Geologists say they do not expect the Colombian volcano Nevado del Ruiz to erupt again in the next few weeks, but they say floods and mudslides, which devastated nearby towns and villages following the Wednesday night eruptions, will continue for at least another two weeks.
"The probability of another big eruption is not very high. In fact, the greatest hazards now are from avalanches of mud coming down the side of that mountain," Robert I. Tilling of the U.S. Geological Survey said yesterday in an interview.
"The glaciers and snow pack on top of that mountain have already partially melted and they are still being heated by the magma molten rock that moved up far enough inside that mountain to touch off two nasty explosions," said Tilling, a specialist who has observed many volcanoes.
Tilling said the mud and water, which flooded four rivers in the valley below the mountain's northeast flank, will almost surely continue to flow because that part of central Colombia is in one of its two annual rainy seasons.
Also, he said, the deluge of warm water -- triggered by the melting of the mountain's snow and ice pack -- will not stop for at least another few days.
"So much debris from old mudflows has been exposed by the first avalanche that whenever a fresh rush of warm water goes down the mountain it shakes that debris loose and you have another mudslide," Tilling said. "Even if there are no more eruptions of Nevado del Ruiz, the trouble is not over."
The volcano erupted with two large explosions between 11 p.m. and midnight in the midst of a torrential rainstorm, which Tilling said was one reason the mudslides were so devastating. The heavy rains loosened the soil along the northeast flank of the mountain, making it easier for the melted ice and snow to carry mud when it roared into the valley below.
Tilling said he had no idea how much ice and snow was melted by the heat of the two eruptions, but that it was in the thousands of tons. He said the snow pack at this time of year might be 30 or 40 feet deep and the glaciers as much as 300 feet thick.
"The top of that mountain is so steep that I can imagine the mudslides starting down the mountain at 90 miles an hour," Tilling said. "That's enough momentum to carry a lot of mud into the valley by the time that avalanche has slowed down."
Tilling said geologists believe the two eruptions took place through the same vent in the mountain, although he said he had no eyewitness confirmation. He said geologists still do not have a good eyewitness report of what took place on top of the mountain, the shape of the mountain's summit now, and whether ash and steam are still exploding.
"Everything happened in such pitch dark and in such heavy rain that we don't have any good visual observations of the event," Tilling said.
The two eruptions were much more explosive than originally thought, sending ash and hot rock onto the city of Bucaramanga 235 miles away, Tilling said. Early reports had the ash falling no farther away than 90 miles.
"Hot ash to a distance of 235 miles tell us these eruptions were very explosive," Tilling said. "I have to assume that the ash and rock vented into the atmosphere by the eruptions were hot enough to be somewhere on the order of 1,200 degrees Fahrenheit."
One reason Nevado del Ruiz waited almost 400 years to explode -- its last major eruption was 1595 -- is the nature of its rock chemistry. Tilling said the magma in the volcanoes under the Andes Mountains is rich in silicon, sodium and potassium, which makes the rock so thick that it tends to trap explosive gases and plug up its own conduits.
"Hawaiian volcanoes are rich in sulfur and iron, which form magmas that are more fluid and which allow gases to escape and relieve some of the pressure," Tilling said. "Hawaiian volcanoes may glow all the time with lava flows but they don't explode the way Nevado del Ruiz did."
Tilling said that if reports of the death toll in Colombia are accurate, Wednesday's eruption would be the second worst volcanic disaster in the 20th century. The worst came in 1902, when an eruption of Mount Pele in Martinique killed 39,000 people, including 29,000 in the town of St. Pierre.