Scientists predict second Ruiz blast.

Scientists predict second Ruiz blast

Mass evacuation of the Nevado del Ruiz mountainside began June 13, the 39th day in a row that scientists detected ominous rubling inside the volcano. Colombian officials -- remembering the eruption on NOV. 13, 1985, when rivers of mud killed more than 22,000 people (SN:11/23/85, p.326) -- want everyone out of the way before the next flare-up. They have told 25,000 people to move to refugee caps in Libano, 17 miles southeast of Ruiz.

The prolonged seismic activity in the mountain suggests that magma (molten rock) is moving near the surface. Increasing sulfur dioxide spewing from volcanic vents also indicates upward-moving magma. And recently, scientists have measured subtle changes in the volcano's shape, which may represent the heaving and sighing that precede a great volcanic cough.

"We think the likelihood of a further eruption within days or weeks is very high," says Hansjurgen Meyer, scientist in charge of the Observatorio Volcanologico de Colombia in Manizales, 18 miles northwest of Ruiz.

But scientists may be reading the signs wrong, Meyers says, because they know so little about Ruiz. "You can only do a very basic kind of forecast when you haven't been able to observe several cycles of a volcano's activity, and so far we have only seen one," he says.

Geologists try to infer the mountain's behavior from that of other subduction volcanoes -- created by the melting of oceanic plates as they descend under continents -- and from the geologic record in the rocks of Ruiz. They know,

for example, that Ruiz belches clouds of ash and rubble, as Mt. St. Helens does, but it does not send out racing rivers of lava as do the rift volcanoes of Hawaii. They also know the 18,000-foot Ruiz has erupted 10 times in 10,000 years, and it almost always sets off mudflows, called lahars.

Direct measurements of Ruiz date only from late 1984, when mountain climbers began feeling earthquakes and seeing large plumes of gas. And it wasn't until July 1985 that four portable seismographs were put on the mountain.

On Sept. 11 there was a minor eruption of steam, ash and rock. Nobody was hurt, but the event led to increased surveillance of the volcano.

By October, geologists had installed additional seismographs and set up tiltmeters to record changes in the mountain's shape. But the volcano gave no sure signals that it would erupt. Even on Nov. 12, when geologists climbed to the summit to collect gas samples, they saw no signs of an imminent explosion.

The Nov. 13 eruption was much smaller in volume than the largest blast of Mt. St. Helens in 1980. But the lahars made it the second-worst volcano disaster of the century. (The worst was the 1902 eruption of Mt. Pelee in Martinique, in which 28,000 died.) Hot ash and gravel seared about 10 percent of the mountain glacier. Melted ice flooded down, gathering dirt and rubble on its way. In three hours, the towns of Armero and Chinchina were washed out.

Ruiz then was quiet until late April, when seismic activity began to increase. On May 4, there began a series of harmonic tremors that lasted until June 14. On June 15 and 16, the tremors were replaced by a swarm of small earthquakes. On June 17, the tremors resumed.

It's difficult to tell exactly what the latest seismicity means, because it did not occur the same way last fall. Rumbling before the September eruption lasted only a week, according to David Harlow of the U.S. Geological Survey (USGS) in Menlo Park, Calif. Between Oct. 1 and Nov. 11, there was little seismicity. Tremors began again only two days before the Nov. 13 eruption.

Recent changes in the mountain's shape are puzzling as well, because there are few comparison data. Deformation measurements were taken for only one month before the November eruption. "We should have started measuring a year and a half ago so we would know how much the magma store grew before it erupted," says Norman G. Banks of the Cascades Volcano Observatory in Vancouver, Wash., who is project chief of the USGS team studying Ruiz. "We don't know if here is a much larger amount there still to be tapped." Generally, the more magma present, the more volcanic activity expected.

Some scientists have speculated that the lahars could be worse next time because they can flow through fresh, clean pathways. On the other hand, the ash layer from November might protect the ice cap from melting as quickly next time. Ultimately, the lahars will be determined by the size of the eruption, which scientists cannot predict, Harlow says.

As scientists study signs of eruption, Colombian government and Red Cross officials try to persuade mountain residents to move. "Some insist on staying where they are, and some wait for government promises that they will be given other homes," Meyer says. Once the people are resettled, he adds, they are likely to stay away for years, until Ruiz completes its volcanic cycle.