At Ruiz: small but ominous eruptions.

At Ruiz: Small but ominous eruptions

The Colombian volcano Nevado del Ruiz erupted two Sundays in a row in late July. On the 20th, it belched a stream of ash and dust for about two minutes; on the 27th, it erupted for a full 90 minutes, but not as energetically as it had the week before. Neither episode was nearly as violent as the eruption on Nov. 13, 1985, which melted part of the mountain glacier, causing mudflows that killed more than 22,000 people (SN:11/23/85, p.326). But scientists are viewing the July events as further evidence that another catastrophic eruption may be on the way.

"The more eruptions you have, the more you know that the conduit is open,' says Norman G. Banks of the Cascades Volcano Observatory in Vancouver, Wash., who is project chief of the U.S. Geological Survey team studying the Colombian volcano.

Ruiz has been rumbling continuously for three months now. At this point, the mountain tremors are so strong and steady that they are overloading seismicity measuring instruments on the scene, according to Stanley N. Williams, a volcanologist at Louisiana State University in Baton Rouge, who has spent much of this year studying the volcano.

Another ominous sign is the great quantity of gas venting from the crater. Ruiz is releasing more than 5,000 tons of sulfur dioxide each day, Williams says. There is some hope that this activity is serving to "de-gas' the magma inside the mountain and make the volcano less volatile, but it may instead indicate only that magma is moving underground.

"Since May 5, the volcano has had almost uninterrupted tremors, inflation and gas release,' Williams says. "This is considered activity premonitory to an eruption, and it just keeps going and going and going.'