BOSTON — Massive forces deep within the earth are trying to tell us something. Beware of volcanoes. Heed the warning signs and take evasive action.
Rumblings north of Los Angeles, near Mexico City, and in the Caribbean, for example, are demanding close attention, volcanologists say. Last week Mt. Ruapehu in New Zealand erupted in the country's largest volcanic activity in half a century. And over the weekend, seismic activity increased under Washington State's Mt. St. Helens.
Geologists' ability to detect those warning signs and help people take life-saving precautions is better than they have ever been. But the wealth of new information has also increased the known areas of volcanic activity, and scientists' estimates of the risk that volcanoes pose to people around the world are rising.
It's not that volcanoes are becoming more active. ''What's changing is our understanding,'' says Stanley Williams of Arizona State University at Tempe. He notes that as populations grow, people are ''surrounded by unknown volcanoes'' in some places. The number of volcanoes causing concern increases as geologists find more of them. The number conventionally quoted for the world's active volcanoes is between 500 and 600. Dr. Williams says that the number is more likely to be at least several times that. T he United States Geological Survey projects that some 500 million people - mostly in developing countries - could be at risk from dangerous eruptions by the year 2000.
Today's volcano watchers have a space-age perspective. US polar orbiting weather satellites and the Landsat Earth observer satellite track volcanic ash plumes in the atmosphere and monitor lava lakes. Ground mapping radar such as that on the European Space Agency's ERS-1 satellite can measure vertical movements to within a centimeter - useful accuracy for monitoring Vesuvius in Italy.
The most important monitoring still takes place on the ground, however. Volcanologists study local earthquakes and mild eruptivity. But gases seeping up from below provide the best clues. Williams explains that ''gases are such volatile things that they give us signals [early].''
Geologists go on the theory that, as magma (molten material) begins to rise from 15 or 20 kilometers deep, carbon dioxide emissions should increase at the surface. Later, as magma nears the surface, emissions of acidic gases - especially sulfur dioxide - should also rise.
That's what's going on at Mammoth Mountain, 250 miles north of Los Angeles. Tree-killing carbon dioxide seepage is ''waving flags,'' Williams says. It indicates active magma below. While there is no imminent danger, Williams cautions that ''we should pay attention.''
Meanwhile, at Popocatepetl near Mexico City, active eruption has been going on for 10 months. Although ash falls have caused no significant threat so far, Michael Sheridan, volcanologist with the State University of New York at Buffalo, calls the volcano ''dangerous.''
And on the Caribbean island of Montserrat, geologists are watching the Castle Peak vent of the Soufriere Hills Volcano. Dr. Sheridan says there's a ''fairly high probability'' that the action ''will become serious.''
And, as though to make the volcanologists' point for them, Mount Ruapehu began erupting in New Zealand.
This fiery action is the hallmark of our geophysically active planet. Earth's outer layer is broken into a number of plates that slide over partly molten material beneath them. Some volcanoes, such as Mammoth Mountain, lie in plate interiors. But 80 percent of them occur where these plates override one another; another 15 percent form where plates diverge.
Material welling up between diverging plates in the ocean, for example, forms new sea floor and volcanoes like those in Iceland. Alternatively, when a plate bearing old sea floor plunges back into the earth beneath a plate bearing continental crust, it forms deep sea trenches such as those off the island arcs of Japan and the Philippines and along the western coast of South America. The descending material partly melts to supply new magma for other volcanoes.
Sheridan says that public education, combined with local monitoring, is the best way to minimize disaster when a volcano erupts. He says people ''should know their volcano ... [and] identify safe areas'' ahead of time. He notes that this strategy ''worked well'' with the 1992 eruption of Mt. Pinatubo in the Philippines where evacuation ''was the key to mitigation.''
Joy Crisp of the Jet Propulsion Laboratory in Pasadena, Calif., says she is looking forward to the proposed international Earth Observing System of advanced satellites that are due to be launched in a few years. It is to have instruments that give a global view of volcanoes in what she calls ''near real time.'' That means data are to be available within a day or two of the observations. Data processing computers can be programmed to alert volcanologists by pager immediately when a volcano has unusually hot temperatures or high amounts of sulfur dioxide gas. She explains that ''satellite monitoring may save lives when people are not stomping around volcanoes.''
For his part, Williams says, ''I don't buy that.'' He acknowledges that the satellite data will be useful, especially for research. But, he explains that if the purpose is to save lives, he says, the money would be better spent helping developing countries, where the greatest risk lies, to do their own monitoring and to educate the public.
Likewise, premature evacuation and misunderstood early warnings can be counterproductive. After an early warning over a decade ago with no eruption in Long Valley where Mammoth Mountain is located, ''people were very hostile to scientists,'' Sheridan says.
This highlights volcanologist's toughest task: Keeping public attention and confidence while preparing for an eruption that may not come for a long time. Scientists have to deal with the ''chamber of commerce effect,'' Williams says. Businesspeople, and local officials, who don't want to hear about risk have often been ''successful in preventing scientists from being helpful,'' he explains.