An active volcano in Ecuador has collapsed twice in the past 250,000 years, causing vast debris avalanches that reached over 60 kilometres away. It could happen again, but it isn’t possible to predict when – although there is no reason to think it is imminent.

Sangay is Ecuador’s most active volcano. It lies on the eastern edge of the Andes mountains, overlooking the western edge of the Amazon rainforest.

“It’s a volcano that’s in the jungle,” says Viviana Valverde of the Geophysical Institute in Quito, Ecuador. As a result of its remote location, for decades it wasn’t regarded as a major risk, but she says that is now changing.

Since the late 1990s it has been known that “the volcano has two scars”, says Valverde. These suggested that the eastern mountainside had collapsed twice within the past 250,000 years. But the extent of the resulting debris avalanches of volcanic rocks wasn’t known.

Valverde and her colleagues investigated 541 rocky hummocks to the east of Sangay, and found that many of them contained rocks from the volcano. These were left behind by at least two debris avalanches that took place when part of the volcano collapsed. The debris avalanches reached at least 60 kilometres from the volcano’s crater – much further than such debris avalanches were thought to be able to travel from Sangay. Several towns lie within the debris avalanche zone.

Most volcanoes experience occasional flank collapses, but the debris avalanches don’t usually travel so far. Valverde says Sangay’s height is key – it rises 4000 metres above the surrounding landscape. “And that’s something unique,” she says.

The earlier debris avalanche cannot be precisely dated, but based on the ages of the volcano’s rocks the team estimates it happened between 250,000 and 100,000 years ago. The second debris avalanche happened around 30,000 years ago, based on the carbon dating of a woody branch buried among the rocks.

Sangay remains highly active, so Valverde says a third collapse and debris avalanche is possible – but there is no way to say when. “We say it might happen, but it can mean a few years or like a hundred years,” she says. It is difficult enough to predict major volcanic eruptions, but a further complication is that the debris avalanches may have been partly triggered by earthquakes, which are also hard to predict.

Journal reference: Journal of Volcanology and Geothermal Research, DOI: 10.1016/j.jvolgeores.2021.107172