The three largest Yellowstone eruptions produced a rain of ash that spread over much of western and central North America and beyond; these ash deposits are greater than 2 m thick near their eruptive sources and as much as a meter thick in surrounding areas.
Each of these three eruptions produced a caldera, or deep crater-like depression, tens of kilometers wide, formed by collapse of the ground surface into a partly emptied subterranean magma chamber. The latest of these three great eruptions formed the Yellowstone caldera. Renewed magma influx beneath the Yellowstone caldera in central Yellowstone National Park uplifted parts of the caldera floor and produced voluminous intracaldera lavas, the youngest of which extruded in a series of eruptive episodes about 164,000, 152,000, 114,000, 102,000, and 72,000 years ago.
Available data suggest a highly episodic behavior of past eruptions of this sort, periods of a few thousand years characterized by numerous eruptions being separated by longer intervals of about 12,000 to 38,000 years without eruption. One statistical measure of eruption probabilities based on this episodic behavior suggests an average recurrence of 20,000 years. The fact that no such eruption has occurred for more than 70,000 years may mean that insufficient eruptible magma remains beneath the Yellowstone caldera to produce another large volume lava flow.
Of the hazards, the most likely to occur are hydrothermal explosions, with an average annual probability from as high as 0.5 (equivalent to an average recurrence of 2 years) for small explosions to perhaps 5x103 (an average recurrence of 200 years) for explosions large enough to form a 100-m-diameter crater. Hydrothermal explosions that might result in potential risks to people have a probability no greater than 0.1 per year (an average recurrence of 10 years) for small explosions. Potential for a volcanic eruption is much lower than that for hydrothermal explosions.
