One way of looking at the power of volcanoes is what scientists call the Volcano Explosivity Index (VEI) — sort of a Richter scale for eruptions. And like the Richter scale used to measure earthquakes, the power of an eruption increases exponentially from number to number in the VEI index.
The VEI scale runs from zero to eight. The higher the VEI number, the bigger — and less frequent — the eruptions. On one end there are the burbling, rather gentle eruptions that happen on the big island of Hawaii. These happen daily on Earth, and even with their occasional impressive fountains of lava, they rate a zero on the VEI.
At the other extreme is the Yellowstone eruption of 2.1 million years ago, which is described on the VEI as an eight: mega-colossal, with a towering ash cloud 10 miles high that pours out at least a thousand cubic miles of ash. That Yellowstone eruption had 10 times the ejected material as a VEI 7 volcano, which modern humans have never seen either.
In fact, the last VEI 7 eruption was in Toba, Indonesia, 74,000 years ago, and it caused such global cooling that some scientists think it nearly drove humans to extinction.
The largest known eruption in the last several thousand years is believed to be that of Tambora, Indonesia, in 1815. It was tens of times more massive an eruption than Mount St. Helens in 1980. Despite pouring out 7 cubic miles of ash and causing short-term global cooling, Tambora was small fry compared with any of Yellowstone's big eruptions, or even the eruption of Toba 74,000 years ago.
No eruptions of this magnitude have happened since the dawn of civilization, about 10,000 years ago — which is lucky for us, and perhaps one reason civilization has been able to develop.
As with Yellowstone, none of the other caldera-based supervolcanoes around the globe fit the classic volcano image.
Calderas are broad, sunken areas often filled with lakes, ringed with hot springs and landscaped with domes of lava. They are something like cauldrons, after which they were named, and tend to be the largest volcanoes on the planet — hence the less formal name they are sometimes given: supervolcanoes.
Like other calderas worldwide, the Yellowstone caldera landscape was created by the "roof collapse" on a subterranean chamber after molten rock — called magma — was ejected in massive prehistoric eruptions. It's almost as if there was a giant magma balloon under the surface that suddenly deflated. The deflation itself is the super-eruption, and the sunken land left behind is the caldera.
Also, as with many other calderas, there is still hot material not far underground at Yellowstone, which is why there are so many hot springs and geysers today.
As for what caused the land to inflate with magma and explode in the first place, it was a powerful "hot spot" welling up from deep in the Earth and melting rock closer to the surface into magma, says Smith.
But what caused the hot spot? And what can explain it today? Scientists are still learning answers to these questions.
"A hot spot is a long-lived point spot of magmatism," explains geologist Paul Ihinger of the University of Wisconsin at Eau Claire. Among the most famous places made by hot spots are the Hawaiian Islands, Iceland and, of course, Yellowstone.
In the case of Yellowstone, the hot spot has left a 350-mile trail marked by several generations of ever-older and deader calderas marching away to the southwest of Yellowstone.
The oldest is a 15- to 16.5-million-year-old dead caldera straddling the Oregon-Nevada state line near McDermitt, Nev. The trail of dead calderas is evidence that the hot spot has remained in place while the North American continent has moved southwest over it.
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