Yellowstone: the time bomb under America

Deep beneath Yellowstone National Park lies a vast super-volcano which, if it blew up, could devastate much of the US. Recently, it's been a bit too restless for comfort. David Usborne reports

Visitors to Yellowstone National Park in the north-western United States know not to be careless about the bears that roam its pines or the many hissing and sizzling geysers that dot its magnificent landscape. Few ever worry about being blown into space, though.

Startling new geological data published yesterday in the journal Science suggests that it might be a good idea for most of us – and certainly those living in the region – to be aware that there is more to Yellowstone than grand vistas and abundant wildlife. The hot springs are a clue to what lies beneath: seething layers of molten magma, super-heated gases and hydrothermal liquids.

Yellowstone straddles one of Earth's most studied "hot-spots", where fissures in the crust, created by volcanic eruptions of eons past, have allowed giant streams of molten rock, or magma, to push closer than normal to the planet's surface. In recent years something intriguing – if not to say thoroughly nerve-rattling – has been going on. The magma is on the move. And so is Yellowstone.

Over the past three years, according to the report, the ground in the volcanic caldera that spans about 925 square miles and accounts for much of the park's terrain has been rising towards the sky at the rate of almost three inches per year. That is three times faster than has ever been observed before. It raises the obvious question: what is happening under the park? And what might be about to happen?

The study's authors are aware, of course, that the notion of Yellowstone being some kind of humming volcanic time-bomb is not something that tourism officials will want to advertise. And, indeed, any kind of panic because of the new data, remarkable though it is, would be entirely misplaced, they insist. "There is no evidence of an imminent volcanic eruption or hydrothermal explosion. That's the bottom line," insists Robert Smith, a professor of geophysics at the University of Utah and the lead researcher in this study. "A lot of calderas worldwide go up and down over decades without erupting."

It may also be reassuring to know that no very big bangs have happened at Yellowstone for a very long time. The caldera, the walls of which are easily discernible from some vantage points in the park, was formed by some massive eruption past when a more classic-looking volcanic cone was probably obliterated. And while the park is still technically a "super-volcano", it is estimated that it has not blown its top for 640,000 years. If you are planning to be in the park on a Thursday next March, therefore, the chances of it detonating that particular afternoon are surely slim.

No one is about to take their eyes off the park, however, not least because of these unusual new findings that suggest at least that pressures beneath the ground are rising. Moreover, geologists are well aware that were a major eruption indeed to happen, the impact would rival any natural disaster the world has ever seen. Remember the destruction when Mount St Helens flipped her lid in 1980, turning 240 square miles into a wasteland? The energy released at Yellowstone would be many hundreds of times greater.

Moreover, Yellowstone may be due a massive release. Geologists believe that the super-volcano beneath the park has undergone major eruptions at roughly 650,000-year intervals. There have been about 140 such events over 16 million years. Because the last serious explosion is believed to have taken place 640,000 years ago – although there was a minor flare-up 70,000 years ago – who is to say, really, that another one is indeed not imminent? Scientists have been observing the rising and falling of the ground at Yellowstone since 1923. The last most rapid period of upward movement occurred between 1976 and 1985, but only at a rate of about one inch a year. Professor Smith and his assistants began taking their readings in 2004 with instruments aided by satellite tracking placed at numerous spots across the caldera. They have even observed undulations in the caldera's surface, with some spots rising faster than others one year and then slowing down again while different areas catch up.

In the study, Accelerated Uplift and Magmatic Intrusion of the Yellowstone Caldera, 2004 to 2006, the authors note that while most of the magma remains about 400 miles below the surface, a significant plume rises to about 30 miles deep, where it spreads out horizontally like a pancake that is larger than Los Angeles. It seems likely that the pancake is expanding and causing the floor of the caldera suddenly to rise.

"Our best evidence is that the crustal magma chamber is filling with molten rock," Professor Smith explained. "But we have no idea how long this process goes on before there either is an eruption or the inflow of molten rock stops and the caldera deflates again." In other words, something is afoot, but no techniques exist to forecast what comes next. The prediction is easier for single-channel, cone volcanoes. At a caldera such as Yellowstone, the magma could suddenly blow through at any number of locations. "We use the term 'restless' to describe these systems," Professor Smith said.

And what if the ground at Yellowstone does not start to go down? Well, these calderas, he admits, "occasionally they burp". Let's hope the park's belly-ache resolves itself – such a "burp" would shake half of the planet.