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Hawaii’s Kilauea volcano erupted on 3 May 2018, leading to a massive outpouring of lava that devastated the southeastern part of the island.

USGS Hawaiian Volcano Observatory

Did heavy rains trigger the eruption of the most dangerous U.S. volcano? Scientists are skeptical

In May 2018, Hawaii’s Kilauea volcano let loose its largest eruption in 200 years, spewing plumes of ash high into the air, and covering hundreds of homes in lava. The eruption terrified local residents, but it gave scientists a once-in-a-lifetime opportunity to study the volcano’s explosive behavior. Now, a new study claims that extreme rainfall boosted underground pressures and was the “dominant factor” in triggering the eruption.

It’s not the first time rainfall has been linked to volcanic activity, says Jenni Barclay, a volcanologist at the University of East Anglia who was not involved in the new work. Previous research suggests storms passing over Mount St. Helens may have played a role in explosive activity between 1989 and 1991. And intense rains fell shortly before and during the activity of Montserrat’s Soufrière Hills volcano from 2001 to 2003. Rain may have also triggered eruptions of Réunion’s Piton de la Fournaise volcano. Still, Barclay believes rain is, at best, a contributing factor to volcanic eruptions and not the main driver. “It’s a series of coincident events that have led to the triggering of this larger episode,” she says.

Researchers on the new study used satellite data from NASA and Japan’s space agency to estimate rainfall during the first months of 2018, before the start of the eruption. More than 2.25 meters of rain fell on the volcano in the first months of 2018, the researchers found. They created a model to show how the accumulated rainfall could seep into the pore spaces in rocks deep underground, boosting pressures that eventually caused fissures in the volcano’s flank to open up and release magma. When they looked at records of previous Kilauea eruptions going back to 1790, they found that 35—more than half—started during the nearly 6-month rainy season.

“The more we looked at the data sets, the more things started pointing in the same direction,” says Jamie Farquharson, a volcanologist at the University of Miami. He and his colleagues publish their results today in Nature.

Other researchers are not so convinced. Michael Poland, a volcanologist at the U.S. Geological Survey’s Yellowstone Volcano Observatory, says factors other than rainfall were far more important in building up pressures. In November 2017, less lava started to come out of the overall Kilauea system, which has been erupting continuously since 1983. That slowdown, he says, was a “kinking on the hose” that caused pressures to build beneath the volcano. GPS sensors on the volcano summit captured a rise of a few centimeters in the weeks leading up to the eruption. Poland says this inflation is unrelated to rainfall and shows that pressures were rising “to pretty extreme levels” in the magma chambers beneath the volcano.

Michael Manga, a geoscientist at the University of California, Berkeley, is also skeptical. “The premise behind their hypothesis is not completely unreasonable,” he says, given that changes in pressure—even because of snowmelt—can cause small earthquakes. But pressure changes from rainfall would be so small that they wouldn’t have made much difference. “They’re smaller than the stresses from tides from the Moon,” he says.

Manga worries about the implications for residents living nearby if these conclusions don’t hold up. “Real people live near volcanoes,” he says. “Do we want an observatory to increase the alert level after heavy rain?” Probably not, he says.

Farquharson doesn’t want to be an alarmist. “We were not trying to say that every time there’s a bit of rain in Hawaii, volcanoes are going to go.” However, he thinks rainfall should be monitored at volcanoes, as it’s relatively cheap and easy.

“I think that the more evidence we put forward for it, the more and more people will come around to the fact that this could be something.”

*Correction, 23 April, 11:45 a.m.: This article originally misstated that 1.26 meters of rain fell on Hawaii’s Kilauea volcano in April of 2018.