What Makes Them Blow

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.