Yellowstone's Super Sisters: A
List of Known Supervolcanoes
Written by Discovery Channel June 24, 2007
From the page: "There are other supervolcanoes on Earth, some of which erupted in prehistoric times and could erupt again. At least one has had an eruption bigger than Yellowstone's largest and may have played a critical role in shaping human history. The history -- even the location -- of others is less certain. Some are identifiable only by the deep layers of ash they left behind, such as the more than 1,000 cubic miles of tuff dumped in eastern Africa and the Red Sea by a mystery eruption somewhere in Ethiopia. There are also calderas that have just not been well studied, a prime example being Ethiopia's 460-square-mile Awasa caldera and the 1,000-square-mile Pastos Grandes caldera of Bolivia, which rivals the latter, which in turn rivals the largest in the world: Lake Toba, Indonesia.
LONG VALLEY, CALIFORNIA
"Second only to Yellowstone in North America is the Long Valley caldera, in east-central California. The 200-square-mile caldera is just south of Mono Lake, near the Nevada state line. The biggest eruption from Long Valley was 760,000 years ago, which unleashed 2,000 to 3,000 times as much lava and ash as Mount St. Helens, after which the caldera floor dropped about a mile, according to the U.S. Geological Survey. Some of the ash reached as far east as Nebraska.
"Long Valley's most recent eruption was in Mono Lake just 250 years ago, but it was very small. More worrisome is a swarm of strong earthquakes in 1980 and the 10-inch rise of about 100 square miles of caldera floor. Those developments have geologists concerned that Long Valley is gearing up for another eruption of some sort..."