And yet, scientists estimate that when the supereruption occurred, the magma was at 785 degrees Celsius. To explain the discrepancy, the team suspects that heat infiltrated the system so quickly that the argon didn’t have time to escape. This heat is likely what awoke the cool supervolcano, pushing it toward eruption in less than a few hundred years — perhaps within decades.

That’s a far cry from the lethargic time scales that often define the field of geology. And yet, recent research at the Yellowstone supervolcano in Wyoming and the Taupo supervolcano in New Zealand has also suggested that the events leading up to supereruptions can occur on human time scales.

The findings suggest that the supervolcano had to wake up from an extremely cold state, raising questions about how a solid ocean of magma could melt and mobilize so rapidly.

“The physics of how this all works is still being worked on and debated,” Dr. Andersen says. But many think that an injection of fresh magma further below the volcano likely did the trick.

And given that Long Valley’s magma sits a mere five kilometers below the surface, such an event wouldn’t have gone unnoticed, Dr. Gualda says. Waters circulating in the crust would have started to heat up, generating new hydrothermal features like geysers, hot springs and mud pots as the landscape started to boil over. The ground, too, would have swelled upward as the supervolcano’s magma reservoir grew.

Should other supervolcanoes play by the same rules as Long Valley, the results could help scientists read the signals of unrest preceding future supereruptions or even smaller eruptions.