Caracchi, the lead author of one of the papers, compares the way magma buoys up through the crust to how a balloon buoys up in water -- so powerfully that it's difficult to keep it just under the surface. "It's got to come up," he said.
There is also another factor which helps enlarge the magma chamber itself, at least for Yellowstone, according to the University of Utah's Robert B. Smith, chief seismologist for the Yellowstone Volcano Observatory.
"In Yellowstone we have made a case that (the magma chamber is) so big because it's in an area of lithospheric extension," Smith said. This means the crust of the Earth there is being pulled apart.
"It allows magma to ascend much more easily," Smith said. "It allows it to rise and enlarge."
In fact, GPS stations used to monitor Yellowstone show that it's stretching apart at a rate of 3.5 to 4 millimeters per year, Smith said. That's 3.5 to 4 meters per thousand years or 3.5 to 4 kilometers over a million years.
The last Yellowstone eruptions were 160,000 to 70,000 years ago. That was preceded by eruptions at 2.1 million, 1.3 million and 640,000 years ago. Understanding exactly what sets off these giant eruptions -- none have occurred in human history -- is critical for forecasting them, Smith said.
"You really have to address the physics of magma systems," said Smith, who points to the new studies as examples of just that kind of research. "It's the next big area we have to focus on. We won't be able to forecast eruptions without that knowledge."