• Graph of recent earthquakes

It would be a doozy if it were to happen (watch a bbc docudrama) - think hundreds of times what the eruption of mount st helens was.

If you'd rather have a textual assessment, rather than a video, here's a good few paragraphs by Cecil Adams about the scientific assessment of the yellowstone caldera.

And here's a bunch of nice graphs and stuff about what would happen in the event of a supervolcanic eruption at yellowstone.