IT'S A QUESTION ONLY HOLLYWOOD--which can make up any answer it pleases--would dare ask: why are there no volcanoes in southern California? To the north and south of Los Angeles, the "Ring of Fire" stretches for thousands of miles of Pacific coast; underneath the city and its surroundings, the earth shakes and cracks, but nothing erupts out of it. If "Volcano," which opened last week (review), suggests that the forces of nature were just waiting for mankind to build a subway under Wilshire Boulevard--well, science offers a more reassuring explanation. But geologists also warn that even though, in most years, as many Americans die from volcanoes as are abducted by aliens, a major disaster is only a matter of time--human time, not geologic. It will kill not by fire but by ice, mixed with rocks, mud, dust and ashes--less photogenic than the movie's roiling, 2,000-degree lava but just as deadly, and even harder to escape.
Volcanoes are among the most powerful forces of nature, moving whole mountains in blinding swirls of dirt. The last cataclysmic eruption on U.S. soil was 17 years ago, when the north face of Mount St. Helens blew out, killing 57 people in a sparsely settled portion of Washington state. But in 1985 the Colombian city of Armero was wiped out by a mudflow from the Nevado del Ruiz volcano; 23,000 people died in a few minutes. There are only about 550 active volcanoes on land (and about twice that number undersea), but if they all blew up tomorrow, seismologists say, they could endanger as much as a tenth of the world's population.
And who is to say they won't? Volcanology has made great theoretical progress in the last 20 years, but predicting the movement of molten rock underground is a little like doing meteorology without satellites. In their never-ending quest for data, geologists deploy computer-lined networks of seismographs, global positioning sensors that detect ground movement down to a few millimeters, tiltmeters that can record changes in level as small as the thickness of a penny over a kilometer, and sensors to measure emissions of sulfur dioxide and other gases. But researchers still have to hike up to the craters to take samples, a procedure that Stanley Williams, a geologist at Arizona State University, likens to "walking with a bottle up to the back of a jet as it's taking off." Williams was in the crater of Colombia's Galeras volcano in 1993 when it erupted in a blast of hot gas and rocks that melted the flashlight he was carrying, broke both his legs and nearly crushed his skull. He was lucky; six other researchers and three tourists were killed in the blast.
Last week, as Mount Wilshire erupted on thousands of screens around the country, geologists were monitoring Soufriere volcano on the tiny Caribbean island of Montserrat, which began venting steam about two years ago and in one burst last fall dumped 600,000 metric tons of ash on the surrounding countryside. The island-wide alert, which had been at "orange" since April 11, was downgraded to "amber," but if the volcano does erupt, the island's 7,000 inhabitants would have nowhere to go but into the ocean. On the other side of the globe, geologist Dave Sherrod of the Hawaiian Volcano Observatory was walking across newly crusted lava on the slopes of Kilauea, breathing sulfur dioxide mixed with the smell of burning rubber from the soles of his boots. The brittle rock crumbled underfoot, and orange-red magma (molten rock below ground) glowed in the bottoms of cracks and fissures. Kilauea has been spewing lava almost continuously since 1983, burying almost 40 square miles and 180 houses in flows as deep as 75 feet. But it moves slowly, and no one has been killed. In January observatory scientists accurately predicted a new eruption near an occupied park campground, allowing rangers to evacuate the area just before the lava began spouting.
And in western Washington, geologists from the Cascades Volcano Observatory were keeping their usual watch on Mount Rainier, 50 miles southeast of Seattle--says Kevin Scott, a U.S. Geological Survey hydrologist, "the most dangerous volcano in the whole United States." The danger is not that the mountain will blow up --Rainier's history suggests that any new eruption would probably be fairly gentle--but that it will unleash a cataclysmic flash flood of mud, rock and ice known as a lahar. On its upper slopes, Rainier is covered in thick glaciers; inside, it is rotten with hollows carved by flowing magma and sulfuric acid, creating an unstable situation that could be disrupted by even a minor earthquake. It has happened at least 55 times over the last 10,000 years. Evidence of the last big flow, some 500 years ago, is easy to find; passing a man-made pond near a development within easy sight of Rainier, Scott points out what looks like a stump. "That's the top of a tree," he says. "The rest of it is buried in 30 feet of mud."
Geologists have calculated the likely flows of a lahar off Rainier's slopes and found that several paths head straight for the city of Orting, a burgeoning outpost (population: 2,960) of the Seattle-Tacoma metropolis. Situated 30 miles down a narrow valley from the summit, Orting would be reached by a lahar in an hour or less and buried within minutes. The town has an evacuation plan, but the fallacy, Scott says, is the assumption that anyone would be in a position to sound the alarm. There are seismographs on Rainier, but only an expert geologist could tell if a lahar was on its way. And when the geologists go home at the end of the day, nobody is watching.
One place where geologists aren't watching for an eruption is Los Angeles. Volcanoes typically occur in association with earthquakes, along the fault lines between tectonic plates--which, as everyone knows, is a good thumbnail description of southern California geography. (An exception to this rule is Hawaii, where for unknown reasons a magma "hot spot" keeps erupting in the middle of the Pacific plate.) The movement of plates across the earth's crust causes volcanoes in two ways. Separating plates, usually in the middle of the ocean, open a seam that allows magma to well up. Alternatively, when plates crash into one another and the heavier one plunges beneath the lighter, the resulting increase in pressure squeezes molten rock up through fractures in the overlying plate. The volcanoes of the Pacific Ring of Fire are mostly of this kind.
But--amazingly enough--Los Angeles doesn't lie near such a fault. Its geography is dominated by "strike-slip" and "thrust" faults, where plates grind past one another or butt together without ever overriding. These produce strong earthquakes but no lava. Admittedly--as a geologist remarks to a skeptic in the movie --there is no history of anything, until it happens. But Pomona College geologist Rick Hazlett asserts that there is "no geological reason for a volcano to erupt in Los Angeles." And if he says so, Angelenos can safely scratch volcano insurance from their list of worries. Hazlett was the scientific adviser to "Volcano."