Mount Jefferson, Oregon, USA
(c) 2005 Andrew Alden, licensed to About.com (fair use policy)
Mount Jefferson, in north-central Oregon just south of Mount Hood, hasn't erupted in some 15,000 years, but no one has declared it extinct. Its flows date back about 300,000 years, after all, and its ash has been detected in deposits as far away as southeastern Idaho. Interestingly, its most recent lava flows display the flash-frozen walls so well known from Iceland, where volcanoes today are still effectively in the Ice Age. Apparently this peak had its own ice cap at the time.
Mount Jefferson and a large region around it are built primarily of relatively high silica rocks—dacite and andesite. These contain a goodly share of quartz as opposed to the nearly quartz-free basalt of the rest of the Cascade volcanoes. The area lies near the active part of the Newberry hotspot, and maybe there's a connection.
