Kīlauea got wet, then went off

The dramatic eruption of Hawaii’s Kīlauea Volcano in 2018 was likely activated by extreme rainfall, according to new analysis outlined in the journal Nature. 

Jamie Farquharson and Falk Amelung from the University of Miami, US, suggest that several months of anomalously high precipitation infiltrated the volcano’s subsurface, increasing pore pressure to the highest in nearly 50 years.

This weakened the volcano’s structure, allowing magma to intrude, resulting in the eruption.

Perhaps not surprisingly, the pair also suggest that improving our understanding of the relationship between rainfall and volcanic eruptions might help us forecast future rainfall-induced activity. 

“We knew that changes in the water content in the Earth’s subsurface can trigger earthquakes and landslides. Now we know that it can also trigger volcanic eruptions,” says Amelung. “Under pressure from magma, wet rock breaks easier than dry rock. It is as simple as that.”

Kīlauea’s eruption was complex and multi-stage, involving the collapse of the summit caldera, but its trigger has been unknown.

It reached a peak on 3 May, throwing incandescent lava more than 60 metres into the air and over 33 square kilometres of Hawaii’s Big Island, destroying hundreds of homes. It ceased in September.

Using a combination of ground-based and satellite measurements of rainfall, Farquharson and Amelung modelled the fluid pressure within the volcano’s edifice over time – a factor that can directly influence the tendency for mechanical failure in the subsurface.

They propose that very high fluid pressure facilitated magma movement beneath the volcano and also explains why there was no widespread uplift in the months prior.

“An eruption happens when the pressure in the magma chamber is high enough to break the surrounding rock and the magma travels to the surface,” says Amelung. 

“This pressurisation causes inflation of the ground by tens of centimetres. As we did not see any significant inflation in the year prior to the eruption we started to think about alternative explanations.”

While small steam explosions and volcanic earthquakes have been linked to rainfall infiltration at other volcanoes in the past, this is the first time that this mechanism has been invoked to explain deeper magmatic processes.

“Interestingly, when we investigate Kīlauea’s historical eruption record, we see that magmatic intrusions and recorded eruptions are almost twice as likely to occur during the wettest parts of the year,” says Farquharson.