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Sunset Crater Volcano National Monument, Arizona
North of Flagstaff, (and a great Route 66 detour) the San Francisco Volcanic Field of around 400 volcanoes is prominent on the western horizon from the Hopi Mesas, and its peaks are said to be the home of their powerful kachina spirits. Some of the volcanoes are still active, though the most recent eruption was that of Sunset Crater in around 1066AD. The crater was named by John Wesley Powell for the many colours of its cone.
It's too unstable to walk around the rim (visit Meteor Crater if you wish to do this) but a trail passes through the lava tubes around its base that you can explore. There's also a very interesting visitor centre at the monument where you can learn about the history and geology of this fascinating area.