No one expected this," said Caltech geophysics professor Mike Gurnis, a study co-author. "It's strange. I guess that's why we find it so interesting," he said.
Ni speculates that the superplume may be rich in iron compared to the surrounding mantle because it comes from near the core.
The study also confirmed earlier results showing the plume tilts to the northeast, the same direction the African plate has drifted for the past 100 million years.
Another superplume is posited to exist beneath the central Pacific Ocean, and Ni predicts his results will lead to further study of the Pacific superplume.
"In the future people will be spending a lot of time exploring these kind of structures," Ni said.