Yellowstone National Park is a wonder of the natural world. Its geysers, hot springs, and bubbling mud pools are some of the most impressive examples of geologic activity, and amaze even those who have never visited the park (including myself). Yellowstone became the first National Park in the world when President Grant set it aside for all of posterity in 1872. The dawning of the age of the automobile brought a huge surge of interest in the park, and was the inspiration for the Yogi Bear cartoon.

But those early visitors, and the scientists who came just as eagerly, had no idea what they were really coming to see. It wasn’t until the late 1960s that Bob Christiansen, a geologist with the US Geological Survey (USGS) began to realize that the cliffs surrounding the park were really the rim of volcanic caldera. A volcanic caldera forms when the ground collapses because the magma beneath it erupted. Why did it take geologists so long to recognize Yellowstone for what it is? Because usually volcanic calderas are at most 2-4 km in diameter. The cliffs of the Yellowstone caldera stand 65 kilometers apart.

The Yellowstone Caldera sits next to two more ancient volcanic calderas to the south west. It just so happened that right when Christiansen began to trace out these enormous ancient calderas, other scientists began to use radioactive dating techniques on ash-fall deposits throughout the central United States. Three major ash-falls blanket much of the country from California through Idaho, Iowa, and down to northern Louisiana. These blankets were dated at approximately 2.2 million, 1.3 million, and 640,000 years old; exactly the same age as the three Yellowstone calderas.

By carefully recording the thickness and extent of each ash layer, scientists determined that the three eruptions had released between 300 and 2,500 cubic kilometers of ash. Mt. St Helens released a paltry-by-comparison 1 cubic kilometer. The largest of the three eruptions stands as the fifth largest eruption in all of known geologic history. That is how Yellowstone became known as a supervolcano.

Interestingly, the three Yellowstone eruptions have been between 650,000 and 900,000 years apart. Despite the very limited statistics, one could say that we may be due soon for another eruption, considering that it has been 640,000 years since the last. If one were so inclined, one could calculate the odds of such an eruption occurring each year: approximately 1 in 700,000. But, since it has been quite a while since the last eruption, it would seem that the odds are something more like 1 in 100,000 or so. Given that an average American lives to be about 70 years old, that would put each of our odds of witnessing another giant Yellowstone eruption at about 1 in 13,000. That folks, means you are far more likely witness such an eruption than you are to die in a plane crash.

Now those crude calculations are a might speculative, and any geologist would be quick to point out that there are no signs that an eruption at Yellowstone is imminent. There are, however, signs that Yellowstone is working up another blow. Since observations began in about 1925, the caldera floor has been slowly rising each year as magma refills the chamber 5 km below the surface. Our abilities to monitor the supervolcano have improved over the decades, and now we can detect the swarms of earthquakes that signal the movement of huge magma flows far below.

From observations of much smaller volcanic eruptions elsewhere, we know that earthquake swarms grow more frequent and intense as an eruption grows near. So geologists expect that any eruption at Yellowstone, even one much smaller than those that formed the three calderas, would show these signs as well. In 2001, the USGS and the University of Utah formed the Yellowstone Volcano Observatory to monitor seismic activity in the area. Hopefully the Observatory can provide enough warning to save the residents of the central United States. But, a nation-wide evacuation order would most certainly not be universally obeyed.

What would an eruption on the scale of the Yellowstone caldera eruptions be like? First, the eruption itself might last for weeks to years. During that time it would release steam and ash into the atmosphere that would partially block the sun, chilling the entire world. For thousands of miles, a blanket of ash would rain down on the land killing crops, poisoning water, and destroying mechanical and electrical equipment. Unless evacuated beforehand, people across the midwest would probably not get enough warning to leave before they are buried or suffocate on the ash. The casualties would no doubt be in the millions.

The good news is, that just like the stock market, past geologic events are no guarantee of future activity. But even so, if I lived in the part of the US that is likely to be affected by a massive eruption, I would listen to those evacuation orders pretty closely.