What are the chances of a devastating supervolcano in Yellowstone?

Tuesday 6 January 2009 14.23 GMT

Intense seismic activity in Yellowstone National Park in the last two weeks has led to speculation that a cataclysmic eruption is imminent. Bill McGuire of the Hazard Research Centre at University College London assesses the evidence.

Even the smallest supervolcano would produce enough ash to bury the entire UK to a depth of 1 metre.

Imagine a volcanic blast so devastating it obliterates at a stroke the English county of Yorkshire or the US state of Connecticut; a detonation so titanic it buries a continent in ash and plunges the entire world into a bitter volcanic winter. Volcanic super-eruptions or supervolcanoes may sound like the stuff of disaster movies, but they have happened many times before and scientists monitoring the giant Yellowstone volcanic system in the US state of Wyoming are beginning to wonder if the next one will happen on their patch.

The Yellowstone volcano is probably most famous for its spectacular Old Faithful geyser, which shoots a column of boiling water to half the height of Big Ben every 90 minutes or so. This, and 10,000 other hot springs, geysers and bubbling mud pots are testament to the enormous heat generated here from molten magma stored deep within the crust. On three occasions over the past two million years, prodigious volumes of magma have blasted their way to the surface, leaving behind three enormous volcanic craters, or calderas, which coalesced to form a vast depression 75km across.

Like other volcanic caldera systems, such as Long Valley in California and Campi Flegrei in Italy's Bay of Naples, the Yellowstone caldera is 'restless'. In other words, the ground surface is constantly rising and falling, while the region is frequently shaken by swarms of small earthquakes. Over the past two weeks, however, the caldera has become increasingly agitated, inviting talk of an imminent and cataclysmic eruption.

What are the chances of another super-eruption at Yellowstone in 2009? Certainly, the more than 500 small earthquakes recorded since 27 December are unusual and the seismic activity is now more intense than anything recorded in the last few decades. On top of this, the ground surface has swelled more than 20cm since 2004: a rate three times faster than other episodes of uplift measured since monitoring began in 1923.

Against this, however, super-eruptions at Yellowstone happen every 600,000 to 800,000 years, meaning that the probability of such an event happening in any single year is extremely small. In fact, eruptions of any size at all are rare, and the last time magma breached the surface was a good 70,000 years ago.

The current bout of twitchiness at Yellowstone may be the result of new magma breaking rock as it heads towards the surface, but there are other possible explanations too. The quakes may be related to movements along a fault due to the simple relief of stresses accumulated in the crust, or caused by rock fracturing associated with the subterranean movement of super-heated water.

In fact, explosions of boiling water could well present the main threat, and looking back over the past 15,000 years, such hydrothermal explosions have punched their way through to the surface on more than 20 occasions, leaving behind impressive craters that are now largely water-filled. Around 13,000 years ago, a huge steam blast opened up a hole as wide as inner London, an event that would certainly spoil the day of any hikers in the vicinity should it happen today.

Even if it does turn out that the recent unrest is due to the movement of a new batch of magma, the chances are that its volume is small and that it will cool and solidify before it reaches the surface. Then again, we can never be absolutely certain. The problem is that we don't know exactly what signs to expect before a super-eruption, because – fortunately for all of us – they happen on average only every 50,000 years or so, with the last one battering much of New Zealand's North Island 26,000 years ago.

As before all eruptions, we would intuitively expect the ground to swell as new magma made space for itself. Swarms of earthquakes would also be anticipated as the magma broke rock to open a path to the surface.

Both phenomena are now being seen at Yellowstone, though not on any great scale. With a thousand cubic kilometres of magma involved in even the smallest super-eruptions – sufficient to bury the entire UK under a metre of ash – we might expect unprecedented swelling of the ground surface – perhaps by tens of metres – and huge numbers of earthquakes, with both the swelling and shaking increasing in intensity until the magma eventually blasted its way out.

But there is another rather worrying possibility. What if instead of forcing its way upwards, the magma stays where it is until natural tectonic forces open a deep fault or fracture that intersects it and provides instant access to the surface? Then we may see almost nothing in the way of warning signs until it is far too late.

Notwithstanding this possibility, the frequency of super-eruptions at Yellowstone is so low that the chances of the current burst of activity heralding such an event are microscopically small. It is possible that an eruption on a smaller scale could occur, while a steam explosion would be a fascinating and scientifically curious incident, but hardly a world-shattering event.

A far more likely outcome is that the earthquake swarm will gradually fade away, the swelling that has accumulated over the last few years will subside, and restless Yellowstone will turn over and go to sleep for another few decades.

Bill McGuire is director of the Aon Benfield Hazard Research Centre at University College London. His latest book, Seven Years to Save the Planet, is published by Weidenfeld & Nicolson