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How Earth’s devastating supervolcanoes erupt

Thank goodness Mount Sinabung isn’t a supervolcano. Binsar Bakkara/AP

Devastating supervolcanoes can erupt simply due to changes that happen in their giant magma chambers as they slowly cool, according to a new study. This finding marks the first time researchers have been able to explain the mechanism behind the eruptions of the largest volcanoes on Earth.

Geologists have identified the roots of a number of ancient and possible future supervolcanoes across the globe. No supervolcano has yet exploded in human history, but the rock record demonstrates how devastating any eruption would be to today’s civilisation. Perhaps most famous is the Yellowstone supervolcano in Wyoming, which has erupted three times in the past two million years (the last eruption occurred 600,000 years ago).

These giant volcanic time bombs seem to explode once every few hundred thousand years, and when they do, they throw huge volumes of ash into the sky. At Yellowstone, the eruption that happened two million years ago ejected more than 2,000km3 of material – enough to cover Greater London in a mile-thick layer of ash.

It is estimated that a super-eruption like that would drive a global temperature drop of 10˚C for more than a decade. Such a dramatic change in global climate is difficult to comprehend. Aside from the instant local devastation, there would be global impacts such as crop failures, followed by large famines.

Despite their potential threat, comparable to a large asteroid impact, the mechanisms and origins of super-eruptions have remained obscure. Modestly sized volcanoes operate on different time-scales and magnitudes, and their eruptions appear to be triggered by pulses of molten rock (magma), which increase the pressure in the underground magma chambers that feed their vents.

Two papers recently published in the journal Nature Geoscience try to solve the mystery of how super volcanoes are formed and how they erupt.

Using experiments and computer modelling, scientists have discovered what drives a super-eruption. They find that, over time, the underground magma becomes increasingly more buoyant. Eventually, it becomes a bit like a beach ball held down beneath the waves—when it is released, it shoots into the air, forced up by the dense water around it.

In the first paper, a team led by Wim Malfait and Carmen Sanchez-Valle of ETH Zurich used a synchrotron (an accelerator that can generate intense X-rays) to measure the density, temperature, and pressure of molten rock held in conditions resembling those of a magma chamber several kilometres below the surface. This required them to mimic deep Earth conditions in the lab at the European Synchrotron Radiation Facility, holding samples at temperatures up to 1,700˚C and the pressure of 36,000 atmospheres.

An artist’s impression showing the magma chamber of a supervolcano with partially molten magma at the top. The pressure from its buoyancy is sufficient to punch through 10km or more of the Earth’s crust above it. ESRF/Nigel Hawtin

To feed a supervolcano you need a huge magma chamber. The Zurich team’s results show that, as the magma chamber cools, it begins to solidify and crystals grow in it that are denser than the magma. As these fall to the base of the chamber, the remaining molten rock gets progressively less dense. If there is enough of it, their measurements indicate that the magma eventually becomes light enough that it can force its way through more than 10km of Earth’s overlying crust.

Co-author Carmen Sanchez-Valle, also at ETH Zurich, said: “Our research has shown that the pressure is actually large enough for the Earth’s crust to break. As it rises to the surface, the magma will expand violently, which is a well known origin of a volcanic explosion”.

The second paper by Luca Caricchi and colleagues at the University of Bristol, describes computer simulations of the same processes, finding that the buoyancy of melt in maturing magma chambers is also key to these huge events.

This contrasts with the way that more familiar smaller volcanoes erupt. There, blasts follow directly from rapid injections of magma, or from external events that release the pressure on it, such as earthquakes or even the melting of overlying glaciers, as seen in Iceland recently.

The results indicate that supervolcanoes just require a steady accumulation of molten rock that remains hot enough that it does not completely solidify—a massive eruption is then simply a matter of time. Thus, the eruption of massive supervolcanoes seems to be an inevitable part of their “life cycle”. Just as a sufficiently large star will necessarily generate a supernova, so a huge magma chamber should eventually become a massive eruption.

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14 Comments sorted by

  1. Fred Pribac

    logged in via email @internode.on.net

    Great explanations - thanks!

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  2. Craig Read

    logged in via Twitter

    Great explanation Simon.

    I did find the title a little confusing though. The article addresses how an eruption of the Yellowstone caldera might occur, but (as I understand it) there is zero doubt that it will occur. It's just a question of when.

    Given that the last eruption happened 600,000 years ago and they occur every few hundred thousand years, what are the odds of it happening within our (or our children's) lifetimes?

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    1. Simon Redfern

      Professor in Earth Sciences at University of Cambridge

      In reply to Craig Read

      Good point Craig, I think I'll edit out the "might", which refers to the 'how' more than the 'when'. The latest news from Yellowstone observations is that it doesn't pose an immediate threat, I understand. There are probably more higher-risk man-made threats that we should worry about first!

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    2. Craig Read

      logged in via Twitter

      In reply to Simon Redfern

      Thanks Simon.

      Ars Technica also covered this last night (pretty much word-for-word), and the question of drilling to relieve pressure came up in the comments. Somebody replied stating that even if we could drill down through 10km of rock, it'd be like relieving the pressure of a full water balloon with a pin.

      As you said: there are other man-made threats that we can do something about. I guess it's a case of some extinction events we can avoid and some we can't.

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  3. Henry Verberne

    Former IT Professional

    A question for Dr Redfern: given the probability that one day an eruption of a super volcano such as Yellowstone will occur- with devastating consequences for perhaps much of North America, how practical would it be to "relieve the pressure" if instrument data indicates it is building?

    Given the potential for catastrophic impact such an endeavour seems worthwhile. I suspect that we may not have the capability, at least for the foreseeable future?

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    1. Simon Redfern

      Professor in Earth Sciences at University of Cambridge

      In reply to Henry Verberne

      An interesting question, Henry. The roof of the Yellowstone magma chamber is around 8km below the surface - the deepest bore hole drilled anywhere was in Russia and went down more than 12km, so we have the capability. But this in itself would be a hugely risky venture, and it is difficult to predict exactly what would happen - could this in itself trigger an eruption by weakening the roof? We don't know what the pressure in the chamber is right now. The hazard is high, but the risk is low. Alongside the potential for dabbling in something and making it worse, and given that nothing is likely to happen on the time scale of our life times, much less on the time scale of a government's life time, my guess is that this isn't a high priority. The destruction of LA, on the other hand, is much more likely and imminent, for example ... http://www.bbc.co.uk/news/science-environment-25317823

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    1. Henry Verberne

      Former IT Professional

      In reply to Charles Powell

      The definition of "super-volcano" is not supplied in the article but given the global impact when it exploded in 1816 I would think it fits the implied definition.

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    2. Simon Redfern

      Professor in Earth Sciences at University of Cambridge

      In reply to Charles Powell

      Great article, thanks Charles. It shows how Tambora erupted something in the range of 100s of cubic kilometers of ash, whereas supervolcanoes like that at Yellowstone would erupt an order of magnitude more ash, in the range of 1000s of cubic kilometers. There is a Volcanic Explosivity Index which is somewhat akin to the Richter scale for earthquakes, and Tambora is put at a VEI of 7. Supervolcanoes are those eruptions of VEI of 8 or more.

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  4. David Arthur

    resistance gnome

    Thanks for this simply-expressed explanation that even we lay folk can comprehend.

    Could an analogy be drawn with bubbles forming then rising through a dense medium to burst at the medium's surface?

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  5. John Doyle
    John Doyle is a Friend of The Conversation.

    architect

    I only just noticed this article [there are an awful lot of articles in The Conversation now]
    Yellowstone is famous and it took a long time to discover it because the crater is so vast that no one could see it as a crater.
    Easier to see is Toba, a recent event even bigger than the famous Yellowstone eruption. It occurred about 86,000 years ago, well within human existence. It threatened the very survival of humans in Asia. Lake Toba is over 100 km long. Apparently it all went off in about 1 week. Unimaginable!

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