NARRATOR: Vulcanologists believe it would all start with the magma chamber becoming unstable.
BILL McGUIRE: You'd start seeing bigger earthquakes, you may see parts of Yellowstone uplifting as magma intrudes and gets nearer and nearer the surface.
ROBERT SMITH: And maybe an earthquake sends a rupture through the brittle layer, you've broken the lid of the pressure cooker.
BILL McGUIRE: This would generate sheets of magma which will be probably rising up to 30, 40, 50 kilometres sending gigantic amounts of debris into the atmosphere.
http://www.bbc.co.uk/science/horizon/1999/supervolcanoes_script.shtmlWhat makes me uneasy about super-volcanoes (as opposed to terrorists) is we can fight the terrorists but you can't fight Mother Nature. It's that "helpless" feeling that there is nothing you can do about it.