If the mods will permit my two cents on what is probably the most viewed thread in PPRuNe history...
Yellowstone National Park in the US is a huge volcanic caldera waiting to go off again someday. Check out the Old Faithful Geyser!
Yellowstone Caldera - Wikipedia, the free encyclopedia
I flew over Mt Saint Helens as SLF a month after it blew. I've never seen such a huge fresh hole in a mountain before or since, but aside from the occasional puff of smoke and steam, it's been pretty quiet.
As covered by many others in this thread, the Icelandic volcano seems to produce an especially abrasive ash that the jet stream carries over much of European flight paths. (I wonder if it's affecting migratory birds?)