Volcano: In the Belly of the Beast: Scientists know what makes a volcano blow but still cannot say when

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To perform short-term divination, geologists examine a number of clues. Mild seismic activity in the area, rumblings or the emission of ash and gases are all harbingers of greater things to come. Changes in the snowcaps that cover tall volcanoes may also indicate trouble. In Iceland, for example, the sight of a sagging, snow-covered mountaintop, which indicates that hot magma is pushing upward and melting the ice cap, warns knowledgeable residents to head for safety. More sophisticated techniques include tiltmeters or laser ranging devices to detect deformations in the volcano cone, also caused by magma oozing upward. Seismometers are used to measure harmonic tremor around the volcano, a series of highly rhythmic shock waves associated with the motion of magma inside or beneath the cone. In Hawaii, a harmonic tremor lasting for some four to five minutes nearly always precedes an eruption.

Armed with such signposts, geologists have targeted several volcanoes that may erupt in the near future. More than 500 volcanoes are classified as active, most of them lying in the so-called Ring of Fire, a broad circle that more or less coincides with the boundaries of the Pacific, where oceanic plates are diving under continental plates. Of particular concern to scientists are some of the peaks in the Cascades, the mountain range that includes Mount St. Helens. The Mammoth Lakes ski-resort area in California is another area of potential volcanic activity.

Yet the researchers stress the still primitive nature of their craft. In the case of Mount St. Helens, one of the most heavily instrumented volcanoes ever, experts predicted many aspects of the 1980 eruption, yet they were caught off guard by both its fury and the extent of the mudflows it generated. And at Nevado del Ruiz, warning signs had abounded since Dec. 22, 1984. At that time a series of earthquakes were detected, followed by 30 minutes of harmonic tremor. Mild tremors continued throughout the spring and summer, and on Sept. 11, ash spewed forth for seven hours, accompanied by a roaring sound and electrical discharges. But for all these red flags, experts were unable to pinpoint when the big boom would come, or even if it would. Lacking that precision, Colombia, perhaps reluctant to disrupt the lives of residents with a false alarm, was unable to act in time. In a heavily populated area, says Gail Mahood, a geologist at Stanford University in California, "if you make a prediction and you're wrong, you could cost billions of dollars to an economy."

Until they can eye a volcano and declare with certainty that it is ready to burst, scientists will remember with a wince their warning nearly ten years ago about Soufrière, a volcano on the Caribbean island of Guadeloupe that began to spout a heavy plume of ash. Goaded by the geologists' alarms, authorities evacuated more than 70,000 people from the area and kept them away for 3½ months. The result: the mountain continued to sputter smoke and cough volumes of ash for a while, but it never blew. --By Natalie Angier. Reported by Christine Gorman/New York and Charles Pelton/San Francisco