New York, Nov 5 (IANS): Supervolcanoes with massive eruptions with potential global consequences become active when the roof above them cracks or collapses, not because of internal pressure building, suggests new research.

Knowledge of triggering mechanisms is crucial for monitoring supervolcano systems, including ones that lie beneath Yellowstone National Park and Long Valley, California, US, the researchers pointed out.

"If we want to monitor supervolcanoes to determine if one is progressing toward eruption, we need better understanding of what triggers a supereruption," said lead researcher Patricia Gregg, professor of geology at University of Illinois.

"It is very likely that supereruptions must be triggered by an external mechanism and not an internal mechanism, which makes them very different from the typical, smaller volcanoes that we monitor," Gregg noted.

Considered five hundred times larger than a typical volcanoe, a supervolcano is classed as more than 500 cubic kilometres of erupted magma volume.

"Typically, when we think about how a volcanic eruption is triggered, we are taught that the pressure in the magma chamber increases until it causes an explosion and the volcano erupts," Gregg said.

"This is the prevailing hypothesis for how eruptions are triggered. At supervolcanic sites, however, we do not see a lot of evidence for pressurisation,” Gregg noted.

According to the new model that the researchers developed, if a crack or fault in the roof penetrates the magma chamber, the magma uses the crack as a vent to shoot to the surface. This could trigger a chain reaction that "unzips" the whole supervolcano.

The study was published in the Journal of Volcanology and Geothermal Research.