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Mixing Earth's mantle with a delayed flush.

Journeying deep into computer versions of the Earth, two research teams independently have found evidence that could force a compromise in a divisive debate about the currents of rock flowing inside the mantle - the thick layer separating the planet's metallic core from its thin veneer of a crust. Over geologic time, these currents send Earth's continents slowly careening around the world, slamming together to form mountain ranges and rifting apart to create ocean basins.

For decades, geoscientists have argued over whether convection currents stir the entire mantle or whether the mantle is layered into upper and lower parts that do not mix. The new supercomputer simulations suggest that the real world may combine elements of both ideas, with a generally stratified mantle that occasionally flushes great masses of rock across i the boundary and down toward the core.

Although the two groups have taken different routes in creating numerical versions of the mantle, both sets of calculations show flushing patterns, a correspondence that lends credence to the concept. "It seems to be something that might really be happening inside the Earth," says Paul J. Tackley of the California Institute of Technology in Pasadena, a member of one of the modeling teams. Tackley and his colleagues published their findings in the Feb. 25 NATURE.

The other modeling group, led by Satoru Honda of the University of Hiroshima in Japan, discuss their simulations in the Feb. 26 SCIENCE.

Both models are three-dimensional representations of the mantle that depart from previous ones by including a critical transition at a depth of 670 kilometers -- the boundary between the upper and lower mantles. Seismologists discovered the boundary when they noticed that seismic vibrations speed up when descending past 670 km. To explain the speed change, mineralogists have theorized that the transition marks a phase change: Rock below the boundary is thought to have a denser crystalline structure than rock above the boundary,

In the model simulations, as rock at the top of the mantle cools, it sinks until it reaches the 670 km boundary, Initially, the cooler, downwelling rock is not dense enough to sink into the lower mantle, so it pools right above the boundary Eventually, however, the puddle of cooler rock grows heavy enough to break through the boundary and cascade into the lower mantle, flowing toward the core.

While the flushing pattern appears in both simulations, it takes different forms in the two models. Tackley's group found three or four breakthroughs occurring around the world at any one time. Honda and his colleagues saw the cascades developing one at a time and affecting the entire Earth. As it sinks into the lower mantle, the down flowing material would send plumes of hot material from near the core rising into the upper mantle.

The discrepancy may stem from basic differences in the models. Tackley's team uses a spherical mantle, whereas Honda's group represents the mantle as a wide fish-tank-like box.

The modeling results may help explain observations made by seismologists who study slabs of ocean floor that get pushed down into the mantle during collisions with other pieces of ocean floor or continents. In some places, the boundary at 670 km appears to deflect the slabs, preventing them from sinking into the lower mantle. In others, the slabs seem to penetrate the boundary That pattern may match the simulations, which show flushing occurring only in limited locations, says geophysicist Scott D. King of Purdue University in West Lafayette, Ind.

While the new models show promise, everyone involved realizes that the present generation of numerical simulations lacks important elements that could alter the mantle picture considerably, Researchers are currently trying to add the effect of tectonic plates, which are much stiffer than the mantle rock.
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Title Annotation:mixing of upper and lower mantle
Author:Monastersky, Richard
Publication:Science News
Date:Feb 27, 1993
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