Trace a line on the map from northern Nevada through southern Idaho and up into northwest Wyoming, and you'll follow an intermittent scar of volcanic mayhem extending 350 miles (560 kilometers) and stretching back 18 million years. The chain of volcanoes grows progressively younger as you move along this west-to east line, each one marking an area where magma pressure from a lone hot spot broke through. The chain, like the hot spot, dead-ends in Yellowstone National Park [sources: Achenbach; Geological Society of London].
Actually, it isn't the hot spot that moves. Rather, the North American plate grinds overhead at around 1.8 inches (4.6 centimeters) per year. Every so often, the hot spot bursts through. During its more than 2 million years beneath Yellowstone, it has produced three jumbo-sized events [sources: Achenbach; Robinson; Tyson; USGS]:
- 2.1 million years ago, the Huckleberry Ridge event blasted out 588 cubic miles (2,450 cubic kilometers) of material and created a caldera the size of four Manhattans
- 1.3 million years ago, a supereruption at Mesa Falls kicked out an estimated 67 cubic miles (280 cubic kilometers) of ejecta (a category 7 VEI, but often treated as a supervolcano)
- 640,000 years ago, the Lava Creek supervolcano erupted with 240 cubic miles (1,000 cubic kilometers) of material, with a possible ash pillar altitude of 100,000 feet (30,480 meters). Debris from the event scattered across the American West and down to the Gulf of Mexico.
Today, the Yellowstone hot spot has assumed a mellower mien, as far as we know. It heats the park's famed geysers, hot springs, steam vents and mudpots, and takes some of the chill off of Yellowstone Lake -- itself partly formed from a collapsed supervolcano caldera. But it also occasionally causes the ground above to dome unsettlingly and remind us that a sleeping dragon is still, after all, a dragon [sources: Achenbach; Encyclopedia Britannica; USGS].
Although researchers monitor Yellowstone for earthquakes, ground deformation, stream flow and temperature, how much warning a supervolcano might provide before erupting remains unclear [sources: Geological Society of London; Tyson; USGS]. Earthquakes, of which Yellowstone has 1,000-3,000 annually, might warn of a volcanic event, but they might also release pressure and thereby help prevent one [sources: Achenbach; USGS].
Supervolcanoes also periodically release pressure via smaller eruptions. In the 640,000 years since Lava Creek, Yellowstone has experienced roughly 80 nonexplosive, lava-producing eruptions, and the next Yellowstone eruption will more likely be of the scale of Pinatubo -- far from insignificant, but not a supervolcano [sources: Achenbach; USGS].
But what if the dice don't roll our way? What might a supervolcanic eruption at Yellowstone be like?