Supervolcanoes may be able to form in just a few hundred years - far, far more quickly than previously believed.
These devastating eruptions spew out tremendous flows of super-heated gas, ash and rock capable of blanketing entire continents and creating decade-long winters. There's evidence that one such super-eruption, which took place in Indonesia 74,000 years ago, may have come remarkably close to wiping out the entire human species.
Geologists believe that a super-eruption is produced by a giant pool of magma that forms a couple of miles below the surface and then simmers for 100,000 to 200,000 years before erupting.
But a new study suggests that this period could be as short as a few hundred years.
"Our study suggests that when these exceptionally large magma pools form they are ephemeral and cannot exist very long without erupting," says Guilherme Gualda, assistant professor of earth and environmental sciences at Vanderbilt University.
The team focused on the Bishop Tuff super-eruption that occurred in east-central California 760,000 years ago. And they found several independent lines of evidence that indicate the magma pool formed within a few thousand years - perhaps just a few hundred - before it erupted, covering half of the North American continent with hot ash.
These giant magma pools are 10 to 25 miles across and one half to three miles deep. At first, they're largely free from crystals and bubbles. Later, however, crystals and bubbles form gradually, and progressively change the magma's physical and chemical properties.
The good news is that, as far as geologists can tell, there's no such giant crystal-poor magma body currentlycapable of producing a super-eruption. The bad news, though, is that this is probably because they can form so quickly.
Gualda reckons the reason previous estimates for the lifetimes of these pools is so high is an artifact of the method used to make them.
The measurements have been made using figures for radioactive decay within zircon crystals; and, says the team, the fact that they can withstand the heat and forces found in a magma chamber means they're not good at recording the lifetimes of crystal-poor magma bodies.
Gualda and his colleagues instead determined crystallization rates of quartz. They found four independent lines of evidence indicating that the formation process took less than 10,000 years and most likely between 500 to 3,000 years.
They suggest that the zircon crystal measurements record the extensive changes that take place in the crust required before the giant magma bodies can begin forming as opposed to the formation itself.
"The fact that the process of magma body formation occurs in historical time, instead of geological time, completely changes the nature of the problem," says Gualda.
"Instead of concluding that there is virtually no risk of another super-eruption for the foreseeable future because there are no suitable magma bodies, geologists need to regularly monitor areas where super-eruptions are likely, such as Yellowstone, to provide advanced warning if such a magma body begins to form."