Colombia volcano: what next?

In the valleys below Nevado del Ruiz, the effects of the Colombian volcano's eruption last week have been devastatingly clear. Torrential floods of mud and water, triggered when two blasts Nov. 13 melted snow and ice crowning the mountain, raged down the volcano's steep northeast flanks at speeds of up to perhaps 90 miles per hour and flowed into river channels, engulfing the town of Armero and killing more than 22,000 people. But as to the volcano itself--how the blasts occurred, if there were precursory signals and whether the volcano is gearing up for more eruptions -- scientists have been considerably less certain.

"We really don't know what happened on the mountain yet," says Patrick Muffler of the U.S. Geological Survey (USGS) in Menlo Park, Calif. "Before we can ask the scientific questions, we need hands-on knowledge" of the volcano.

And that is just beginning to happen. Darrell Herd of the USGS in Reston, Va., was able to fly over the volcano last weekend in spite of bad weather. According to USGS colleagues, he saw mudflows in all valleys draining the volcano and spotted two separate plumes within the enlarged crater. He estimated that only about 5 percent of the snow and ice had melted during the eruption, so the greatest worry is that more mudslides could be triggered even if the volcano remains relatively tame. Based on the behavior of other subduction zone volcanoes (which are driven by the melting of an oceanic plate as it descends under a continent) such as Mt. St. Helens, scientists say the chances of another large eruption soon are not great. But they really can't be sure. That is why they have begun to set up real-time monitoring of the seismic rumblings and deformation of the volcano. Two teams of USGS specialists, for example, arrived in Colombia last weekend with 3,000 pounds of equipment to replace and upgrade the seismic stations that had been placed on the volcano when it first began to release steam and ash on Sept. 11.

On a broad geologic time scale, the eruption was not a surprise. Herd had shown in 1974 that major eruptions of Nevado del Ruiz occurred about every 300 years, and since the last large blast took place in 1595, another was due. The Sept. 11 eruption of steam and ash was another clue that alerted the Colombians. It also produced a relatively small mudslide, which raced down the volcano so fast that it leaped across waterfalls and ran up the sides of the channel bobsled-style, according to Herd.

Hazard maps, drawn up in October as a result of studies conducted by an international team, apparently predicted very well where the November mudslides would occur. But the eruption happened before the Colombians could implement emergency plans, says Herd. USGS scientists still don't know whether there were recognizable precursory signs in the days prior to the eruption. Compared with many other places, "there's virtually no volcanic monitoring in South America," notes Muffler. Adds Herd, "I'm sure this eruption will heighten Colombian, as well as international, concern about the potential for other possible dangerous volcanoes.... Are there some long-term lessons that we can learn from this tragedy that can be applied elsewhere?"