Among the volcanoes in the Cascade Mountains, the long-term average is two eruptions a century.

"That doesn't sound like a lot," John Ewert, a Geological Survey scientist who was involved in assessing the risk posed by volcanic eruptions. "But when you consider the size of the volcanoes and consider most of them are covered with snow and ice, it becomes a much more significant number."

An eruption can melt the snow and ice, setting off avalanches and gargantuan flows of debris rolling down the side of the mountain. But for now, the Geological Survey has few instruments keeping watch over them.

At Mount Shasta in Northern California, there are two seismometers within 10 miles of the volcano and another four within 20 miles. "And that's about it," Mr. Ewert said.

Satellite monitoring might be able to give earlier warning of volcanic activity. For example, satellites were able to detect a swelling of the South Sister volcano in central Oregon in 2001.

Swarms of small earthquakes -- usually the first sign of a reawakening volcano -- did not start until three years later.

However, the technique does not work flawlessly; it did not work at Mount St. Helens, for example.

Many of the scientists now observing Mount St. Helens were there when it erupted in 1980 and continued to observe the mountain as a series of 16 smaller eruptions, some lasting only a few days, continued through 1986. Then the mountain fell quiet, and the scientists did not expect another eruption in their lifetimes.

Last September, a swarm of small earthquakes started shaking the volcano. The first eruption of ash and steam rose upward a couple of weeks later, followed by a flood of reporters who crammed news conferences, asking if another blast like the one of 1980 was imminent.