What Actually Causes A Supervolcanic Eruption?

Yellowstone National Park conceals a supervolcanic magma chamber. Nina B/Shutterstock
Robin Andrews 05 Nov 2015, 17:06

There’s a worrying amount of scientific data being released at the moment regarding supervolcanic eruptions: In particular, one study shows that they only take 500 years to go from “recharging” their vast magma chambers to violently erupting. The reason that these gargantuan volcanoes erupt was thought to be due to the ability of the magma to rise within the chamber, but a new study – published in the Journal of Volcanology and Geothermal Research – disagrees, claiming that the magma plays second fiddle to the surrounding strength of the rock itself.

The mechanisms behind each supervolcanic blast were previously thought to be relatively simple: Once its enormous magma chamber is full to bursting point, the enormous internal pressure overcomes the mechanical strength of the surrounding rock, causing it to crack open and release its contents in a cataclysmic depressurization event.

However, things are not so simple – for one, the magma chamber spends most of its life as a “crystal mush,” not a molten liquid, which requires re-melting by the addition of new gas-filled magma from below. In addition to this, previous studies have shown that it is the ability of the magma to migrate up through this mush to the surface of the chamber – a measure of its buoyancy – that appears to determine whether or not an eruption will occur.

Magma rises upward when it is less dense than the surrounding magma: This can be achieved by heating the magma up, or by introducing more gas into it, both achievable through a deeper magma injection. This magma bobbing up at the top of the chamber – like an ice cube in a glass of water – will increase the pressure on the roof of the chamber, eventually causing it to burst.

This new study suggests that it is actually external influences – factors outside of the magma chamber – that control when the eruption will occur, rather than internal processes. Specifically, the roof of the chamber has to become structurally weak before the magma chamber can violently depressurize, according to this new study. Regional tectonic activity – including an earthquake – or the slow, continued expansion of the magma chamber itself could trigger a mechanical failure of the roof. The actual buoyancy of the magma makes little difference, according to Patricia Gregg, the author of the study.

The researchers used computational models in an attempt to calculate the various physical characteristics of supervolcanic magma chambers. They looked at previous studies focused on calculating the upward buoyant force of the magma on the roof of the chamber, comparing their new model to these older ones.

“What they miss in the buoyancy model is [that] the magma may push up, but the roof pushes back down,” Gregg said in a statement. These authors argue that the magma buoyancy makes little difference, even in a huge chamber with a large density change, which would permit a large migration of magma upwards towards the roof.

Not everyone agrees with the findings of this paper. Professor Kathy Cashman, a volcanologist at Bristol University not involved in the study, found the modeling “has some very odd results,” telling IFLScience that the external (roof structure) and internal (magma dynamics) processes involved in causing a caldera to rupture and erupt are not mutually exclusive – meaning that both are involved in causing an eruption.

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