The May 18 eruption of Mount St. Helens in Washington State—a Cascade range volcano dormant for 127 years—leveled forests over 150 square miles, obliterated the picturesque tourist retreat of Spirit Lake, pushed the Toutle River out of its bed and sent a killing flood as far as 30 miles downstream. As the mountain continued to rumble with violent aftershocks last week, at least 18 people were known dead, 70 were missing. St. Helens itself had shrunk from the fifth tallest mountain in Washington (9,677 feet) to the 30th (less than 8,400). The volcanic ash from the eruption, the worst on this continent in 6,500 years, piled up four inches deep in the streets of Ritzville, 170 miles a way—and began a slow drift around the planet.

The United States Geological Survey, a branch of the Department of the Interior, has been monitoring St. Helens since its first tremblings in March. One USGS scientist five miles from the peak was killed when the blast destroyed his listening post. The service has continued to follow Mount St. Helens closely, and Jack Fincher of PEOPLE last week talked to volcanologist Don Swanson about his findings:

Was Mount St. Helens the largest eruption in recent memory?

It was very large, but the blast at Kamchatka, in the Soviet Union, in 1956 was twice as big, and Krakatoa in Java was also worse. The Mount Mazama eruption, which created Crater Lake in Oregon 6,500 years ago, was bigger in terms of materials ejected. Mount St. Helens is the most violent we have any evidence for in North America.

How will Mount St. Helens affect people in the Northwest in the long run?

If there is another big eruption, there could be serious damage to timber, landscape and surrounding areas, but I think we've seen the worst. The ash can disrupt nature and people's lives for a short time, but I don't believe it could lay waste to an area. In the immediate future we hope to see a series of diminishing ashfalls and the growth of a dome on the crater.

Is the ash itself dangerous?

From a chemical standpoint, no. The blanketing, gritty aspect of it is the problem.

Will it affect the weather?

No—although if much more ash is expelled high into the stratosphere, there could be brief periods of cool weather because the sun's rays would be blotted out. This has happened in major eruptions before.

Do the recent earthquakes in the area mean eruptions are more likely now?

There is no evidence of a link between earthquakes and eruptions. But it is evident that at some time in the future most of the Cascade volcanos are going to erupt again. Volcanologists have been saying that for years—and Mount St. Helens just brought it home.

Do other volcanos threaten America?

Not immediately, although there are other historically active areas like Yellowstone National Park and Craters of the Moon on the Snake River Plain in eastern Idaho. They have been the site of fairly recent activity—within a few thousand years.

Is it a true that volcanologists can learn about volcanos only from violent eruptions?

Well, I think we can learn as much from smaller ones. We did learn that it pays for the public to heed warnings; two of my friends lost their lives, and they were five to seven miles away. We should remember the number of lives that were saved by putting up the roadblocks and enforcing an unpopular evacuation. On a normal mid-May Sunday with sunny weather like that week, I would imagine that thousands of people ordinarily would have been in the area. All of them would have ended up dead or missing.

Why did St. Helens not produce a lava flow?

In this kind of mountain the lava is gas-rich. The gas builds up until it finally blows, like a pressure cooker. When that happens, the lava's heat and pressure break up the rock around it. The end result is ash and pumice.

Given the loss of life and the tremendous destruction, did any good come from this eruption?

Yes. Much of the soil of this country is derived from volcanic ash. It's very rich agriculturally, and much of the farmland in the Midwest harkens back to such eruptions in prehistoric times.

After his helicopter tour of the Mount St. Helens area President Carter predicted that Mount St. Helens would eventually become "a tourist attraction equal to the Grand Canyon." What's your reaction?

It would be very beneficial to the public. Visitors to such an area would have a far greater understanding of just how severe natural disaster can be. It would make them more humble about nature. You're just overwhelmed by the magnitude of it.