What Makes Them Blow

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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.