There are two answers to this. The obvious one is, yes of course they could. The annoying one, is that it has to be negotiated, world wide flight maps would hav

There are two answers to this. The obvious one is, yes of course they could. The annoying one, is that it has to be negotiated, world wide flight maps would have to be updated, flight traffic control reprogrammed, and it is just easier by far in terms of bureaucracy not to do this stuff. After all, people must have known when they chose to live there... The flight plan into Keflavik in Iceland goes directly over Mount Hekla (the way point is Mount Hekla, which probably made more sense back in the days of visual navigation). Mount Hekla is a fairly active Volcano, and is a little overdue for its next explosion, for which there might be at most about half an hours warning time. After all, people must know the risks when they choose to fly...