Taal volcano spews ash and debris kilometres into the sky.Credit: Domcar C Lagto/PACIFIC P/SIPA/Shutterstock

Researchers in the Philippines are monitoring the Taal volcano closely for signs of a major eruption. The volcano’s activity has eased since it began spewing steam and ash more than a week ago, but the threat of a large-scale eruption remains, say scientists. In addition to the immediate risk to life, such an event could contaminate water supplies and disrupt power generation for millions of people, and halt ground and air travel.

At 2:30pm local time on 12 January, Taal — which is on an island about 60 kilometres south of the capital Manila — started ejecting lava and blew a giant plume of rock fragments, known as tephra, up to 15 kilometres high. Ash travelled as far north as Quezon City, some 70 kilometres away, forcing tens of thousands of people living on Taal’s Volcano Island and in nearby provinces to evacuate.

The volcano’s activity has stalled, but this does not mean the worst is over, says volcanologist Mariton Bornas, who heads the division responsible for monitoring and predicting eruptions at the Philippine Institute of Volcanology and Seismology (PHIVOLCS), a government agency in Quezon City, just north of Manila.

The volcano remains at level 4, the second-highest level on the country’s volcano-alert system, which means a hazardous eruption could happen in hours or days. Scientists say the threat of a major eruption remains high because PHIVOLCS has reported some 450 volcanic earthquakes in the past 24 hours, as well as the emergence of fissures, and receding of the surrounding lake. The ongoing seismic activity suggests that magma is continuing to rise to the volcano's surface from deep within the Earth, says Bornas.

A hazardous past

Since the sixteenth century, Taal has erupted about 30 times, including four major events. Increased seismic activity, like that seen since 12 January, preceded most of those eruptions, says Perla Reyes, a volcanologist at PHIVOLCS who, with Bornas and others, recently published an assessment1 of Taal’s eruptions. But she says not all cases of increased seismic activity have resulted in a major eruption.

Reyes’s study also reconstructed a major eruption that occurred in 1754, and mapped the extent of the damage. The researchers found that tephra had dispersed as far as Manila.

A repeat of the 1754 eruption would be the worst-case scenario, says Bornas. That event involved almost seven months of activity: the volcano cycled through low-key unrest, escalated to a highly explosive eruption, and then activity lulled before repeating the process. An eruption that lasts that long today would be devastating for the locals displaced from their homes, she says. Some 2 million people live within 35 kilometres of the volcano.

Jacques Zlotnicki, a geophysicist at the French National Centre for Scientific Research (CNRS) in Aubière, says that he does not think the crisis will de-escalate any time soon. “This eruption may last months or more,” says Zlotnicki. His studies suggest that the volcano is due for a large eruption. “Large plumes expelled at [heights of] 10 to 15 kilometres or more could reach Manila within a few hours, depending on the winds,” he says.

The current lull has given many locals a false sense of security, says Reyes, and some of the more than 6,000 people living on Taal Volcano Island have returned to their homes and animals. “Unfortunately, people tend to return in quiet times,” she says. “This is very unsafe.”