The only volcanic arc in the lower 48 states continues to be pretty darn quiet according to the USGS. The Cascade range that spans from Lassen Peak in the south to Mt. Baker in the north (and maybe Mt. Garibaldi in Canada) has been remarkably quiet in terms of eruptions for the last century. In fact, only Mt. Saint Helens and Lassen Peak have erupted since 1900 - and most of the volcano have shown very few signs of even coming close to eruption beyond minor seismic swarms, steaming or land deformation. This is in contrast to the reports from the 1800s that suggest that Hood, Shasta, St. Helens, Baker, South Sister(?) and Rainier all experienced at least minor ash emissions. Now, these historical accounts are notoriously sketchy, but few people would argue that compared to other arcs such as the Aleutians or Kamchatka, the Cascades are a mighty quiet arc. The reasons for this are anybody's guess at this point, with suggestions varying from the migration of the Mendocino Triple Junction to the shallow dip of the Juan de Fuca plate to random chance. It is definitely a fascinating question to consider, especially with the potential hazards a "reawakening" of the Cascade Arc might have on the Western U.S.
