But in the mantle, the layer beneath Earth's crust, a huge 18-mile-wide (30 km) region of very high conductivity reaches down to a depth of 20 miles (35 km), well below the 12-mile-thick (20 km) crust. This giant magma zone isn't one big pool, but a series of interconnected pockets, scientists think.
The findings were bolstered by research in geochemistry, rock composition and seismology from other teams, Whaler said. "The results from standard electrical conductivity get a huge range, so the additional information gives additional bounds. I suspect nobody would have believed us without some supporting evidence from other techniques."
For example, a study published in the journal Nature on July 4 shows the mantle under the Afar region is about 212 degrees Fahrenheit (100 degrees Celsius) hotter than it should be. And although the crust is thinner than in other spots around the planet, it is actually thicker than models predict. (What is Earth Made Of?)
Taken together, the recent discoveries suggest researchers still don't understand how the final stages of breakup occur in continental crust, Whaler said.
"Most people have said we can look at the Afar Rift and it's a good on-land analogue of mid-ocean ridges," she said. "But what this result says is, there is still quite a distinct difference between the crust and upper mantle beneath a fully formed spreading ridge and the Afar Rift."
For Roger Buck, a geophysicist at Columbia University's Lamont-Doherty Earth Observatory who was not involved in the study, the discovery raises questions about what controls the timing of magmatic activity such as dike intrusions.
"A commonly held view is that long, quiet periods occur because there is no magma available in the crust to trigger dike opening and volcanism at spreading centers," Buck wrote in an accompanying editorial published in Nature Geoscience. "However, the results ... bring into question this standard view. Instead, there may always be large quantities of magma available in the mantle and shallow crust at many spreading centers."
Email Becky Oskin or follow her @beckyoskin. Follow us @OAPlanet, Facebook & Google+. Original article on LiveScience's OurAmazingPlanet.
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