Supervolcanoes likely triggered externally, study finds

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Redoubt Volcano on March 31, 2009. View to the east of the summit crater of the volcano, heavily covered with deposits from recent eruptions, many of which were preceded by harmonic tremor. Credit: Game McGimsey

Supervolcanoes, massive eruptions with potential global consequences, appear not to follow the conventional volcano mechanics of internal pressure building until the volcano blows. Instead, a new study finds, such massive magma chambers might erupt when the roof above them cracks or collapses.

Knowledge of triggering mechanisms is crucial for monitoring supervolcano systems, including ones that lie beneath Yellowstone National Park and Long Valley, California, according to the study led by Patricia Gregg, University of Illinois professor of geology, in collaboration with professor Eric Grosfils of Pomona College and professor Shan de Silva of Oregon State University. The study was published in the Journal of Volcanology and Geothermal Research. Gregg also presented the findings this week at the annual meeting of the Geological Society of America.

“If we want to monitor supervolcanoes to determine if one is progressing toward eruption, we need better understanding of what triggers a supereruption,” Gregg said. “It’s very likely that supereruptions must be triggered by an external mechanism and not an internal mechanism, which makes them very different from the typical, smaller volcanoes that we monitor.”

A supervolcano is classed as more than 500 cubic kilometers of erupted magma volume. For comparison, Gregg said, Mount St. Helen’s ejected about one cubic kilometer of material, so a supervolcano is more than five hundred times larger.

“A typical volcano, when it erupts, can have lasting impacts across the globe,” Gregg said. “We’ve seen that in Iceland when we’ve had large ash eruptions that have completely disrupted air traffic across Europe. A supereruption takes that to the nth degree.”

The new study’s findings are contrary to a pair of papers published in the journal Nature Geoscience in 2014 that claim a link between eruption likelihood and magma buoyancy. The magma byouancy hypothesis suggested that magma may be less dense than the rock surrounding it and therefore could push up against the roof, like an ice cube bobbing in water, increasing the pressure within the chamber and triggering an eruption.

“Typically, when we think about how a volcanic eruption is triggered, we are taught that the pressure in the magma chamber increases until it causes an explosion and the volcano erupts,” Gregg said. “This is the prevailing hypothesis for how eruptions are triggered. At supervolcanic sites, however, we don’t see a lot of evidence for pressurization. When I incorporated buoyancy into my numerical models, I couldn’t reproduce the 2014 studies.”

Gregg’s numerical model incorporates all of the physics – conserving mass, energy and momentum – to calculate what would happen if a large buoyant magma body were to form in the shallow crust. The model showed that even when the chamber was huge and the difference in density was very large between the magma and he surrounding rock – an unlikely scenario – buoyancy added very little pressure to the system.

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