'Super-eruptions' of major volcanic systems such as Yellowstone are triggered by a combination of the temperature influence and precise shape of the magma chamber, say scientists.
And the good news is that Yellowstone, at least, appears in no danger of an imminent eruption.
Such super-eruptions occur about every 100,000 years and are some of the most catastrophic natural events on Earth. They've have been implicated in the start of ice ages.
The eruption of the Huckleberry Ridge in Yellowstone Park about two million years ago was more than 2,000 times larger than the 1980 eruption of Mount St Helens in Washington.
Other super-volcano sites include Lake Toba in Sumatra, the central Andes Mountains, New Zealand and Japan.
The Oregon State University team says that super-volcanoes don't erupt for the same reasons as ordinary ones - generally, a trigger from within.
"Most models have simply taken small historic eruptions and tried to scale the process up to super-volcanic proportions," says geologist Shanaka de Silva.
"Those of us who actually study these phenomena have known for a long time that these eruptions are not simply scaled-up Mt Mazamas or Krakataus – the scaling is non-linear. The evidence is clear."
Instead, says the team, the creation of a ductile halo of rock around the magma chamber allows pressure to build up over tens of thousands of years.
This gradually lifts the roof above the magma chamber - until, eventually, faults from above trigger a collapse of the caldera and an eruption.
"You can compare it to cracks forming on the top of baking bread as it expands. As the magma chamber pressurizes at depth, cracks form at the surface to accommodate the doming and expansion. Eventually, the cracks grow in size and propagate downward toward the magma chamber," says post-doctoral researcher Patricia Gregg.
"In the case of very large volcanoes, when the cracks penetrate deep enough, they can rupture the magma chamber wall and trigger roof collapse and eruption."
Gregg says it doesn't appear that Yellowstone is primed for another super-eruption anytime soon.
"The uplift of the surface at Yellowstone right now is on the order of millimeters," she says.
"When the Huckleberry Ridge eruption took place, the uplift of the whole Yellowstone region would have been hundreds of meters high, and perhaps as much as a kilometer."