Waiting to explode

0 Comments | Insight on the News, July 29, 1996 | by Phil Berardelli

With 1,500 active volcanoes worldwide, scientists are struggling to predict eruptions.

Volcanoes, perhaps the most awesome manifestations of geologic forces, also are the messiest and most inconvenient. When they erupt, they can despoil large areas of sky and landscape for years. But since they often slumber for millennia, they lull civilizations into false senses of security. Large cities have sprung up within the shadow of a scenic, snowcapped cone, naively risking its deadly ashfall and massive landslides when it returns to life.

Such is the case with Popocatepetl, a three-and-a-half-mile-high frustum that looms 40 miles southeast of Mexico City's teeming populace. Since January 1994, "El Popo," as the volcano is called by its human neighbors, has threatened a new cycle of cataclysms. Periodic eruptions have ejected hot gases and ash, and it occasionally belches large stones. A mudslide triggered by lava-melted snow almost buried Cholula, one of Mexico's oldest towns.

Within the last few months, the incidents have become more frequent. One eruption killed several climbers who were on the mountain's slopes. No major explosions have occurred as yet, however, so authorities have attempted to downplay the potential hazard. Still, some volcanologists are worried: They have detected a large lava dome forming inside El Popo's crater, indicating an increase in pressure from the underlying magma. "Virtually all volcanoes that form domes have explosions," says Stanley Williams, professor of geology at Arizona State University.

What is uncertain is the next eruption's timing and scale. This is why predictions frequently are controversial among volcanologists and disaster-management specialists. The stakes are high. About 25 million people live within El Popo's danger zone. Warnings issued too soon and too emphatically could jade the population's response to a real emergency. Property damage will be substantial no matter how wellprepared the people. The buildings, highways, houses and precious ancient ruins that ring the mountain stand vulnerable to the volcano's will.

Williams knows about the power of volcanoes perhaps better than any living human. He and a scientific party were visiting the crater of Nevado del Galeras in Colombia in 1993 when the volcano suddenly exploded, instantly killing his five colleagues. Williams was badly injured but survived by "pure chance," as he describes it.

So whenever one of the Earth's 1,500 active volcanoes (including 40 within the continental United States) threatens a populated area, Williams takes an urgent interest. Along with Popocatepetl, he's concerned about Long Valley, an area in eastern California near Yosemite National Park -- one of the most popular recreation areas in America.

Long Valley actually is a 10-by-15-mile caldera -- an ancient crater -- that was created 700,000 years ago when a mountain collapsed in an eruption 500 times more powerful than the one that shattered Mount St. Helens in 1980. Trees there are dying in large numbers as carbon dioxide exudes from a swelling body of subsurface magma. (Several deaths also have occurred, the victims suffocated by accumulations of carbon dioxide in their cabins.) April brought a swarm of earthquakes.

As frightening as this is, it's all part of normal geologic processes. "There is a basic misconception that the Earth is cold and quiet," Williams says. "But it's still very hot and dynamic." There are about 15 continually active volcanoes worldwide. The rest have erupted periodically during the last 10,000 years and remain potential hot spots. The average number of eruptions yearly remains curiously constant -- 50 to 60.

Lee Siebert with the Smithsonian Institution's Global Volcanism Program cautions that the most dangerous factor in volcanic eruptions is nearterm unpredictability. Among individual volcanoes, behavioral variances can be large, and a sudden resumption of activity does not guarantee a major event. "Nobody knows for sure about Long Valley," says Williams, although "the most explosive events have come from volcanoes thought to be extinct."

When scientists concentrate their attention on certain volcanoes, how ever, they often are able to predict their behavior. Trouble spots such as Popocatepetl and Long Valley require intensive monitoring and data correlation -- which are not done routinely.