This whole subject encouraged me to fire up the google machine to look up a little more info about the Yellowstone supervolcano.
One thing that has fascinated me for some time about Yellowstone is the concept of how HUGE the caldera is. If you drive through the park, a good share of the current topography of the landscape is actually the bottom of the crater left over from the last giant eruption, which was about 630,00 years ago.
Then, if you drive from Ashton, Idaho, up to Big Sky, Montana, about fifty miles of that drive will be through the bottom of the crater left over from the PREVIOUS eruption, which was about 1.3 million years ago.
And if you look at this map (from the following incredibly interesting site http://www.swisseduc.ch/stromboli/perm/yellowstone/geol-en.html):
Those blue, red, yellow, and brown splotches are identified calderas from explosions of the Yellowstone supervolcano going back as far as 16.5 million years. The hotspot underneath present-day Yellowstone Park stays constant as the North American plate slowly (I mean REALLY slowly) drifts over it toward the southwest on its way to the San Andreas Fault where it dives under the Pacific plate, I guess.
Every 700,000 years or so, the Yellowstone supervolcano explodes, leaving another caldera to slowly march its way toward California.
And speaking of ash, this image from the same site:
shows the portion of North America that was heavily covered in ash by the last explosion of the supervolcano, 630,000 years ago. If you read the accomanying text, it explains that THIS explosion was pretty mild compared to the third-to-the-most-recent one 2.1 million years ago. Here's what it says, with my bolding added:The previous caldera forming eruption of 1.3 million years before present produced 280, the one of 2.1 million years a staggering 2450 cubic kilometers of ash. These and the Toba caldera of Sumatra are among the greatest quartenary volcanic eruptions known worldwide. In comparison, the 1980 eruption of Mount St. Helens seems minute: only 1.3 cubic kilometers of lava was erupted. The most recent volcanic eruptions took place at Yellowstone 70'000 years ago. For more detailed information go to "Volcanic History of the Yellowstone Plateau Volcanic Field" by the USGS and University of Utah.
This is just fascinating stuff.