Geologists Spar Over Model

Harvard Team Reports Hot Sports Deeper Than Believed

As lava and "black smokers" heat up the depths of the oceans, a controversy over whose model correctly predicts the shape of the earth's crust and mantle is heating up geologists at Harvard and around the country.

According to Professor of Geology Adam M. Dziewonski, hotspots underneath mid-oceanic ridges in the earth's mantle can reach depths of 300 or even 600 kilometers. This suggests that mid-oceanic ridges are active suppliers of crust-forming material, rather than simply passive openings from which hot mass flows.

Dziewonski and graduate student Wei-jia Su are challenging a claim made last January by two California researchers that such hotspots extend to depths of only 100 kilometers. Evidence for the Harvard geologists' theory, published in today's issue of the British science journal Nature, is drawn from a new three-dimensional model of the entire mantle.

But University of California at Santa Cruz researcher Yu-Shen Zhang--who co-authored a Nature paper last January with Toshiro Tanimoto, now at the school's Santa Barbara campus--suggested yesterday that the differences in the two findings were due to inferior resolution in the Harvard team's measurements.

Dziewonski and Su created the model using a technique known as seismic tomography, based on many of the same principles as CAT scans.

Waves created by seismic disturbances travel to and from points on the earth at different speeds depending on the material between the points. For instance, hot areas like mid-ocean ridges slow down waves, while cooler areas, such as continents, allow them to travel at faster-than-average speed.

By analyzing tens of thousands of waves to determine their trajectories and velocities, the Harvard team extrapolated a model of hot and cool spots in the mantle. They were able to prove that hotspots underlying mid-oceanic ridges affect wave speed more than 100 kilometers below the crust, and therefore that the hotspots must extend to such depths, some up to 600 kilometers.

Zhang said yesterday that while Harvard data was taken from a large-scale model of the entire earth, his study focused on only the upper reaches of the mantle. He also suggested that a difference in the waves measured by the two teams could result in different sets of data.

Dziewonski speculated that some parameter may have been over- looked in Zhang's measurements, since somevelocities were slowed by up to 10 seconds in thenew model, but only two seconds in Zhang's.

"There was a significant disagreement with ourresults," Dziewonski said. "We felt it wasimportant to inform the scientific community thatthe world is different from what Zhang andTanimoto said, to demonstrate they were wrong, andto show what we think the correct model is."

Su and Zhang said they would be in contact totry to reconcile the differences in theirfindings. But Dziewonski said similarities in thetechnique used by both teams were more importantthan the differences he has with theZhang-Tanimoto model.

"The fact that we can get down and discussdifferences of depths is an expression of thesuccess of the project and of the technique [ofseismic tomography]," Dziewonski said.

Geologists hope to drill into the ocean floorsoon to plant seismographs whose measurements willnot be affected by poor conduction in water.

"We have a chance within the next 10 years tosolve most of the mystery of the dynamics of theearth on a large scale," Dziewonski said