BY ROBERT COOKE
 Newsday

 Fair warning, this time from a French volcanologist:

 The tall mountain called El Misti, overlooking Peru's second-largest city, Arequipa, could soon pop its lid in a major, dangerous eruption.

 If and when it goes, Jean-Claude Thouret said, El Misti could be extremely hazardous, posing as much danger for the 750,000 residents of Arequipa as Italy's Mount Vesuvius holds for the people of Naples. El Misti, just 11 miles from Arequipa's center, has emitted ash falls every 500 to 1,500 years, and even worse events have occurred every 2,000 to 4,000 years.

 Vesuvius erased the coastal communities of Pompeii and Herculaneaum, burying them completely in a two-day period in the year AD 79. It took 1,600 years before
 Pompeii was rediscovered and uncovered.

 Thouret bases his warning for Peruvians on the 19,000-foot-tall volcano's eruptive history. According to the Geological Society of America, studies have shown that El Misti produces ``alternative explosive events'' that lead to ``pyroclastic flows, surges, tephra (airborne ash) falls.'' The last such ``event'' occurred about 600 years ago.

 Especially ominous is the danger from pyroclastic flows. These fearsome events consist of broiling hot clouds of ash, gas and airborne debris that come rolling down the volcano's flanks. Pyroclastic flows can pour downhill at 100 mph, destroying everything they contact. No one can outrun them.

 Thus, ``the possible impact of Misti on Arequipa is as worrisome as that of Vesuvius near Napoli,'' Thouret and five colleagues wrote in a recent issue of the Geological Society of America's bulletin. Thouret is a faculty member at Blaise-Pascal University in France. His co-workers include volcano researchers from Peru and Germany.

 Despite years of work and the rather obvious hazards, it is still not possible either to predict exactly when an eruption will come from El Misti or how large it might be. But by studying the layers of rocks, ash and pumice in and around the tall cone, geologists can discern the mountain's behavioral patterns through thousands of years.

 SEEING THE FUTURE

 ``We cannot forecast the magnitude of the future eruptions unless we detect changes in the volcano's behavior a few weeks or a few months in advance,'' Thouret said. Even so, clues scientists collected from the volcano's history suggest what kinds of eruptions can occur, roughly how often they come and what the consequences have been.

 Geologist Tom Casadevall, regional director of the U.S. Geological Survey in Denver, said he has visited Peru -- and El Misti -- several times. ``It's a monster volcano. The cone is very youthful; it looks as if it had erupted yesterday,'' he said.

 MOST RECENT EVENTS

 The latest eruptive event from El Misti hit about 600 years ago, and another occurred about 2,000 years ago, Thouret said. Both episodes, though of moderate size, would have been hazardous for people living nearby. And now that almost a million people live there, the toll in lives and property damage could be huge.

 Similar studies of a volcano's violent history have been accomplished in the past, and the information has been valuable. In the mid-1970s, for example, geologists Don Mullineaux and Rocky Crandall, with the U.S. Geological Survey, warned that Mount St. Helens, in Washington state, seemed ready to erupt. And, just a few years later, in May 1980, it erupted -- with a bang.

                                    © 2002