Supervolcanoes likely are triggered externally
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.
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.
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.
Gregg's numerical model inco - 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.
The new study found that the size of the magma chamber is a much greater factor in generating supervolcanic eruptions. The buoyancy studies suggested that this correlation was due to having more material pushing up, but the Illinois-led study found that the size of the chamber affects the stability of the rock containing the chamber.
According to the model, if a crack or fault in the roof penetrates the magma chamber, the magma uses the crack as a vent to shoot to the surface. This could trigger a chain reaction that "unzips" the whole supervolcano.
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