Spanning 1800 square miles, the San Francisco Volcanic Field has produced over 600 volcanoes in north-central Arizona. In this area along the edge of the Colorado Plateau, differences in crustal thickness disrupt the flow of material in the underlying mantle. This has promoted melting, leading to the continuous formation of volcanoes here over the last six million years. The vast majority of these volcanoes are cinder cones, and one of the best examples is preserved at Sunset Crater Volcano National Monument.
Sunset Crater is the youngest volcano in Arizona at around 900 years in age. At 1000 feet in height and a mile in width at its base, it has undergone very little erosion since its relatively recent formation.
As with other cinder cones, Sunset Crater was created during repeated Strombolian eruptions of basaltic lava. Moderately viscous and gas-rich, the lava was sprayed into the air in fountains of red-hot molten rock. Blobs of lava cooled and hardened as they fell, and these cinders accumulated around the main vent. Finer ash particles were spread much farther. Later on as gas was depleted, denser lava flows burst from the sides of the cone and spread over the landscape.
The Bonito Lava Flow at Sunset Crater is one such deposit. Covering about two square miles, this dark basalt is up to 100 feet in depth in some places.
As a testament to the high gas content of this lava, the basalt here is rich with vesicles. These structures were formed when the rock hardened with gas still trapped inside:
As the lava flow cooled on the surface and formed a crust, breaks in the crust sometimes produced spatter cones like the one shown below. Here gas ejected smaller fountains of lava, and it fell back to the ground before it fully cooled and solidified. The semi-molten lava collected in blobs around this spatter vent:
As part of the larger San Francisco Volcanic Field, the area around this national monument is also rich with volcanic features. Many other cinder cones fill the skyline of the plains that extend to the north and west:
Nearby Wupatki National Monument preserves the structures built by ancient native Americans, who used both local Moenkopi sandstone and volcanic basalt in the walls of their dwellings:
At Wupatki I also came across this lava bomb, a large fragment of solidified lava several feet in length that was ejected from one of the cinder cones:
Just to the west of Sunset Crater are the San Francisco Peaks, making up the rim of a massive stratovolcano:
The tallest spot here is Humphrey’s Peak, the highest point in Arizona at 12,633 feet. These mountains dominate the skyline over much of northern Arizona, and can be seen from both Grand Canyon National Park to the north…
…and from near Winslow to the east:
For anyone interested in volcanism, Sunset Crater and the larger San Francisco Volcanic Field are must-see destinations. Although there are currently no active volcanoes here, the sheer number of volcanic features make it worth the trip. And if the past is any indication, this area will again one day host active volcanoes.