Even when physically standing in the crater of a supervolcano, such as Yellowstone National Park, Montana, it can be hard to grasp exactly how much larger they are than regular volcanoes. Patricia Gregg, assistant professor of geophysics at the University of Illinois, points out that while the famous 1980 eruption of Mount St Helens, Washington, produced one cubic km of erupted material, a supervolcano eruption produces at least 500 times more than that. ‘A Yellowstone-sized super-eruption would be 2,500 cubic km, that’s 2,500 Mount St. Helens,’ she emphasises. ‘It’s just a completely different order of magnitude.’

However, supervolcanoes are not simply larger volcanoes. They also have a significantly different eruption triggering mechanism to regular volcanoes, which are principally caused by pressure from a build-up of magma below the surface. While supervolcano eruptions are very rare even in geological terms, evidence suggests that when they do erupt, it can happen over just a few weeks, or even days.

“If the trigger is external, what we want to do is look at seismicity patterns. That’s going to be a really important indication”

A 2014 study suggested that supervolcano eruptions are caused by the effect of buoyancy, with relatively less-dense sub-surface magma forcing its way up through the denser surrounding rock. However, a recent study, led by Gregg, found little evidence to support this theory. ‘Our goal with this paper was to test some of these previous assumptions that you could generate overpressure due to buoyancy,’ says Gregg. ‘We had read these papers that basically said, “We’ve solved the problem, it’s done”. But I could not reproduce their results at all. I could not find any evidence for eruption triggering due to buoyancy.’

Instead, she suggests that supervolcano eruptions are triggered by external factors, monitoring of which would therefore require looking for entirely different signals. ‘There are a couple of supervolcanic systems – Yellowstone and Toba [Sumatra], being the really obvious ones – where we know there’s magma accumulating,’ she continues. ‘If it’s buoyancy that triggers it, then we should be very concerned when we see a large magma body accumulating. But if the trigger is external, what we want to do is look at seismicity patterns, generation of new faults, reactivation of old faults, basically the structural stability of the region, because that’s going to be a really important indication.’

Gregg suggests that by having a better understanding of what triggers eruptions, vulcanologists can be more targeted in how they are monitoring these systems.

This article was published in the January 2016 edition of Geographical magazine.