The Classic Caldera
Now comes the tricky part of working out which one is more likely to destroy the world – or get near enough. Place your bets now!
In order to do this, we need to look at the eruption history of these monsters. So let’s start with the classic, Yellowstone. This caldera is 72 kilometers (45 miles) across – so large that you can only really see it from space.
The three eruptions at Yellowstone – 2.1 million, 1.3 million, and 640,000 years ago – formed distinct, interwoven calderas, with the latest being named the Yellowstone Caldera. The most powerful of the three was the first, registering at a VEI 8, which produced 2,500 times the volume of volcanic debris as the 1980 cataclysm at Mount St. Helens.
The least powerful blast, still a VEI 6-7, was the second, with the most recent being the second-most powerful, another VEI 8. Smaller, crater-forming, and lava-effusing blasts have happened, but one day, at some point, another super-eruption will likely take place.
Speaking of which, if you’ll peek at those dates, you might have noticed that it erupts once every 660,000-800,000 years, which suggests that the next blast will take place around 50,000 years from now. Some scientists, however, think Yellowstone’s already overdue for another blast by around 20,000 years, but there’s not enough data to say either way.
Today, the magma source under Yellowstone is unbelievably massive. For years, geophysicists thought there was just a shallow, fairly sizable magma cache under the National Park – but there’s actually another, deeper one that was discovered in 2015. In total, there’s enough molten rock down there to fill up to 14 Grand Canyons right up to the brim.
When it erupts, pretty much the entire chamber will empty out onto the surface in an explosive decompression event. Pyroclastic flows will essentially wipe out anything in the National Park, but the real danger to the nation – and the world – is the ash fallout.
Yellowstone's former ash fallouts, mapped. USGS
Based on the ancient eruptions, thousands upon thousands of cubic kilometers of ash will blanket about 60-70 percent of the US within a day or two, with much of this area ending up smothered in a layer of ash 1 meter (3.3 feet) thick. This will bring agriculture to a grinding halt, cause millions of buildings to collapse under the weight, and cause millions of people to suffer from respiratory problems, perhaps fatally.