Scientists are stepping up their watch of the Popocatepetl volcano on the southern outskirts of Mexico City. The volcano has been spewing gases and vapors in an ominous sign of a possible eruption.
Although most of the steam rising from fissures in the snow-capped 17,887-foot volcano, known as Popo, is water vapor, scientists have also detected gases emitted by the magma - the term for the lava churning inside a volcano.U.S. and Mexican scientists have found that sulfur dioxide emissions have doubled to 3,000 metric tons a day in the past year, signalling a worrisome change in the pressure of the magma.
"That raises the risk (of an eruption), but we can't quantify by how much. It certainly calls for more vigilance," said Servando de la Cruz Reyna, a vulcanologist at the Institute of Geophysics of the Mexican National Autonomous University.
Dr. Stanley Williams of Arizona State University said about 100 volcanoes around the world are currently emitting gases but "only four or five in the world are releasing that much gas, and Popo has joined that club."
"The most probable outcome is a single explosion at the summit, which would be of no danger to anyone - except me if I'm in the crater - but the situation needs to be monitored more thoroughly," Wil-liams said.
Scientists are now looking for the two other warning signs of an eruption - deformations in the crust caused by pressure from the magma and tremors on the mountain that looms over the Mexico City valley populated by more than 20 million people.
"These signs are usually observed before an eruption, but they don't always signal that an eruption is imminent. Each volcano is different, and each situation must be interpreted differently," de la Cruz said.
The government of Mexico has donated $300,000 for topographic studies to measure any deformations and for a network of seismic monitors to be installed over the next six months on and around the mountain, said Roberto Quaas of the National Center for Disaster Prevention.
Layers of earth and rock around Popocatepetl reveal that the volcano, whose name in Nahuatl means "mountain that spews smoke," has had a varied history of eruptions, the last deadly one occurring at the start of the 14th century, before the Spanish conquest.
Scientists are also watching other volcanoes in less densely populated parts of Mexico for possible eruptions, de la Cruz said.
"We're giving special attention to Colima and Tacana, which had an explosion in 1986 that opened a 65-foot crater," he said.
Two months ago, 700 people were evacuated from their homes near the Colima volcano in western Mexico after a small explosion destroyed the lava cap formed during the volcano's last eruption in 1991.