Months of earthquakes would culminate in a huge quake, creating fissures to the magma chamber 7km below. As pressure is released, gases dissolved in the magma come out of solution, turning the magma into a boiling froth.
The total energy released would be equivalent to an 875,000 megaton explosion. The shockwave would kill 90,000 people. Most of the lava would fall back into the crater. Any flows would be slow and only spread 40-50km or so.
3. Ash flows
More dangerous are the pyroclastic flows – clouds of dense, hot ash flowing outwards from the vent.
A 2016 study found that the flows would probably move at 16-72km/h. That’s slow enough that you could outrun the ash in a car.
4. Volcanic winter
The US would be blanketed with ash – up to a metre deep in Salt Lake City. This would cause respiratory problems. Two hundred million tonnes of sulphur dioxide carried into the stratosphere would cool the climate for a decade.