Yosemite's a volcanic crater (or, to be more precise, a caldera).
Every so often, the whole damn park explodes, with enough style to make Krakatoa look like a bottle rocket. Kind of screws up the atmosphere for a while, and we're right about due for another explosion.
http://www.bbc.co.uk/science/horizon...olcanoes.shtml
http://en.wikipedia.org/wiki/Supervolcano