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Earth sciences

Hawaiian volcano erupted ‘like a stomp rocket’

20 Jun 2024 Isabelle Dumé
The new 'stomp rocket’ mechanism
Eruption: A sketch of the Kilauea magma chamber and the mechanism proposed for the volcano's eruptions in 2018. (Courtesy: Ben Holtzman/Eos)

A series of eruptions at the Hawaiian volcano Kilauea in 2018 may have been driven by a hitherto undescribed mechanism that resembles the “stomp-rocket” toys popular in science demonstrations. While these eruptions are the first in which scientists have identified such a mechanism, researchers at the University of Oregon, US, and the US Geological Survey say it may also occur in other so-called caldera collapse eruptions.

Volcanic eruptions usually fall into one of two main categories. The first is magmatic eruptions, which (as their name implies) are driven by rising magma. The second is phreatic eruptions, which are prompted by ground water flash-evaporating into steam. But the sequence of 12 closely-timed Kilauea eruptions didn’t match either of these categories. According to geophysicist Joshua Crozier, who led a recent study of the eruptions, these eruptions instead appear to have been triggered by a collapse of Kilauea’s subsurface magma reservoir, which contained a pocket of gas and debris as well as molten rock.

When this kilometre-thick chunk of rock dropped, Crozier explains that the pressure of the gas in the pocket suddenly increased. And just like stamping on the gas-filled cavity in a stomp rocket causes a little plastic rocket to shoot upwards, the increase in gas pressure within Kilauea blasted plumes of rock fragments and hot gas eight metres into the air, leaving behind a collapsed region of crustal rock known as a caldera.

A common occurrence?

Caldera collapses are fairly common, with multiple occurrences around the world in the past few decades, Crozier says. This means the stomp-rocket mechanism might be behind other volcanic eruptions, too. Indeed, previous studies had hinted at this possibility. “Several key factors led us to speculate along the line of the stomp-rocket, one being that the material erupted during the Kilauea events was largely lithic clasts [broken bits of crustal rock or cooled lava] rather than ‘fresh’ molten magma as occurs in typical magmatic eruptions,” Crozier tells Physics World.

This lack of fresh magma might imply phreatic activity, as was invoked for previous explosive eruptions at Kilauea in 1924. However, in 2018, USGS scientists Paul Hsieh and Steve Ingebritsen used groundwater simulations to show that the rocks around Kilauea’s summit vent should have been too hot for liquid groundwater to flow in at the time the explosions occurred. Seismic, geodetic, and infrasound data also all suggested that the summit region was experiencing early stages of caldera collapse during this time.

First test of the stomp-rocket idea

The new work is based on three-dimensional simulations of how plumes containing different types of matter rise through a conduit and enter the atmosphere. Crozier and colleagues compared these simulations with seismic and infrasound data from previously-published papers, and with plume heights measured by radar. They then connected the plume simulations with seismic inversions they conducted themselves.

The resulting model shows Kilauea’s magma reservoir overlain by a pocket of accumulated high-temperature magmatic gas and lithic clasts. When the reservoir collapsed, the gas and the lithic clasts were driven up through a conduit around 600-m long to erupt particles at a rate of roughly 3000 m3/s.

As well as outlining a new mechanism that could contribute to hazards during caldera collapse eruptions, Crozier and colleagues used subsurface and atmospheric data to constrain Kilauea’s eruption mechanics in more detail than is typically possible. They were able to do this, Crozier says, because Kilauea is unusually well-monitored, being covered with instruments such as ground sensors to detect seismic activity and spectrometers to analyze the gases released.

“Our work provides a valuable opportunity to validate next-generation transient eruptive plume simulations, which could ultimately help improve both ash hazard forecasts and interpretations of the existing geologic eruption record,” says Crozier, who is now a postdoctoral researcher at Stanford University in the US. “For example, I am currently looking into the fault mechanics involved in the sequence of caldera collapse earthquakes that produced these explosions. In most tectonic settings we haven’t been able to observe complete earthquake cycles since they occur over long timescales, so caldera collapses provide valuable opportunities to understand fault mechanics.”

The study is detailed in Nature Geoscience.

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