SIX YEARS AFTER THE eruption on Galeras I stood again at the crater's rim, scarcely recognizing the blasted pit spread out before me. The ledge on which Igor and Nestor knelt and sampled gases had disappeared. The western rim, where Geoff, Fernando, and Carlos had stood, had been partially blown away. Portions of the southwestern lip had collapsed.

As I gazed into the crater, I was struck by how tiny, in a geological sense, the fatal eruption had been. It was a mere hiccup, a blast so small that geologists decades hence will find no sign of it. Yet the eruption killed nine men, injured five others, and continues to ripple through the lives of dozens more. It nearly killed me.

What had happened? Galeras was not quiet, as we had thought, but merely plugged. The crater floor had effectively been welded shut, trapping the rising gas pressure. Eventually it gave way.

In hindsight, my colleagues and I realized that Galeras had been sending out clues—the tornillos, the screw-shaped seismographic signals indicating the slow movement of magmatic gas and fluids. Bernard Chouet, a seismologist with the U.S. Geological Survey, had noticed small numbers of them before the July 1992 eruption and had written a report stating that the same thing could happen again. He sent it to INGEOMINAS, the Colombian geological survey. I never saw a copy, and Chouet's Colombian colleagues, who had noticed 17 tornillos in early January 1993, never warned that this was a threat.

Only after further eruptions in 1993 did we finally come to understand that small numbers of tornillos at Galeras—even as few as one or two per day—might presage an eruption. But based on all available evidence at the time, the consensus at the observatory was that Galeras was safe.

Colleagues assured me I had done nothing wrong, that there was no way to have foreseen the eruption. What I didn't know was that a few of them had been saying quite the opposite to others, contending that I had recklessly led my colleagues to their deaths. When I first heard these accusations, I was too stunned to react. Now I shake my head in wonder. How easy it is to apply the knowledge we have now to the events of 1993. But for me, Marta, and the other scientists in Pasto, there was no such 20/20 vision. We studied the best available data. We made what looked like a sound decision. And just when we were on the cone, Galeras behaved capriciously, as natural forces are wont to do. I was fooled, and for that I take responsibility. But I do not feel guilty about the deaths of my colleagues. There is no guilt. There was only an eruption.   

Stanley Williams is a professor of geology at Arizona State University. Fen Montaigne is the author of the fly-fishing memoir Reeling in Russia. This article is excerpted from their forthcoming book, Surviving Galeras, to be published this month by Houghton Mifflin.

Next Month: Outside reports on the simultaneous publication of Surviving Galeras and rival geologist Victoria Bruce's No Apparent Danger: The True Story of Volcanic Disaster at Galeras and Nevado Del Ruiz. Did Williams, as Bruce charges, ignore warning signs that an eruption at Galeras was imminent and endanger his fellow scientists?