Annabel grinned brightly, pleased that she was going to get to explain the horror and wonderful of this to him. She leaned in towards him, her expression animated.
"A supervolcano is defined as is any volcano with an ejection volume greater than 1,000 km cubed. Which, uh, is science talk to say it's thousands of times bigger than any normal volcanic eruption. They happen where magma rises into the crust from a hotspot and then pressure builds until the crust can't contain it anymore. The size of them means that when they erupt they send lava and ash for thousands of miles and change the weather of the planet, and can even trigger ice ages and mass extinction. When Yellowstone goes it'll destroy almost the entire United States and lead to years of volcanic winter that will probably kill off almost of all mankind, if we're still around."
Annabel was smiling brightly as she spoke, quickly and with excitement, as though this was all wonderful news.