An ash plume from Soufriere Hills on Montserrat, taken from the ISS on October 11, 2009.
If you ever wonder what might happen to the U.S. if a large volcanic eruption, lets say from Rainier or Long Valley or Shasta, occurred, you can look at the island of Montserrat for some of the potentials problems. The renewed activity at Soufriere Hills (video link) that started in October is causing problems with the power infrastructure of the island – specifically the ash from the eruption is falling on power lines and damaging them. Ash has a minor electrical charge, so it will coat anything with a charge that attracts it, so power lines could be coated with ash very easily during a volcanic eruption. The same can be said for transfer stations and transformers – the power grid can potentially be very vulnerable to an ash-rich event. Also noted on Montserrat is disruptions caused by lightning generated by pyroclastic flows, which add another element of hazard during an eruption.
The current wind pattern has been pushing ash into “safe zones” on the island. You can get an idea of just how thick the current ash plume is from new NASA Earth Observatory images of the volcano. The plume from the dome heads off to the west, shadowing almost the entire lower third of the island.