It was this “dome avalanche” that Indonesian volcanologists believe triggered the eruption, according to the country’s geological chief, Eko Budi Lelono, from the Ministry of Energy and Mineral 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 a heavy spell of rain that day,” he said.
The role of rain in this case has raised questions around 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, scientists told CNN.
Saturday’s blowout created what’s known as pyroclastic flow, which are fast-moving clouds of lava, gas and ash. The temperature of these clouds are typically between 800 to 1,000 degrees Celsius, Eko said.
It can move fast — sometimes around 10 kilometers an hour, but up to 100 km/h — and can be impossible to run from.
More than 30 people were killed in the weekend eruption and searchers are looking for dozens more people missing. Thousands of buildings were damaged, many buried under heavy piles of ash that shrouded homes in entire villages.
The force of this eruption was bigger than usual. 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 5km, Eko said.
Millions of Indonesians live near the foot of volcanoes, where the soil is particularly fertile and good for growing crops. More than 8,000 live within 10km of Semeru.
They sometime get warnings that Semeru — one of Indonesia’s most active volcanoes — will erupt as its activity increases, but rain-triggered events like this are harder to predict, Heather Handley, a volcanologist at Monash University in Australia, told CNN.
Continued global warming is projected to bring more extreme rain events to many parts of the world, raising concerns that these larger eruptions could come with little or no notice at all.
Scientists don’t know this is necessarily going to happen, but more have been asking that question since 2018, when Hawaii’s Kīlauea volcano erupted after days of heavy rainfall.
“People have been thinking about the relationships between climate and triggers of volcanic eruptions,” Handley said.
“There’s still a lot to explore that we don’t know yet, so but it’s good to think about other external driving mechanisms of volcanic eruption that we might otherwise have missed.”
Handley explained that there are several ways increased rain and global warming more generally could impact volcanic eruptions.
“If heavy rainfall is making it easier for magma to get to the surface, we could see an increase in the frequency of eruptions that we get,” she said.
“There’s also been a lot of thought around the effects of melting ice and snow, which is often at the top of a volcano. When it melts, it takes the pressure off the top, which can cause even more melting, which can cause more frequent eruptions,” she said.
“But overall, we don’t have a very full understanding of the impacts of climate change on volcanic eruptions.”