Each program at each university will have a different character. I would recommend deciding what aspect of volcanology that you want to study, i.e. volcano seismology, stratigraphy, modeling, etc., then research the universities carefully to find out where the people are that are doing this sort of work.
Be sure to talk to prospective advisors to know if you will get along with them and that your interests overlap. Also, more importantly, talk with the current and former students of your prospective advisor to see how they feel about both their advisor and the university. Most universities will set up meetings to allow you to do this if you ask.
Also, ask about what your potential projects will be and where, or if, you will be doing field work. Also, check into the availability of teaching and research assistantships and what the requirements for your degree will be.
For more information and a partial list of universities that offer graduate programs related to volcanology follow this link: