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While the stories the studios are telling are mostly make-believe, the danger is real. Increasingly, however, scientists can do something about it. They did so most famously in 1991, when they took the pulse of Mount Pinatubo in the Philippines, predicted it was about to erupt and persuaded officials to evacuate 35,000 people two days before it did. Researchers now have at their disposal an arsenal of newly developed volcanology hardware, ranging from satellites to acoustical sensors to highly sensitive gas sniffers. Whether the technology is up to the task of monitoring not just one peak but hundreds worldwide, though, is impossible to say, but the question is becoming pressing. "Someday," says Robert Tilling, chief scientist of the USGS Volcano Hazards Program, "one of these mountains will erupt on a scale many orders of magnitude greater than mankind has ever seen."
For all their fearsomeness volcanic mountains are relatively simple geologic structures--little more than lesions in the earthly dermis that suggest a fever condition far below. Volcanologically active areas generally lie atop clashing tectonic plates, where fractures five or six miles belowground create chambers into which magma rises and pools. The faster the plates collide, the more volcanic chambers are formed, which is why so many eruptions take place in the geologically active area of the Pacific known as the Ring of Fire.
Magma held in the chamber eventually makes its way toward the surface through channels in the overlying rock. As the ascending ooze climbs higher, the pressure on it is dramatically reduced, allowing gases trapped within to bubble out like carbonation in an opened bottle of soda. As this happens, the magma takes on a foamier consistency, increasing its speed and mobility. When this scalding froth rises high enough to make contact with subterranean water, the water flashes into steam, turning the whole hellish mix into a natural pressure cooker. Finally, the explosively pressurized magma blasts out of the earth in an eruption that can send rocks, ash and gases flying out at near supersonic speeds. "The driving force of an eruption is gas," says Tilling. "Pressure builds up, some plug gives, and the whole thing goes."
For the volcanologically untutored, there are worse ways to learn what a volcano looks like than to see Dante's Peak. Though the story line is standard disaster-film fare, the science is generally sound. As the movie reveals, the first debris disgorged by a volcano is often a great gray mass of ash. The opaque cloud, made of pulverized rock and glass, falls like concrete snow on land and buildings miles away and may blot out the sun for days.
After the ash, some volcanoes produce what is known as a pyroclastic flow, a ground-hugging cloud of superheated gas and rock that forces a cushion of air down the mountainside at up to 100 m.p.h., incinerating anything in its path. Other mountains spew that signature substance of the volcano: lava. (On this point Dante's Peak was wide of the scientific mark, concocting a fictitious mountain that produces both substances.) Lava moves at speeds ranging from less than 1 m.p.h. to 60 m.p.h.