The first volcanoes in the San Francisco Volcanic Field began to erupt about 6 million years ago, in an area where the town of Williams is now. Subsequently, a several-mile-wide belt of successively younger eruptions migrated eastward, to the area of modern Flagstaff, and beyond toward the valley of the Little Colorado River. Today, this belt of volcanoes extends about 50 miles from west to east.
Most of the more than 600 volcanoes in the San Francisco Volcanic Field are monogenetic (the source of a single volcanic eruption) basalt cinder cones. The cinder cones are relatively small, usually less than 300 m (1,000 ft) tall, and formed within months to years and are considered single eruptive events. Many of them have adjacent lava flows that formed as lava oozed out from the base of the cone and flowed away for a substantial distance. SP Crater, 40 km (25 mi) north of Flagstaff, is an excellent example of a cinder cone, and its associated lava flow extends for 7 km (4.3 mi) to the north of the cone. Several zones of concentrated eruptive activity have been identified – the more silicic volcanic centers appear to have begun with basalticactivity and then evolved to more silicic compositions.
Why does Northern Arizona have so many geologically young volcanoes? Most volcanoes are located near boundaries of the Earth's tectonic plates, but Arizona is well within the interior of the North American Plate. By looking at seismic waves and studying elements and isotopes within rocks from the region, scientists now believe that the layers of molten material in the mantle circulate beneath the Colorado Plateau. Warm material from the asthenosphere (lower mantle) is able to invade the lithosphere (upper mantle). The influx of hotter lower-mantle mantle material below the crust provides the heat necessary for volcanism.
