[sammyg2]

From wiki:

The major features of the yellowstone caldera measure about 55 kilometers (34 mi) by 72 kilometers (45 mi) as determined by geological field work conducted by Bob Christiansen of the United States Geological Survey in the 1960s and 1970s.

One such caldera, the Bruneau-Jarbidge caldera in southern Idaho, was formed between 10 and 12 million years ago, and the event dropped ash to the depth of a foot 1,000 miles (1,600 km) away in northeastern Nebraska and killed a large herd of rhinoceroses, camels, and other animals at Ashfall Fossil Beds State Historical Park. Within the past 17 million years, 142 or more caldera-forming eruptions have occurred from the Yellowstone hotspot .
(all this talk about polar bears, what about the rhinoceroseseses? )

The last full-scale eruption of the Yellowstone Supervolcano, the Lava Creek eruption which happened nearly 640,000 years ago, ejected approximately 240 cubic miles (1000 cubic kilometres) of rock and dust into the sky.

Geologists are closely monitoring the rise and fall of the Yellowstone Plateau, which averages ±0.6 inches (about ±1.5 cm) yearly, as an indication of changes in magma chamber pressure.

The upward movement of the Yellowstone caldera floor—almost 3 inches (7 centimeters) each year for the past three years—is more than three times greater than ever observed since such measurements began in 1923. From mid-Summer 2004 through mid-Summer 2008, the land surface within the caldera has moved upwards, as much as 8 inches at the White Lake GPS station. The U.S. Geological Survey, University of Utah and National Park Service scientists with the Yellowstone Volcano Observatory maintain that they "see no evidence that another such cataclysmic eruption will occur at Yellowstone in the foreseeable future. Recurrence intervals of these events are neither regular nor predictable."