The San Francisco Volcanic Field includes more than 600 volcanoes extending for approximately 50 miles from west to east. The volcanoes punctuate what would otherwise be a relatively flat high plateau. The field includes both Arizona’s highest mountain – Mt. Humphreys of the San Francisco peaks at 12,633 feet – and the state’s youngest volcano -Sunset Crater, which erupted less than 1,000 years ago and can be visited at Sunset Crater National Monument.
The first volcanoes in the field began to erupt about 6 million years ago and the field is considered geologically active today. Geologists believe that the most probable site of a future eruption is on the eastern edge of the volcanic field near the Little Colorado River.
Lava from nearby Merriam Crater to the west flowed across the gray Kaibab Limestone and red Moenkopi Formation of the Colorado Plateau and pushed little Colorado River’s course eastward.