I think many contributors are missing the point. Comparing volcanoes around the Pacific ring of fire or those in Italy has little value.
This article:
BBC News - Could another Icelandic volcano erupt soon?
quotes: Professor Gudmundsson says there are "no signs yet" of an impending (Katla) eruption. "Our eyes are not glued to Katla, we are thinking about the eruption that is happening now." But Dr Goodenough adds that "substantial amounts of magma" are rising underneath both volcanoes. Katla is Eyjafjallajokull's more active neighbour, and scientists believe that there may be a link between the two volcanoes.
And what makes Eyjafjallajokull and Katla different? Well the Katla "volcano's ice sheet is 600-700m thick", and it's this that makes the difference. Magma that erupts through ice has a very different composition to that of other volcanic activity. Being cooled rapidly the silica produces a form of glass ash that melts well below the temperatures found in most modern engines. My understanding is that when it then solidifies it agregates, not dissimilar to water vapour producing snow. So who wants to try flying when the next time you go to max thrust, one, two or three engines say "No" and die. A bit scary with a full fuel load and lots of SLF on board.
PS. The greatest danger to aviation safety is complacency. Modern engines have become too dependable, to the extent that they are often taken for granted, until they fail!! Boeings and Airbuses don't glide very well.