In 2018, Hawaii’s Kilauea volcano erupted like a stomp rocket

A series of explosions from the Hawaiian volcano Kilauea in 2018 may have been triggered by a never-before-seen style of eruption — one that’s reminiscent of a stomp rocket toy.

In May of that year, plumes of hot gas and rock blasted up to eight kilometers into the sky as the volcano erupted explosively 12 times in succession. The progressive collapse of Kilauea’s summit crater, or caldera, triggered those explosive eruptions, researchers reported May 27 in Nature Geoscience.

Each time large chunks of crater rock plunged into the magma chamber beneath, the sudden compression of air in the chamber sent the volcanic debris shooting skyward, the team says — much like the way stepping hard on the air bladder of a stomp rocket sends its foam projectile flying.

Explosive volcanic eruptions are usually triggered by some combination of two well-known mechanisms, says Joshua Crozier, a geophysicist at Stanford University. Depressurization of hot magma as it ascends releases bubbles of gas that can expand to burst molten rock out of the caldera. Alternatively, a rising magma plume can flash-heat groundwater circulating in the encasing rocks, sending bursts of steam and broken bits of rock shooting skyward.

But neither of those mechanisms seemed to explain what happened at Kilauea from May 16 to May 27 in 2018. Geophysical data collected near the summit of the volcano throughout its 2018 eruption indicated that the strange, repetitive sequence of explosive eruptions couldn’t have been generated by either of the above mechanisms, Crozier says.

For one thing, Crozier says, the erupted material didn’t contain bubbly bits of magma, as might be expected in the first scenario. For another, the rocks in the caldera were already far too hot to contain much liquid water that could then be superheated, eliminating the second scenario.

But Crozier and others suspected that the series of collapses of the volcano’s caldera, beginning in mid-May of that year, might have had something to do with it (SN: 1/29/19). To assess this hypothesis, the team analyzed the abundant geophysical data that is constantly collected at Kilauea.

The volcano is one of the most extensively instrumented in the world. Networks of seismometers keep close watch on its inner workings, while GPS-armed tiltmeters installed near the summit detect subtle changes in the movement and slope of the ground, tracking shifts in strain due to moving magma. The Hawaii Volcano Observatory also has a network of infrasound arrays: low-frequency microphones that measure changes in atmospheric pressure caused by, for example, explosions.

Changes to the frequency of the infrasound waves traveling through the ground revealed a distinct pattern during this short time period: The chamber seemed to enlarge, and then there was an explosion of some sort. The seismic data, meanwhile, showed a series of distinct earthquakes, corresponding to these events, each less than magnitude 5.

What was probably happening, the researchers say, is that the magma chamber drained enough to make the caldera roof over it unstable, causing that rock to drop downward under its own weight. That decreased the volume of the reservoir — like compressing a stomp rocket’s air bladder. About 10 to 30 seconds later, cameras observed eruptive plumes emerging from the summit — the result of air pressurization from the collapsing roof shooting the hot gas and rock debris in the chamber upward.

“This is the first time to my knowledge that such a mechanism has been suggested to drive eruptions,” says Larry Mastin, a volcanologist at the U.S. Geological Survey’s Cascades Volcano Observatory in Vancouver, Wash., who was not part of the new study. “It’s a rather unusual mechanism, but the circumstances of this eruption are unusual. And we had unusually good observations … [that] were very useful in helping narrow down the cause.”

Mastin notes the stomp rocket mechanism was at play only in the very early stages of Kilauea’s caldera collapse, “when the collapse was basically just the roof falling in right above the magma body.” Over time, as the caldera floor’s collapse radiated outward, the tightly focused stomp rocket compression was no longer at play at Kilauea. Eruptions at the summit, meanwhile, largely ended as the vent in the central caldera became clogged with material.

Stomp rocket–style eruptions probably aren’t unique to Kilauea, Crozier says. But, he says, the volcano’s extensive monitoring system made it possible to detect and characterize the new phenomenon. And in turn, knowing how to connect the seismic and infrasound data can help with hazard mitigation from other, less well instrumented volcanos, he says.

“In many cases, the first sign we have of an eruption is a seismic or infrasound signal. So if we can get better at relating those types of geophysical data to what the eruptive plume is doing, the better we can calibrate our models,” he says. That would reduce hazards to aviation as well as communities.