But They Didn’t Know When Volcano Would Erupt : Geologists Had Predicted Devastation

Geologists had predicted within the last month that the Nevado del Ruiz volcano, which had been rumbling and belching steam for nearly a year, would erupt and cause widespread devastation, but they had no way of knowing exactly when that would happen, U.S. Geological Survey officials said Thursday.

Hazard maps prepared in October by an international team of scientists “fully anticipated what happened,” Darrell Herd of the Geological Survey’s Office of Earthquakes, Volcanoes and Engineering said in a Washington press conference Thursday.

Herd noted, however, that there was no seismic or geological evidence that would have allowed scientists to calculate the precise date of the eruption, and there was no negligence on the part of Colombian government officials.

Herd also said that it is likely that “there will be additional eruptions from (Nevado del) Ruiz, although not necessarily of the same severity.” He predicted that the activity “would continue for several days before it abates.” In Bogota, the Colombian capital, scientists said that lava began to flow Thursday afternoon from the crater.

Scientists first began watching the 17,716-foot volcano in December, 1984, when increased seismic activity and small releases of steam were observed. Similar activity had been observed since the volcano’s last major eruption in 1595, but such occurrences are common and usually inconsequential, according to David Norris of the Geological Survey office in Pasadena.

There was a small steam blast last March and a much larger blast and ash eruption on Sept. 11. The September blast was accompanied by a mud flow, between 60 and 70 feet wide, that traveled about 20 miles down the Acufrado River valley at speeds of “tens of miles per hour,” Herd said. That blast, he said, indicated that a larger eruption was likely to occur soon.

That mud flow caused no damage, but Colombian officials closed off the mountain and called in a team of geologists from the United States, Ecuador and Costa Rica to assess the hazard. Herd, who wrote his doctoral thesis on Nevado del Ruiz, was a member of the team.

Put in Seismic Network

The team began installing a seismic network on the slopes of the volcano, but their work was hampered by the steepness and ruggedness of the slopes. At the time of the explosion, a rudimentary system was in place, but there is no evidence that this system provided any warning of the eruption.

The team also studied the potential effects of an eruption. Based on the team’s work, several Colombian agencies predicted in October that there was a 67% chance that an eruption would produce flooding and mudslides of the type that occurred Thursday.

The report also said there was a 21% chance of a rock eruption, an 8% chance of a lava flow and no chance of a lateral explosion, blowing away the side of the mountain. The agencies had planned to issue a more definitive report at the end of November.

Colombia’s Mining Ministry Commission had said the town of Armero, which was destroyed by mudslides and overflowing rivers, could be “evacuated in two hours without danger” even if rivers rose 38 to 96 feet because mud moves slowly, according to wire service reports from Bogota.

Fraction Reached Safety

When the volcano erupted in the middle of the night, only a fraction of Armero’s residents managed to scramble to high ground or onto the roofs of their houses. Civil defense officials said there appeared to be about 10,000 survivors in the town.

In October, visitors in the area noted frequent tremblings on the mountain and that the air was filled with sulfurous fumes. An average of 35 minor earthquakes a month have been reported at the site in the past year.

The Colombian government was in the process of preparing detailed emergency plans for an eruption when the disaster occurred.

“I suspect the eruption came too soon, before all the plans were implemented,” Herd said. He also noted that a disaster might not have been prevented even if the plans had been completed, given the steepness of the slopes and the size of the ice and snow pack on the mountain.

Plates Collide

Nevado del Ruiz is one of the northernmost peaks in the Andes, a chain of mountains created from the collision of two plates on the earth’s crust--the Pacific plate and the South American plate.

Those plates meet along most of the western coast of the Americas, producing what is known as the Ring of Fire, a region of intermittent volcanic and earthquake activity. Mt. St. Helens in Washington state, whose eruption in May, 1980, leveled an area of 150 square miles and killed 57 people, is also part of the Ring of Fire.

Like Mt. St. Helens, Nevado del Ruiz is a strata volcano, built up from successive releases of lava. The lava is molten rock produced by the intense heat and pressures generated when the South American plate slides over the top of the Pacific plate.

The eruption itself probably occurred near the summit crater called Arenas, Herd said. There is--or was--a steep wall on the northeastern flank of the mountain directly below the crater and above the headwaters of the Guali and Acufrado Rivers.

Internal Pressures

Herd speculated that internal pressures blew off part of the headwall, creating the mudslide that eventually inundated the town of Armero and dammed the Guali River.

Alternatively, he said, the pressures could have simply weakened the headwall, leading to a natural landslide that exposed the core of the volcano. Such a mudslide also triggered the eruption at Mt. St. Helens.

In either case, mud rushed down the river valley from an altitude of about 15,000 feet to near sea level over a distance of about 30 miles, traveling at avalanche speed. The wall of water and mud destroyed everything in front of it.

Given the state of knowledge about geology, Herd concluded, there was probably little more the Colombian authorities could do about the volcano. “It is my strong sense that the people (living in the area) were made well aware of the hazard.”

Times staff writer Rudy Abramson in Washington contributed to this article.