Now, two geologists, from Northwestern University and the University of Illinois at Chicago, have demonstrated that an adjustment to one of those models --- involving reducing the assumed thickness of the tectonic plate --- allows the model to fit the data much more precisely.
They illustrate the accuracy of the new model with projected and actual ocean depths in the area surrounding the Hawaiian Islands in Friday, March 14th's issue of the journal Science. The two researchers, Carol A. Stein, associate professor of geological sciences at the University of Illinois at Chicago, and Seth Stein, professor of geological sciences at Northwestern University, believe they have demonstrated their model provides a "significantly better fit to the data" than other models currently in use.
Scientists first determined in the late 60s that the earth's surface is covered with some 14 giant plates that move in different directions at a rate of a few inches a year. They have since determined that these plates are formed at mid-ocean ridges by an upswelling of molten rock and move slowly away from these ridges in both directions.
As they inch away from the ridges and age, the plates become deeper and colder, eventually sliding back into the earth's heated interior. This process accounts for some 70 percent of earth's loss of heat from its interior. Another 5 percent is accounted for by "plumes" of molten earth that spout up at a few "hot spots" on the ocean floor, forming chains of islands such as Hawaii.
The plates themselves are called the "lithosphere," and estimates have been that they averaged some 125 kilometers, or 75 miles, in thickness. When geologists developed a model of their movement, based on their speed, age and temperature, the model worked well for regions up to 70 million years old. But seafloor areas older than that were consistently shallower than predicted by the model.
The husband and wife team of Seth and Carol Stein first proposed in an article in Nature in 1992 that if the assumption of the plates' thickness was changed from 125 kilometers to 95 kilometers, the model proved to be far more accurate. Since that time, however, many geologists have continued to use the old models and have expended extensive energy trying to explain its discrepancies.
The Science article refers to this as the "Lake Wobegon Effect." In the radio show "Prairie Home Companion," the mythical town of Lake Wobegon is "where all children are above average." This suggests that there might be something wrong with the standard being applied, as is the case with geologists who attempt to explain the "anomalies" of shallow sea depths while using the wrong model, the authors wrote.
The Science article is accompanied with four charts of a region of the Pacific Ocean that includes Hawaii and the Marshall Islands. One chart is of the actual depth of the ocean floor, and the other three are of projections of that depth using three different models of plate movement. The Stein model, assuming a lithosphere thickness of 95 kilometers, precisely matches the observed depths, except for the chain of islands themselves (which were formed by plumes).
3/12/97
(Media contact Chris Chandler at 847-491-3115, or by e-mail at c-chandler@nwu.edu. Stein can be reached at 847-491-5265 or by e-mail at s-stein@nwu.edu)
( Color charts can be obtained from the website at http://www.earth.nwu.edu/research/stein)
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