Explaining supervolcanoes: big, hot and dangerous

http://www.theguardian.com/world/2014/jan/09/supervolcanoes-yellowstone-bill-mcguire

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Scientists have revealed that the enormous magma chamber under Yellowstone National Park is more than twice the size previously thought, while papers just published suggest catastrophic eruptions may need no external triggers. But can we predict when these sleeping behemoths will wake? We ask volcanologist and author of Waking the Giant: How a Changing Climate Triggers Earthquakes, Tsunamis and Volcanoes, Professor Bill McGuire of University College London, to explain.

What makes a supervolcano?

Volcanic eruptions come in all different sizes, and are measured using the "volcano explosivity index". The key is how much material is ejected. A super-eruption, or VEI-8, ejects at least 1,000 cubic km of ash and debris. That's enough to bury greater London to a depth of 1km.

How bad can a supervolcano be?

One of the biggest supereruptions that we know about, Toba in Sumatra, occurred about 74,000 years ago. That left behind the biggest volcanic crater on the planet, about 100km across. It pumped so much sulphurous gas into the atmosphere that it triggered a volcanic winter. If one of these happened today and we had global harvest failure for two, three, four years, then you are potentially looking at billions of deaths.

What is the biggest supervolcano mystery puzzling scientists?

How the magma is stored – hot – at these huge volumes. With normal volcanoes you get magma accumulating in the volcano and it reaches a particular volume and comes out. With supervolcanoes, you need to keep down more than 1,000 cubic km of hot magma and allow it all to come out all in one go. The question is, how is it kept there until it comes out in one massive great surge.

Is it possible to defuse a supervolcano by drilling?

I've always said it would be like sticking a drawing pin in an elephant's bottom. The energy, the volumes and the heat involved are so tremendous. If we did have a concerted effort to allow them to "let off steam", either it would do nothing or, if it did do something, it's more likely to actually bring forward the timing of the eruption.

Can we predict a super-eruption?

There are always warning signs before an eruption because magma, as it approaches a surface, has to make space for itself. That results in swelling of the surface and, because it has to break rock to get to the surface, that generates earthquakes. But with super-eruptions there's a possibility that, as that huge body of magma expands, it might open up a fracture through the crust very suddenly. So we might see swelling and a bit of earthquake activity but instead of seeing this accelerating and getting worse before an eruption, the eruption could happen much earlier than we expect. We have no idea if that eruption would be big, small or medium sized.

Are there any signs of a super-eruption brewing?

There is a volcano in Bolivia called Uturuncu that has had super-eruptions in the past and has been swelling continuously since 1992. The swelling could die again, or it could build to an eruption in a few years or decades, or do nothing for millennia. We just don't know.

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