New mechanism for volcanic eruptions discovered – Knowledge

A ramming rocket works according to a simple principle: if you step on an air cushion, the air shoots through a tube, at the end of which is a rocket. This rocket is shot into the air by the pressure wave. The principle of this children’s toy can also be applied to volcanoes, for example to Kīlauea in the southeast of Hawaii’s largest island.

The shield volcano is actually a tourist attraction. It is one of the most active volcanoes in the world, but its lava usually flows out very slowly. For this reason and because of its good accessibility, it is sometimes referred to as “The only drive-in volcano in the world” designated.

But in May 2018, Kīlauea showed a different face: After almost a hundred years, a major explosive eruption event occurred again – even a whole series: ash clouds piled up to eight kilometers high, blew away and rained ash down on settlements; lava fountains shot up to 90 meters into the sky and lava bombs struck the surrounding area – accompanied by the strongest earthquakes in almost 50 years.

An explosion at Kilauea in May 2018. (Photo: University of Oregon)

In addition, two dozen fissures opened up on the volcano’s eastern fault zone, from which large quantities of lava bubbled, rolled over streets and houses and finally flowed steaming into the ocean. 2,000 people had to be brought to safety. No volcano in the USA had caused more destruction since the eruption of Mount St. Helen in 1980.

However, the eruption is likely to find its way into the textbooks for another reason: it simply did not fit the pattern of known explosive eruptions. These occur either because molten magma rises or because groundwater evaporates explosively and shatters crustal rock. That is the current opinion. However, neither of these can explain the eruption of Kīlauea in 2018. The material that the volcano spewed out contained hardly any fresh magma. And according to simulations, the rock around the vent was too hot to allow groundwater to penetrate.

Geoscientists from the USA and China have now in the journal NatureGeoscience a third category of explosive eruptions was modelled: the “rocket stomping mechanism”. According to this, a magma reservoir up to two kilometres deep was emptied and lava erupted 40 kilometres away from the eastern fracture zone of the volcano. As a result, the kilometer-thick roof of the volcano sank by up to half a kilometre over months and the crater grew sevenfold.

But already at the beginning of this process in May, the gradual collapse of the caldera created such pressure on the remaining magma reservoir and the gas bubble above it that the magmatic gases shot up an explosive vent in Kīlauea’s crater, carrying rock with them. This took less than half a minute per eruption event. The comparison with the stomping rocket is “a useful analogy to explain how the ash-rich explosions were generated during the 2018 eruption,” says seismologist David Shelly of the US Geological Survey, who was not involved in the study.

The scientists were able to measure the collapse of the volcano and the eruption events in more detail than ever before. This was made possible by the accessibility and the network of instruments. Thanks to the dozen eruption events, the researchers were able to collect large amounts of radar, infrasound and seismic data and thus simulate the rise of carbon dioxide and water vapor along with the basalt rock that was carried along with it. “This was the first time that such a rocket-like propulsion mechanism for an eruption had been investigated,” says lead author Josh Crozier from the California Volcanological Station of the US Geological Survey USGS.

Crozier and his colleagues assume that this eruption mechanism is not that rare; it just has not been discovered as such yet. Candidates include previous explosive eruptions such as that of Pinatubo in the Philippines in 1991, whose summit collapsed by more than 250 meters during the eruption, resulting in the formation of two calderas. Or that of Kīlauea in 1924. “Eruptions of collapsing volcanoes are relatively common,” says geologist Crozier. “We now know that they can lead directly to explosive eruptions.”

Geologist Christoph Helo from the Johannes Gutenberg University in Mainz describes the study as “very interesting”. “It will help us to better understand the fundamental dangers of explosive volcanic eruptions,” he says. “And that is a prerequisite for being able to make better predictions and advance warnings in the future.” Josh Crozier recommends always thinking about using a ramming rocket in the event of an impending caldera collapse.

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