In the Belly of the Beast

By Natalie Angier

The blowout that was so evidently seen and heard . . . gave off three muffled cannon rounds like a bombardment, so loud that they could be heard for more than 30 leagues around the base. In the region there were two rivers, the Guali and the La-gunilla . . . both were flooded with melted snow. It didn't really seem like water, but masses of ash and soil, with such a pestilent odor of sulfur that it couldn't be tolerated even from afar.

That vivid eyewitness account could be a description of last week's furious eruption of the Nevado del Ruiz in Colombia. Yet it was written by a Spanish monk, Father Pedro Simon, who observed on March 12, 1595, the 17,716-ft. volcano's previous major convulsion. The similarities between the two events point up both the always present menace of an active volcano and its lethal unpredictability. Although scientists were convinced that Nevado del Ruiz was due for a major burst, they could not pinpoint the time with sufficient accuracy to allow large-scale evacuation of the surrounding towns.

The Colombian eruption, like most volcanic events, is the result of continental wanderlust. According to the widely accepted theory of plate tectonics, the earth's crust forms the top layer of about a dozen major plates and several smaller ones, which range in thickness from 20 miles to 150 miles. These sections float on a gooey layer of partly molten rock known as the asthenosphere. As they move in different directions at an average speed of several inches a year, the plates collide, dive under and buckle against one another, crinkling up into a mountain range here, yanking apart to form a rift valley or oceanic ridge there. Such tectonic clashing was responsible for the violent earthquake that shook Mexico City two months ago, when the Cocos plate of the Pacific, temporarily stuck in its slow but inexorable plunge under the North American plate, suddenly jarred loose and lurched ahead. Last week's burst involved a similar movement of plates, but the result was entirely different. Extending along most of the coast of South America, the dense Nazca plate of the Pacific, moving eastward, subducts, or descends beneath, the lighter mass of the South American plate, which is moving westward. As the oceanic plate dives deeper into a region of high temperature and pressure some 60 miles to 125 miles below the earth's surface, rock in the area begins to soften and form magma, molten rock. Says Robert Christiansen, a volcano specialist with the U.S. Geological Survey (USGS) in Menlo Park, Calif.: "This is one area in which our knowledge is the least advanced."

Christiansen and others suspect that magma is produced in the subduction zone, the border between the diving plate and the lower mantle. In that complicated layer, a variety of phenomena, including high temperatures, changes in pressure and the influx of water, may act to melt the already softened rock. Minerals and water then coalesce with the molten material into viscous, tear-shaped packets known as diapirs. Because they are more buoyant than surrounding rock, the diapirs percolate upward, like bubbles rising through honey, melting more rock as they go. Eventually they accumulate in pockets called magma chambers, located two miles to 15 miles underground. If the magma is very liquid and gases can escape gradually, a volcano may lie fallow for long periods of time.

Employing a sort of tectonic checklist, geologists divide volcanoes into a few large families, ranging from the potentially explosive subduction type, like Nevado del Ruiz, to the milder plume variety, like the famed Kilauea volcano in Hawaii. In the subduction volcano, the magma is composed largely of andesite, which is a melt relatively rich in silica, water vapor and other gases. These gases are trapped until the magma approaches the surface, at which point, explains Richard Hoblitt, a USGS volcanologist in Denver, the pressure is released and "the gases tend to escape explosively." Another renowned example of an explosive volcano caused by subduction forces is none other than Mount St. Helens. Alfred Anderson, a geologist at the University of Chicago, says the two are almost identical.

Kilauea, on the other hand, frequently belches forth bright orange ribbons of glowing lava (the word for magma once it reaches the surface) in eruptions that are relatively gradual and benign and seldom cause loss of life. This plume magma contains less silica, is lighter than subduction magma, and allows expanding gases in the incandescent mixture to escape continuously as it moves toward the surface.

The Colombian eruption was made devastating by the large amounts of ice on the volcano's cap. "Nevado" is a Spanish word for snow, and though the mountain is near the equator, it is so tall that it is topped by snow and partly sheathed by glaciers year round. When the volcano blew, it tossed out hot material of varying sizes, from tiny grains to large rocks. The glowing debris hit the ice and melted it, causing torrents of scalding water to cascade down the slopes, mixing with volcanic debris and forming lahars, or mud-flows. That slurry, says Kevin Rodolfo of the University of Illinois at Chicago, "goes tearing down the banks of the volcano" at speeds of 30 m.p.h. In prehistoric times lahars raced down the sides of Washington State's Mount Rainier and traveled as far as 50 miles. "That's where the problem comes in," Rodolfo notes. "People don't expect to be affected 30 miles from the summit, and there's no time to warn them."

Being able to warn people to get out of the way long before the lahars descend is one goal of volcanologists. Right now the business of volcano predicting is a hodgepodge of high technology, empirical evidence, common sense and a certain amount of luck. As with earthquakes, geologists can often say that something will occur; the question always is exactly what and when. For long-range predictions, scientists try to reconstruct the history of a volcano, using such techniques as radiometric dating of volcanic deposits and studying tree rings (trees damaged by lahars will record the event with thinner, abraded rings). Says Hoblitt: "A volcano's behavior in the recent geological past is our best guide to behavior in the near future."

To perform short-term divination, geologists examine a number of clues. Mild seismic activity in the area, rumblings or the emission of ash and gases are all harbingers of greater things to come. Changes in the snowcaps that cover tall volcanoes may also indicate trouble. In Iceland, for example, the sight of a sagging, snow-covered mountaintop, which indicates that hot magma is pushing upward and melting the ice cap, warns knowledgeable residents to head for safety. More sophisticated techniques include tiltmeters or laser ranging devices to detect deformations in the volcano cone, also caused by magma oozing upward. Seismometers are used to measure harmonic tremor around the volcano, a series of highly rhythmic shock waves associated with the motion of magma inside or beneath the cone. In Hawaii, a harmonic tremor lasting for some four to five minutes nearly always precedes an eruption.

Armed with such signposts, geologists have targeted several volcanoes that may erupt in the near future. More than 500 volcanoes are classified as active, most of them lying in the so-called Ring of Fire, a broad circle that more or less coincides with the boundaries of the Pacific, where oceanic plates are diving under continental plates. Of particular concern to scientists are some of the peaks in the Cascades, the mountain range that includes Mount St. Helens. The Mammoth Lakes ski-resort area in California is another area of potential volcanic activity.

Yet the researchers stress the still primitive nature of their craft. In the case of Mount St. Helens, one of the most heavily instrumented volcanoes ever, experts predicted many aspects of the 1980 eruption, yet they were caught off guard by both its fury and the extent of the mudflows it generated. And at Nevado del Ruiz, warning signs had abounded since Dec. 22, 1984. At that time a series of earthquakes were detected, followed by 30 minutes of harmonic tremor. Mild tremors continued throughout the spring and summer, and on Sept. 11, ash spewed forth for seven hours, accompanied by a roaring sound and electrical discharges. But for all these red flags, experts were unable to pinpoint when the big boom would come, or even if it would. Lacking that precision, Colombia, perhaps reluctant to disrupt the lives of residents with a false alarm, was unable to act in time. In a heavily populated area, says Gail Mahood, a geologist at Stanford University in California, "if you make a prediction and you're wrong, you could cost billions of dollars to an economy."

Until they can eye a volcano and declare with certainty that it is ready to burst, scientists will remember with a wince their warning nearly ten years ago about Soufriere, a volcano on the Caribbean island of Guadeloupe that began to spout a heavy plume of ash. Goaded by the geologists' alarms, authorities evacuated more than 70,000 people from the area and kept them away for 3 1/2 months. The result: the mountain continued to sputter smoke and cough volumes of ash for a while, but it never blew. --By Natalie Angier. Reported by Christine Gorman/New York and Charles Pelton/San Francisco

With reporting by Reported by Christine Gorman/New York, Charles Pelton/San Francisco