The three ancient Yellowstone supervolcano eruptions were only the latest in a series of more than 140 as the North American plate of Earth's crust and upper mantle moved southwest over the Yellowstone hotspot, starting 17 million years ago at the Oregon - Idaho - Nevada border. (sciencedaily.com)
They estimate the annual chance of a Yellowstone supervolcano eruption is 1 in 700,000. (sciencedaily.com)
Hans - F Graf of the University of Cambridge, who has also modelled the effects of supervolcanoes on climate, agrees. (newscientist.com)