The United States is one of the most volcanically active countries in the world. According to the global volcanism data-base of the Smithsonian Institution (Simkin and Siebert, 1994, 2000; Siebert and Simkin, 2009), the United States (including its Commonwealth of the Northern Mariana Islands) is home to about 170 volcanoes that are in an eruptive phase, have erupted in historical time, or have not erupted recently but are young enough (eruptions within the past 10,000 years) to be capable of reawakening. From 1980 through 2008, 30 of these volcanoes erupted, several repeatedly (Diefenbach and others, 2009).