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Magma "Ocean" May Have Flowed Inside Early Earth

Richard A. Lovett
for National Geographic News
December 5, 2007
 
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."

Missing Elements

In particular, the previously known layers of Earth don't have as much uranium and other heavy, radioactive elements as meteorites have.

The new theory suggests that these elements might have collected in the lower magma ocean, which could explain their relative paucity in upper layers of the mantle.

That would also mean that the buried magma ocean was relatively radioactive, producing heat that would have kept the magma liquid for billions of years and would also have delayed the core from cooling enough to generate Earth's magnetic field.

The magnetic field is thought to be the result of liquid iron circulating in Earth's outer core.

Under the new theory, such conditions in the core may not have existed until Earth was about a billion years old, leaving our world unprotected from solar wind and other cosmic effects.

"The solar wind … will have shaped the Earth's atmosphere very early on if there was not a magnetic field in this period of time," Hernlund said.

(Read related story: "Earth's Magnetic Field Is Fading" [September 9, 2004].)

Slow Zones

Remnants of the magma ocean exist, Hernlund said, in "ultraslow zones" seen by scientists who use seismic waves from earthquakes to probe the boundary between the core and the mantle.

These zones, where seismic waves bounce and bend oddly, appear to represent what Hernlund calls "mushy patches" at the base of the mantle.

These soft patches are easily identifiable because they reflect seismic waves in a manner similar to the way in which a thin sheen of oil reflects light.

Norm Sleep is a geophysicist at Stanford University who was not part of Labrosse's team.

"It's an attractive hypothesis," he said of the new model, but he added that it is still speculative.

"Their hypothesis could well be right," he said, "[but] it's something that needs to be fleshed out to where it's coupled with the overlying mantle throughout the Earth's history."

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