for National Geographic News
A thick magma "ocean" may once have slowly flowed deep beneath Earth's surface, scientists say.
The new theory challenges the widely held notion that Earth's mantle—the thick layer of rock between the outer crust and the inner core—had been solid throughout.
"For about ten years, seismological evidence has accumulated [that there are] dense, thin [magma] pockets at the bottom of the otherwise solid mantle," said Stephane Labrosse of the Ecole Normale Supérieure de Lyon in France.
"We know that the Earth's interior has been cooling down for most of its history, and this means that if there is [magma] now, there should have been more in the past."
In a study published in tomorrow's issue of the journal Nature, Labrosse's team proposes that this magma ocean was one of two layers of molten rock generated by the blast-furnace heat that characterized early Earth. (Learn more about the inner Earth.)
The top layer was about 600 miles (1,000 kilometers) thick, said John Hernlund, a geophysicist at the University of British Columbia and one of Labrosse's collaborators.
That layer was formed by the heat caused by multitudes of asteroid impacts. But it was not liquid for very long, having solidified within about 10 million years.
The new theory proposes that the inner heat that melted the core also melted the bottom layer of the mantle. This produced a layer of solid rock sandwiched between the short-lived upper magma ocean and the longer-lived lower one.
The existence of such a magma ocean would solve a long-standing riddle: why inner Earth's mix of elements doesn't quite match that of meteorites, which are composed of the same materials that formed our world.
"When you add up all of the Earth's elements in the core and the mantle, they should equal something like meteorites," Hernlund said. "But there were some very major discrepancies."
In particular, the previously known layers of Earth don't have as much uranium and other heavy, radioactive elements as meteorites have.
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