BioEd Online

Indonesia's Mount Merapi, which has been belching lava since 4 May, sprang back to life today after a few days of relative calm. Scientists are warning that it is imperative to evacuate many thousands of people who remain in the area.

Gunung Merapi, or Fiery Mountain, lies on the island of Java and is the most active volcano in Indonesia. Scientists at the Merapi Volcano Observatory in Yogyakarta have seen an increase in seismic tremors since August 2005, and although activity declined at the start of this week, lava has continued flowing down the mountain's sides, and clouds of ash now stretch four kilometres from its peak.

Scientists say that a dome of solidified lava perched on the top of the mountain poses a serious danger. This dome has been growing at the relatively rapid rate of about 150,000 cubic metres a day.

Blocks of lava are cracking off its sides and tumbling down the mountainside, breaking apart as they fall. The volcanic activity could die down and these blocks remain small. But there is a possibility, experts say, of a catastrophic collapse of the dome.

This would release hot gasses from inside the rubble, heating the air and releasing clouds of ash. The barrage of rock, dust and heat is known as a pyroclastic flow, and currently presents the greatest threat to life at Merapi, says François Beauducel, director of the Volcanological and Seismological Observatory of Guadeloupe, who has previously studied Merapi.

"The best thing to do is to get everyone out of the way and let the volcano do its thing," adds Jim Luhr, director of the Smithsonian Institution's Global Volcanism Program in Washington DC.

Different kettle of fish

Whereas seismic measurements and deformations in the mountain have successfully forecast the impending threat, the cracking of the lava dome itself is unpredictable.

At other volcanoes, such as Mount Etna in Sicily, Italy, most eruptions are caused by magma seeping up through fissures in the sides of the volcano. Seismometers that measure Earth tremors can map the growth of these fractures inside Etna, so that the eventual eruptions can be predicted very accurately. "It's like shooting fish in a barrel to monitor Etna," says Luhr. "But Merapi is a completely different situation."

Merpai has been intermittently and regularly active over the past century, with about 1,300 people dying in an eruption in 1930. In 1994, a wedding party of about 60 people were killed by the scorching ash and gas of a pyroclastic flow from Merapi. Six years later, Luhr visited the same village with Indonesian vulcanologists to speak with the people still living there, to explain why they should leave the area. The answer from the village elders was that they already had a solution that allowed them to remain: teams of men would run naked around the village at night to keep it safe from the fiery mountain's wrath.

Bridging this gigantic cultural gulf is the biggest problem faced by the Indonesian authorities, says Luhr.

Close watch

John Pallister, who leads the US Geological Survey's Volcano Disaster Assistance Program, returned from Merapi last week, where his team helped to install new digital cameras to monitor the lava dome, and provided advice on the probability of the dome's collapse.

The dome is not yet as large as the one that collapsed in 1994 but it sits in a different position on the summit, so when it will fall "is anybody's guess", he says. Watching closely as the dome changes shape could at least predict which areas are most likely to be affected by a collapse, he adds.

Pallister estimates that there may be 100,000 people remaining in the highest-risk areas, "but it's very hard to get precise figures".

Indonesian vulcanologists advised immediate evacuation of the area on 13 May, but local politicians had been recommending that people leave the area for weeks before that. "The problem is that there's a short tolerance for evacuations there," says Pallister. "People have been sitting in camps for two or three weeks, they've got tired and frustrated, and have started going back."

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