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October 7, 1996

Research Update

Earth sciences

A peculiar type of seismic wave has unveiled a major surprise deep within the earth: The base of the planet's mantle, long assumed hard as a rock, instead may be partially molten.

UCSC researchers studied seismic waves that skim along the sharp boundary between the earth's outer core and lower mantle. Under the south-central Pacific Ocean, something bogs down the speed of the waves by about 10 percent--a huge amount by geophysical standards. The most likely cause, the researchers claim, is that melted material bathes the mantle rock and transforms it into a thick mush.

If true, this conclusion violates the geophysical canon, which holds that the earth consists of four distinct layers: crust, solid rocky mantle, outer core of liquid iron, and inner core of solid iron. The upper mantle melts in regions where hot material rises to the planet's surface, but no one suspected the lower mantle--even in the cauldron of the inner earth--might behave the same way.

"This layer could fundamentally change our understanding of the mechanics of the core-mantle boundary," says mineral physicist Quentin Williams, an associate professor of earth sciences. "Suddenly, the bottom of the mantle might not be as stable as we thought. It's a provocative result."

Williams and seismologist Edward Garnero, a postdoctoral researcher, published their results in the September 13 issue of the journal Science.

The seismic evidence points to a partially molten layer between 5 and 40 kilometers thick in the zone under the South Pacific. Williams and Garnero believe the layer probably encircles the globe, but it may be as thin as a few kilometers or even a few hundred meters in most areas. That's a tiny fraction of the 2,900-kilometer-thick mantle. But because the suspected layer touches the dynamic outer core, the consequences would be dramatic.

For instance, says Williams, the layer may help heat flow from the core into the mantle, a process that ultimately drives the relentless march of earth's crustal plates.

--Robert Irion