Waiting for Popo … to blow

0 Comments | Insight on the News, Sept 28, 1998 | by Bart McDowell

Popocatepetl, the world's most watched volcano, could affect 20 million Mexicans if it explodes--or rather, when it explodes, for scientists say it's only a matter of time before Popo pops.

The busy farm town of Americana, Mexico, boasts a skyline of lofty grain elevators, spires of antique churches and--looming overhead--a very dangerous volcano, the 17,000-foot Popocatepetl.

"Popo," as Mexicans call it, spews more than 8,000 tons of sulfur dioxide into the sky each day (Popocatepetl is Aztec for "Smoking Mountain"). No other volcano in the world expels such quantities of gas. And the mountain has been chain-smoking since December 1994.

Will Popo erupt? There's no doubt about it. The questions are when and how violently? The 20 million people who live nearby and are sometimes peppered by gritty volcanic ash live in an official state of Yellow Alert--not as urgent as the "run-for-your-life" Red Alert, but more worrying than a Green Alert.

Popocatepetl is the world's most watched volcano. Scientists monitor the crater night and day with a wide array of sensors. An unblinking television camera, installed at an elevation of 13,000 feet on a neighboring peak, keeps tireless watch on the crater and its plume of gas. The image is transmitted via the Internet to volcanologists at the National University on the outskirts of Mexico City and to the nearby National Center for Disaster Prevention, known as Cenapred, its acronym in Spanish.

"We can't get away from our work" says Hugo Delgado, nodding at his computer screen. "The wind is strong today." The plume leans sharply eastward, showing the prevailing wind until the summertime rainy season.

Delgado keeps close tabs on that gas plume. Twice a week, he or his assistants drive around Popo in a car equipped with a device called a Cospec that analyzes the chemistry of the plume by means of refracted light, the same technique astronomers use to learn the composition of distant stars. Once a month, Delgado takes the Cospec aboard a small plane and circles the crater for a more precise reading.

"We watch the ratio between sulfur dioxide and carbon dioxide" he says. "Changes are significant--a good tool for predictions."

Other installations on the volcano's flanks record changes in the shape or movement of the Earth: tilt-meters, monitors for navigational signals sent by satellites and a dozen seismic stations.

Geologists estimate Popocatepetl is holding back nine times as much magma, or molten rock, as Mount St. Helens when it erupted in Washington state 18 years ago. The local population takes this statistic seriously.

Road signs along rural highways designate routes for evacuation. Schoolchildren regularly take part in volcanic-emergency drills. And Indian shamans address the mountain in respectful rituals to divine its intentions.