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Taking the measure of volcanic eruptions.

The size of a volcanic eruption - and its potential to threaten nearby population centers - depends on processes that occur deep underground, hidden from the eyes of curious scientists and anxious disaster planners.

But chemical analysis of lava may allow scientists to forecast the size of ongoing or future eruptions, say geochemist Donald J. DePaolo of the University of California, Berkeley, and his collaborators in the United States and Japan.

In a study presented this week at the Geological Society of America meeting in Cincinnati, the researchers argue that certain isotopic ratios in lava - notably, high neodymium-143 to neodymium-144 -- can indicate whether a volcanic system is undergoing a major eruption.

Although using lava composition to infer the behavior of vast underground magma systems is not new, DePaolo's model is the first to link certain combinations of elements in lava directly to eruption volume, he says.

DePaolo's group bases its study on lavas disgorged by Japan's Unzen volcano over the past 300,000 years. Unzen, located on the Simabara peninsula of western Kyushu, began its latest eruption in 1990 after lying dormant nearly 200 years.

Chemical analysis of the material ejected by Unzen both before and since its 1990 outburst indicates that the volcano is probably in the middle of a relatively large eruption, says DePaolo.

Unlike large systems that take millenniums to build up enough energy for a major outburst, some volcanoes' cycles can be measured in decades. Thus, chemical analysis of the precursory burps of a small volcano may yield a rough estimate of when the Big One will erupt. Such a forecast could be valuable to current or future generations living near such a volcano.

DePaolo cautions, however, that chemical messages from magma chambers are no sure thing. Although the chemical composition of lava may indicate that a large eruption is imminent, "it doesn't guarantee you're going to get one," he notes.

Geochemist Alexander N. Halliday of the University of Michigan in Ann Arbor says that DePaolo's model of magma chambers is essentially sound, although it may not work for all volcanoes. "There are some systems that I don't think fit so well," he says.
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Title Annotation:chemical analysis of lava may be used in forecasts
Author:Pendick, Daniel
Publication:Science News
Article Type:Brief Article
Date:Oct 31, 1992
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