What Makes Them Blow

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Is all this complex gear necessary? After all, Indonesian volcanologists have established a warning system that makes effective use of dedicated, if often poorly equipped, human observers. The answer is that the better scientists get at predicting eruptions, the less chance of false alarms. In 1976, 72,000 residents of the Caribbean island of Guadeloupe were forced to leave their homes because a nearby volcano seemed about to blow. Several months later, after no eruption occurred, the considerably discomfited evacuees returned home. And ever since 1980, the California resort area of Mammoth Lakes has fretted over recurrent clusters of small earthquakes. The resort abuts a huge depression caused hundreds of thousands of years ago by an exploding volcano. "What the earthquakes mean is that the volcanic system is still alive and dynamic," notes Robert Tilling of the U.S. Geological Survey. "But we don't know enough yet to be able to predict if, or when, it might again explode."

One of Tilling's colleagues, geophysicist Bernard Chouet, believes he may have found an answer to this dilemma. Prior to many large-scale eruptions, he says, seismometers have picked up tremors that appear to be caused, not by the fracturing of rock, but by low-frequency waves that resonate through the magma itself. While their origin remains a mystery, these vibrations may result from small surges of gas and molten rock. Large numbers of such signals preceded Mount St. Helens' 1980 blast. They also appeared before the unexpected explosion of Mexico's El Chichon in 1982, the blowup of Colombia's Nevado del Ruiz in 1985 and 1987 and multiple eruptions of Alaska's Redoubt. Seismometers positioned at Pinatubo have recorded similar seismic patterns.

The greatest threats to human lives may come from overlooked, long dormant volcanoes. To monitor a volcano requires identifying it beforehand; as recently as 1981, Pinatubo was not even included in the worldwide registry of volcanoes maintained by the Smithsonian Institution. "When a nice little hill covered with lush vegetation finally wakes up," observes Smithsonian volcanologist Tom Simkin, "it's going to cause a lot of damage." Fortunately, scientists were able to see that some nice little hills in the Philippines and Japan were turning nasty while people still had time to get away.