Did Heavy Rain Make Hawaii’s Kilauea Volcano Erupt?

The monthslong eruption of Hawaii’s Kilauea volcano in the spring and summer of 2018 was the most destructive of its recorded history. As the summit crater periodically collapsed, 320,000 Olympic-sized swimming pools’ worth of lava was squeezed out of fissures on its eastern flank. Flaming rivers of molten rock destroyed 700 homes while building new volcanic shorelines, vaporizing lakes and conjuring bizarre weather.

What kick-started such a dramatic sequence? A study published Wednesday in Nature suggests an unusual suspect: rainfall.

In the months before the eruption, Hawaii was inundated by above-average precipitation. This rainfall would have found its way into the pores present within Kilauea’s volcanic rocks. If too much water tries to cram into these pores, the rock fragments from the inside-out.

This new model suggests that, just before the outburst of lava in May 2018, the rocks surrounding the volcano’s cache of magma were experiencing a 47-year peak in pore pressure — enough to break down the walls holding its magma in place until the liquefied rock made a break for it.

It’s a bold claim. “Approaching any new theory with a healthy dose of skepticism is important,” said Dr. Farquharson.

“It’s sort of like a kink in the hose,” Dr. Poland said. “Eventually, the plumbing system just backed up.” And throughout, those monitoring the volcano saw no changes explained by rainfall.

And although this rainfall would have certainly percolated underground, the study’s model is too simplistic to assume it made its way down to the magma’s depth.

The subsurface of Kilauea is a byzantine collection of rocky features, including many impermeable walls of frozen magma. Much of the water there doesn’t move downward, but sideways. “The hydrology of Kilauea is incredibly complex,” Dr. Poland said. “It’s not something that can be well approximated by simple models.”

If the rainwater did fracture layers of rock, starting at the surface and reaching rocks near the magma, this should have produced a downwardly propagating wave of seismicity. This pattern was not seen, said Dr. Manga.

Even if the water did find its way to the depth of the magma cache, the peak stresses in the study’s model are too small to make a difference, even to a volcano ready to erupt. Such stresses are weaker than those caused by the gravitational pull of the moon. “If rainfall is going to trigger a change in behavior, why don’t the tides?” Dr. Manga said.

The study also contradicts a historical record of hundreds of volcanoes around the world that have been saturated by rain throughout recorded history. “We haven’t seen any convincing evidence for rainfall being a trigger for an eruption,” said Dr. Krippner.

The study’s authors are aware that their model simplifies the deep architecture of Kilauea, as is the case for any model trying to virtually reconstruct a complex volcanic system. Dr. Farquharson said they were nevertheless “confident that it is able to reproduce the subsurface movement of water in the rift zone reasonably well.” But as sensors were not tracking fluid pressures at depth during the 2018 eruption, this model’s results are difficult to validate.

This eruption — from its dramatic paroxysm in May 2018 to its abrupt decline that August — “highlights how difficult eruption forecasting is,” Dr. Manga said. Volcanoes are not stand-alone sentinels but components of a complex Earth system, he says, and investigating the forces that could be influencing them is a worthwhile endeavor.

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