What Makes Them Blow

By J. Madeleine Nash/Chicago

When 15,000 anxious Americans were evacuated from Clark Air Base in the Philippines last week, they didn't know what to think. Were they in real danger or the victims of a false alarm? Within 48 hours, they got their answer. Nearby Mount Pinatubo, after sleeping quietly for more than 600 years, suddenly erupted in a series of explosions that shot plumes of steam and ash as much as 30 km (20 miles) into the sky. Debris rained down on surrounding villages, and a giant mushroom cloud was visible 100 km (60 miles) away in Manila.

The confirmed death toll was only six in the first few days, thanks to advance warnings and speedy evacuations. But great dangers remained. Fearing bigger explosions, officials ordered tens of thousands evacuated. An approaching typhoon, moreover, threatened to send destructive mudslides down the mountain. Whatever happens, the swift action by the government reflected the improving ability of scientists to monitor volcanic activity and identify the telltale events that presage eruptions.

Mount Pinatubo's blasts came just one week after Japan's Mount Unzen blew its top, with more deadly results. The red-hot avalanches hurtling down the mountain's slopes killed at least 35 people. But the toll could have been much higher if scientists had not sounded the alarm that an eruption was imminent. In fact, many of those killed were journalists and volcanologists drawn to the mountain by the warnings, whereas most residents of the area fled to safety. They may have to stay away for a long while: Mount Unzen erupted again last week, and the worst may not be over. A series of blasts from the mountain in 1792 created landslides and tidal waves that killed 15,000 people.

Both Pinatubo and Unzen lie along the infamous Ring of Fire, a crescent of volcanic activity that runs around the rim of the Pacific Ocean through the edges of Asia, North America and South America. Washington's Mount St. Helens, which exploded spectacularly in 1980, is part of the ring. It contains three- quarters of the earth's 540 historically active volcanoes. Since such mountains are erupting in one place or another almost all the time, it is merely a coincidence that Pinatubo and Unzen are exploding simultaneously.

The number of eruptions these days is not abnormal, but human populations near the fiery mountains have been growing rapidly. Never before have the volcanoes posed such a serious threat. Some volcanologists believe, for example, that Mount Fuji has entered an active phase, raising the specter of a giant eruption only 100 km (62 miles) from Tokyo.

But scientists hope to foretell most major eruptions, and their record is increasingly impressive. Since 1980, Mount St. Helens has erupted 22 times, and 19 of those episodes were predicted by U.S. Geological Survey volcanologists at the Cascades Volcano Observatory, in Vancouver, Wash. Warnings have also preceded eruptions of Alaska's Redoubt Volcano, which roared to life in 1989.

Unlike earthquakes, which often happen without warning, impending volcanic eruptions generally signal their arrival. Before a blowup, instruments can detect a series of tremors in the mountain, which indicate that molten rock, called magma, is coming up from deep inside the earth. The magma rises gradually, opening fissures that serve as its pipelines to the surface. What happens next depends on the composition of the magma. If it is fairly liquid, it generally produces a stately lava flow that poses more of a threat to property than to humans. Hawaiian volcanoes tend to follow this pattern.

But the volcanoes clustered along the Ring of Fire are more dangerous. The ring traces a geologically active zone where sections of the earth's crust, known as plates, are colliding. Generally the weaker oceanic plates are forced beneath the thicker continental slabs. The friction of grinding rock, combined with heat welling up from the earth's interior, transmutes the lower edge of the oceanic plate into magma. Thick with silica, this type of magma tends to solidify near the surface, forming domes and plugs that seal off the channels through which the magma rises. Such blockages turn a volcano into a giant pressure cooker. At a certain point, when the surrounding rock is no longer strong enough to hold the expanding magma, the mountain blows apart.

The main tools of the volcanologist include seismometers, which record the swarms of tiny earthquakes that occur as the magma rises. Chemical sensors, mounted on airplanes, can detect increases in sulfur-dioxide emissions, indicating that magma has reached the surface. In addition, the physical swelling of mountain slopes, well documented at Mount St. Helens, is a sign of explosive potential. Laser-based devices can pick up minute bulges that are about the width of a nickel and still invisible to the naked eye. In Japan researchers have set up video cameras to monitor the shape and color of fumes at 19 of the country's most worrisome volcanoes.

The Japanese have donated instruments that will enable Mexico to keep a closer watch on Popocatepetl near Mexico City. And shortly after Pinatubo first showed signs of activity in April, the U.S. Geological Survey sent to the Philippines a team of scientists equipped with seismometers, tiltmeters (to measure tiny shifts in the slope of the mountain) and laptop computers to collect and analyze data. Several of the instruments, however, were obliterated by last week's eruptions, hampering efforts to figure out the volcano's next gambit.

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.

With reporting by Seiichi Kanise/Tokyo, Laura Lopez/Mexico City and Nelly Sindayen/Manila