The USGS said: “Hydrothermal explosions are the most likely of Yellowstone’s various volcanic hazards, and the potential for additional future explosions is not insignificant.
“Hydrothermal explosions in Yellowstone that form craters greater than 100 meters (328 ft) wide occur on average every 700 years.
“The largest hydrothermal explosions in Yellowstone occurred after an icecap more than 1km (0.6 miles) thick receded from the Yellowstone Plateau about 13,000 years ago.”
Weaker eruptions are much more frequent but only produce craters several feet wide.
The USGS said these blasts typically erupt once every two years.
source: express.co.uk