Indonesians are used to dealing with natural disasters. Their country sits on the Ring of Fire, a band around the Pacific Ocean that sets off frequent earthquakes and volcanic activity. But Mount Semeru’s eruption on the island of Java Saturday was different. Days of heavy rain had gradually destroyed Semeru’s lava dome(熔岩穹丘), a mound of hardened lava, which partially collapsed.
It was this “dome avalanche” that Indonesian volcanologists believe caused the eruption, according to the country’s geological chief, Eko Budi Lelono, from the Ministry of Resources. “Based on photos and data, we can compare the size of the dome before and after the December 4 eruption. We can see a big mass of the dome’s volume was lost after the heavy rain that day,” he said.
A lava dome can be unstable and collapse for several reasons, but there is growing understanding that heavy rain can be one of them. The role of rain in this case has raised questions about whether climate change could bring more frequent eruptions of this kind. That’s a concern, because eruptions caused by lava-dome collapse tend to be stronger and more destructive than other types.
Saturday’s eruption created what’s known as pyroclastic flow(火山碎屑流), which are fast-moving clouds of lava, gas and ash. The temperature of these clouds are typically between 800 to 1,000 degrees Celsius. It can move fast — sometimes around 10 kilometers an hour, but up to 100 km/h — and can be impossible to run from.
The force of this eruption was bigger than usual. Mount Semeru shot ash some 15 kilometers up into the air, when it’s typically just hundreds of meters, and the pyroclastic clouds reached more than 12 kilometers on land, much further than the usual 5 km, Eko said. Thousands of buildings were damaged, with many buried under heavy piles of ash. More than 30 people were killed in the weekend eruption and searchers will try to rescue dozens more people missing.
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