These supereruptions would have been very dramatic, noted Stephen Self, a volcanologist from Britain's Open University who was not involved in the study.
Such events can produce at least 77 cubic miles (360 cubic kilometers) of basalt: enough to bury Washington, D.C., under nearly 7,200 feet (2,200 meters) of solidified lava, Self said during an October meeting of the Geological Society of America.
Even the largest of the more modern eruptions can't compare, he said. "Supereruptions are very, very much larger. We're talking a factor of a thousand or so."
Luckily for us, eruptions on that scale are rare.
"Compared to our human lifespan, these systems have a very long gestation period, if you will," Self told National Geographic News. "So we're not going to have an eruption immanently, almost certainly."
Furthermore, he said, magma needs to be relatively fluid and gas-rich before it can erupt.
Otherwise, "it's not going anywhere. It could stay down there for a long time, or feed little eruptions, which these caldera systems like Yellowstone have done throughout their history."
Study leader Smith agrees, noting that, for Yellowstone, eruptions the size of Mount St. Helens's 1980 blast or Mount Pinatubo's 1991 explosion happen every few tens of thousands of years.
"They're much more probable, but much smaller," he said.
In addition, he said, about once every thousand years or so, Yellowstone's geyser fields explode in steam eruptions covering up to 250 acres (100 hectares). (See pictures and infrared images of Yellowstone's geysers erupting.)
Magma movements may also play a role in triggering major earthquakes in the Yellowstone area, he added. The most recent Yellowstone quake was a magnitude 7.3 in 1959, which killed 28 people.
Yellowstone Plume Even Deeper?
Smith's team has been studying the Yellowstone caldera for years, using ever-more-sensitive instruments to find the source of the magma that fuels all this activity.
The new model of the plume is largely based on data from an array of super-sensitive seismometers set out in a 400-mile-long (644-kilometer-long) pattern focused on the Yellowstone region.
"A seismometer network is just like an antenna," said Smith, who will be presenting his team's work this week during the 2009 fall meeting of the American Geophysical Union in San Francisco.
"You're listening for seismic waves that come into your antenna. You want it wide enough and long enough that you can detect these things coming up from the interior."
Supereruptions aside, Smith thinks the coolest part of the new study, published in the current issue of the Journal of Volcanology and Geothermal Research, is that the heat source for Yellowstone's famous geyser basins can be traced so deeply.
And Smith suspects that even more sensitive instruments will someday reveal that the magma source goes deeper yet.
"Ultimately," he said, "the plumbing from the geysers may go clear down to the [lower] mantle"—at least 466 miles (750 kilometers) beneath Earth's surface.
SOURCES AND RELATED WEB SITES