"Would you define pyroclastic flow?" asked the young man. "You've used the word twice now."

Pyroclastic flow, of course, is "a mixture of gas and volcanic particles, not lava or liquid, that is going at 60 to 80 miles an hour like a hurricane of ash . . ." according to Richard Fiske, director of the Smithsonian's Museum of Natural History. Pyroclastic flow comes out of volcanoes, which was the subject under discussion yesterday.

Reacting to the immense public interest in the Mount St. Helens' eruption, the museum quickly put together two public discussions yesterday to help people understand what happened and what could still happen.

The four scientists who spoke at the noon forum seemed to say that basically nobody really knows, but this eruption is going to be so thoroughly studied -- more so than any previous "event" -- that in a few years we will know more about volcanoes than ever before.

"A lot of people have died because of the failure to recognize that volcanoes can be active from thousands to millions of years," said Dr. Tom Simkin after the forum. "There can be thousands of years between eruptions." s

Simkin and colleagure William G. Melson agreed that geologists are falling all over themselves to go to Mount St. Helens, but access is being restricted by the U.S. Geological Survey because the volcano is still dangerously unpredictable. One geologist, David Johnston, has already died -- a fund in his memory at the University of Washington was announced to yesterday's audience.

"It would be nice to be out there," said Simkin wistfully. "But there are important things to do here. I try to keep myself from being swept up in this exciting moment."

Melson visited Mount St. Helens on May 4, two weeks before the big blast. At the time, the volcano was moving five feet a day.

"That's more rapid than I've ever seen," he said. "It was unbelievable . . . But once a person has been killed the aspect changes, it detracts from the excitement." Fiske said that once the people who are now considered missing are found, the death toll from Mount St. Helens could reach the "mid-80s."

The noon lecture drew a large crowd to the 500-seat Baird Auditorium, many of them lunch-timers wearing plastic government identification badges. Although there was one couple who privately asked Simkin if their new retirement home in Salem, Ore., was a good buy or should they sell it and move somewhere else (he said they should stick with Salem), most of the questions were quite scientific and knowledgeable.

"Do you expect worldwide climate changes (as a result of the eruption)?" asked one man. Fiske said that not enough volcanic material had gone into the atmosphere to create any climate changes, although evidently the question of how volcanic eruptions affect climate is a subject of considerable controversy in the scientific community.

Although there are about four dozen eruptions a year in the world, Simkin said, they are rarely of the magnitude of this recent one. The most comparable explosion, he said, was Bezymianny in the Kamchatka Islands in 1955.

Geologists who become volcanologists are a rarefied breed. "There are not many hard-core volcanologists," said Simkin. "But people who study volcanic rocks are in every geology department in the country because it's basic. And every one of them is a closet volcanologist."

Simkin, 46, who with his pony tail and beard looks more like a candle maker from Oregon than a scientist, got his Ph.D. from Princeton after originally starting a career as a civil engineer. He first got interested in geology, he said, "probably because I love climbing mountains."

Volcanologists hardly even have an association. There is an International Associatin of Volcanologists, Simkin said, which produces a journal at occasional and unpredictable intervals. A group in the United States has started forming within the last four years, he said, which is determined to remain informal. "We don't want to elect presidents and secretaries and things like that," he said. "We figure we'll piggyback our meetings onto some other scientific conference and have them make all the arrangements."

A volcanologist can specialize in a variety of different approaches, he said: the physics, chemistry, sociological or botanical aspects of volcanoes. "But we are all trying to get at what makes the darn things tick," he said.

Although the U.S. Geological Survey had been monitoring Mount St. Helens very closely, there was no way to predict the first eruption as accurately as the Russians did on Kamchatka in 1975, where it was possible to measure the depth of the earthquake accurately enough to predict when it would move to the surface -- accurately enough so that television crews were waiting. m

The clouds carrying volcanic ash from Mount St. Helens have already been around the globe, he said; particles were recorded here by the Alexandria Health Department's pollution measuring filter. Volcanologists don't have a measuring system as seismologists do, he said, but in general they agree: "It was a darn big eruption."