"For starters, there's the ash: computer modeling indicates that a supereruption (as befits a supervolcano) would spew about 240 miles of it into the atmosphere, effectively shutting down electronic communications and air travel throughout the continent as it dispersed in what's known as an "umbrella" cloud. How it lands depends on wind conditions, but cities closest to the eruption could end up blanketed in up to a meter of ash; those on the East, West and Gulf Coast would get smaller, but still disruptive, ash deposits of their own."