New eruptions at the Popocatepetl volcano near Mexico City have scientists on their toes and busily resurrecting the mountain's rich history of violent geologic activity.
Eruptions that resumed early this month at "Popo" equal the largest since the 17,887-foot mountain entered an active phase 15 months ago, according to a Mexican scientist who is a leading Popo authority.
After a flight over the mountain Tuesday, Claus Siebe messaged U.S. scientists that the top of the volcano is covered with fresh ash and that some dusting of ash has reached the city of Puebla 25 miles to the east.
But Siebe, of the National University of Mexico, and other scientists emphasize that nothing has happened yet that shows that this will turn into a major destructive volcanic event of the kind the volcano has not seen in 800 years.
Michael F. Sheridan, a University of Buffalo geologist who last year joined Siebe and others in preparing a comprehensive hazards map for the volcano, said this week: "The chances of a major eruption are greater when a volcano goes into another strong pulse like we are seeing."
But he added, "Before this could happen, we would see many more warning signs. I would expect before a big eruption there would be a lot more seismicity and ground deformation, and we haven't seen that yet. But the sulfur emission is tremendous."
Popocatepetl has had 16 eruptive periods since the Spanish arrived in Mexico in 1519, but none have caused a major loss of life or property.
More than 20 million people live within a 62-mile radius of the volcano's cone; Mexico City is 40 miles to the northwest.
And in an upcoming scientific article, Siebe notes that earlier disasters have befallen those living in the mountain's vicinity. About 300 years before the Spanish conquest, what is now the site of Puebla, with a population today of 500,000, suffered a Popo-generated mudflow that required a mass evacuation.
Today, prevailing northeasterly winds common at this time of year are carrying ash from the erupting peak toward Puebla and the town of Atlixco. Mexico City television has also shown a fissure that has opened on the eastern side of the mountain.
Within another two months, the seasonal wind patterns will change, and during most days winds will be blowing from the peak toward Mexico City, Sheridan said.
The hazard map shows that about 14,000 years ago, an eruption caused an ash fall that dropped about three inches of ash on most of what is now Mexico City.
In December 1994, Mexican authorities ordered a precautionary evacuation of an area populated by about 75,000 villagers on the south side of the mountain, away from Mexico City. This time, however, there has only been an alert.
Nevertheless, the current activity is causing more than a little concern. "The Mexicans have a problem," said Stanley N. Williams, a volcanologist at Arizona State University. "The activity deserves some respect. Their disaster program mainly has experience with earthquakes, not eruptions. It worries me that the volcano continues to be threatening."