San Francisco Volcanic Field

The C. Hart Merriam elevation gradient traverses the San Francisco Volcanic Field, an approximately 1800 square mile area within which a wide variety of volcanic landforms shape the landscape. Indeed, these landforms create the approximately 2,200 m (~ 6,600 ft.) change in elevation over a relatively short distance of approximately 50 km (~30 miles). This elevation gradient facilitates the existence of diverse life zones over a relatively small area of northern Arizona. The relatively young volcanic rocks of the field were deposited upon much older Mesozoic and Paleozoic sedimentary rocks.

The San Francisco volcanics are named for San Francisco Mountain, the largest volcano in the field and the only stratovolcano. A stratovolcano is a tall, cone-shaped volcano that usually forms by the build up of materials that are extruded by the volcano during successive eruptions over a long period of time. It is believed that the eruptions of San Francisco volcano began about 1,000,000 years ago (1 Ma) and lasted until about 400,000 years ago (0.4 Ma). The oldest eruption to take place within the field was about 6 Ma, in the vicinity of what is now Williams. The youngest eruption to take place within the field was at Sunset Crater approximately 1,000 years ago (.001 Ma).

The lithologies (rock types) of the San Francisco volcanics are diverse as well, ranging from olivine andesite (relatively little to no silica content) to rhyolite (much higher silica content) within San Francisco Mountain alone, suggesting fractional crystallization (heavier minerals “sinking” to the bottom) within the source magma chamber. Many of the landforms are basaltic cinder cones, indicating a deep origin for those lavas, possibly within the earth’s mantle. Some geologists believe that the source of the San Francisco volcanism is a mantle “hot spot,” or upwelling of mantle material from beneath the earth’s crust. As the North American tectonic plate moves slowly west-southwest over this spot, the volcanic eruptions migrate from west to east, resulting in older volcanic rocks to the west and progressively younger rocks to the east.

Landforms

Among the most prominent landforms along the C. Hart Merriam elevation gradient are: San Francisco Mountain; the many cinder cones and lava domes that dot the landscape around San Francisco Mountain; large, lobate flows that form prominent mesas throughout the area; and the paired monoclines and faults that create linear patterns in map view and prominent scarps that are visible from the ground.

An example of a prominent cinder cone is SP Crater, which has a 4 mile-long flow extending to the north of it. A large, lobate flow that forms a prominent mesa is Black Mesa, located in the northeast corner of the gradient map and residing just south of the lowest point along the elevation gradient. This mesa is composed of Pliocene-aged (5-2 Ma) basalt, and the point on the elevation gradient sits on Triassic Moenkopi Formation, a reddish-brown unit that comprises mudstone, siltstone, silty sandstone, and sandstone. A prominent monocline/fault pair appear in the northernmost part of the map area. The fault, Mesa Butte Fault, appears as a sharp, straight line. The monocline is located southeast of the fault, and appears as a folded lip that "hangs" over the Great Basin Desert floor.